Radiation Biology

Question 1

Linear Quadratic equation

Answer 1

S = exp^(-aD-BD^2)

S=surviving fraction
a = alpha, coefficient
of radiation sensitivity for linear component
B = beta. coefficient
of radiation sensitivity for quadratic component
D = dose delivered as single fraction

Question 2

EQD2 equation

and describe the concept

Answer 2

EQD2 allows us to work out an equivalent total dose as if it was in 2Gy per fraction for treatments that were given either hyper- or hypo- fractionated. To be able to calculate if it has reached tolerance dose for noral tissues when adding 2 or more treatments.

EQD2 = D x (a/B + d) / (a/B + 2)

a/B = this is the ratio for tissue/tumour under consideration with the unit Gy

Question 3

Define Biologically Effective Dose (BED) of a given schedule, including the formula for its calculation

Answer 3

i. BED is a measure of the true biological dose delivered calculated using the a/B ratio for the tissue/tumour. BED is mainly used for stereotactic RTx now
ii. BED = nd x(1+d/(a/B))

a/B ratio is for tissue/tumour, unit Gy

Question 4

i. Outline the concept of forgotten dose and its clinical relevance to retreatment of spine

Answer 4

i. Forgotten dose is the concept that normal tissue may have some recovery of their tolerance dose following radiation exposure, such that a proportion of the radiation dose is “forgotten”
ii. For this woman, as it has been 2 years since her previous treatment her normal tissue tolerance is likely higher than it would be if she had just completed treatment.

Question 5

ii. List the factors that need to be taken into account when considering re-treatment in general

Answer 5

i. OARs
i. - Initial radiation dose to OARs and calculating the EQD2 if was not treated in 2Gy fractions.
ii. - Initial radiation volume and coverage of OARs and how this relates to the planned re-irradiation ?degree of overlap.
ii. Concurrent treatments (chemotherapy or immunotherapy) delivered at the time of initial RT which could affect tolerance of OARs
iii. Time interval between the initial treatment and the planned retreatment, is there a potential for recovery of normal tissue tolerance
iv. Patient factors –
i. function of tissue/organ being retreated, i.e. is the underlying organ function good or does the patient have underlying medical conditions such as emphysema and needing to retreat lung
ii. Expected life span of patient or QOL if left untreated: are they at risk of developing late toxicities, or do the benefits of retreating outweigh late toxicity risks
iii. Patient age and ability to tolerate RTx
v. Alternative treatment options – consider if surgery or supportive care would be better options

Question 6

Briefly discuss the re-treatment tolerance for late effects for spinal cord. Include in your answer any relevant clinical and laboratory data where appropriate.

Answer 6

Tolerance dose for spinal cord = 50Gy for 0.02% risk of meylopathy
Late effects for spinal cord is myelopathy which is permanent and Lhermitte’s sign which is usually reversible. Retreatment tolerance for spinal cord can be increased with a time interval. There has been lab studies on rats and Rhesus monkeys that looked at spinal cord myelopathy with retreatment.

In human studies it is estimated a 25% recovery in dose tolerance of the spinal cord has been suggested based on data that looked at a greater than 6 month period between two course of RT

Question 7

Outline the proposed mechanisms of late radiation injury to the spinal cord.

Answer 7

Late damage includes two principal syndromes. The first, occurring from about 6 to 18 months, involves demyelination and necrosis of the white matter from parenchymal cell loss; the second is mostly a vasculopathy and has a latency of 1 to 4 years.

1. Parenchymal cell loss
   i. Occurs 6 months and over after radiation to spinal cord
   ii. Diffuse or focal demyelination and extensive necrosis, probably as a consequence of injury and loss of oligodendrocytes by killing of glial progenitor cells (astrocytes) which cause gliosis and eventuall fibrosis of the cord
2. Vascular injury
   i. Occurs 6 months to years after radiation
   ii. Fibrin rich necrosis of white matter can occur and is thought to be caused by microvascular damage leading to increased capillary permeability and resultant slowing of blood flow to obliteration of vessels
   iii. Probable mechanism is endothelial cell damage, leading to eventual obliteration of small vessels

Question 8

Define and discuss the concept of a serial functional subunit. Include in your answer a brief description of ‘threshold dose’.

Answer 8

In serial FSUs the function of the entire organ depends on the function of each individual FSU.
Serial FSU organ such as spinal cord are not significantly affected by volume effect. Function of the organ is dependent on dose (threshold dose); with a hotspot capable of causing a ‘break’ in the series structure if it reaches over a certain threshold dose.
Serial FSU = must not exceed max DOSE
Parallel FSU = must not exceed max VOLUME
No organ is completely serial or parallel.

Question 9

ii. Below are two cell survival curves for low and high α/β cell lines. (1) Identify the axes and curves labelled A-D

Answer 9

A. Surviving fraction
B. Dose in Gy
C. high a/B ratio cell line
D. low a/B ratio cell line

The dose-responsiveness relationship for late responding tissues (low alpha/beta) is more curved than for early-responding tissues (high alpha/beta) . In the linear-quadratic formulation, this translates into a larger a/B ratio for early effects than late effects.

The a/ratio is the dose at which the linear (a) and the quadratic (B) components of cell killing are equal, that is, aD=BD^2 or D = a/B

Question 10

Define the term α/β value and describe how it can be found from a cell survival curve. (1)

Answer 10

The ratio of the parameters alpha and B in the linear quadratic model; used to quantify the fractionation sensitivity of tissues.
From a cell survival curve the a/B can be found when the components of alpha cell killing are equal to B components of cell killing.

Question 11

iii. List four potential limitations of the Linear Quadratic model in clinical practice. (2)

Answer 11

i. At very low doses per fraction<1Gy, the LQ model could underestimate the biological effect of a given dose, due to the low-dose hyper-radiosensitivity phenomenon.
ii. At very high doses per fraction >8Gy, the LQ model underestimates the biological effect due to factors such as vascular and stromal damage not being taken into account.
iii. The LQ model does not include a time factor and assumes sufficient time between fractions for repair of sublethal damage
iv. It does not take into account tissue /tumour repopulation over time
v. It has not been validated with concurrent chemotherapy

Question 12

i. What is sublethal damage repair?

Answer 12

i. SLD repair is the operational term for the increase in cell survival that is observed if a given radiation dose is split into two fractions separated by a time interval.

Question 13

ii. Using a labelled diagram, outline the key differences in cell survival curves for a single fraction vs. multiple fraction course of radiation therapy. (2)

Answer 13

The concept of an “effective” survival curve for a multi fraction regimen is illustrated. If the radiation dose is delivered in a series of equal fractions separated by time interval sufficiently
long for the repair of sublethal damage to be complete between fractions, the shoulder of the curve is repeated many times. The effective dose-survival curve is an exponential function of dose, that is a straight line from the origin through a point on the single-dose survival curve corresponding to the daily dose fraction (e.g. 2Gy).
The dose resulting in one decade of cell killing (D10) is related to the D0 by the expression D10=2.3xD0

D0 of the effective survival curve (i.e. the reciprocal of the slope), defined to be the dose required to reduce the fraction of surviving cells to 37%, has a value close to 3Gy for cells of human origin

D10 is the dose required to kill 90% of the population.

Question 14

iii. Describe the mechanism of acute skin reaction including how moist desquamation develops. (3)

Answer 14

Skin is an example of a H-type tissue from Michalowski’s classification (hierarchical)
H-type tissues have 3 groups of cells present: Stem cells, Maturing/partly differentiated and functional cells.
The skin is composed of the outer layer, the epidermis, which is the site of early radiation reactions, and the deeper layer, the dermis, which is the site of late radiation reactions
The epidermis is derived from a basal layer of actively proliferating cells, which is covered by several layers of nondividing differentiating cells to the surface, at which the most superficial keratinized cells are desquamated. It takes about 14 days from the time a newly formed cell leaves the basal layer to the time it is desquamated from the surface. The target cells for radiation damage are the dividing stem cells in the basal layer.
Early erythema develops in the second to third week of a fractionated regimen, similar to sunburn, which is caused by vasodilation, oedema, and loss of plasma constituents from capillaries. Reactions resulting from stem cell death take longer to develop and cause desquamation from depletion of the basal cell population.
At lower doses, islets of skin may regrow from surviving stem cells; at higher doses, at which there are no surviving stem cells within the area treated, moist desquamation is complete, and healing must occur by migration of cells from outside the treated area.

Question 15

iv. Briefly describe consequential late effects and how they differ from late toxicities of radiation therapy. Give a clinical example. (2)

Answer 15

If intensive fractionation protocols deplete the stem cell population below levels needed for tissue restoration, an early reaction in a rapidly proliferating tissue may persist as a chronic injury. This has been termed a consequential late effect, that is, a late effect consequent to, or evolving out of, a persistent severe early effect. The earlier damage is most often attributable to an overlying acutely responding epithelial surface—for example, fibrosis or necrosis of skin consequent to desquamation and acute ulceration.

Question 16

Name the structures labelled A to I in the diagram of an animal cell below:
(3)

Answer 16

Question 17

Name the DNA-repair pathway that is involved during / immediately after DNA replication pertaining to incorrectly paired nucleotides. (1)

Microsatellite Instability (MSI) is caused by mutations in the genes responsible for the above pathway. Name at least one affected gene and name the associated syndrome. (1)

Answer 17

Proofreading / Mismatch repair. Proofreading, which corrects errors during DNA replication. Mismatch repair, which fixes mispaired bases right after DNA replication

Lynch syndrome caused by mutation in the DNA mismatch repair gene

Question 18

Define the term “doubling dose”.

Answer 18

Doubling dose is the dose required to double the incidence of spontaneous anomalies within a population.
Esitmated doubling dose fir humans is about 2 Sv from atomic bomb data.

Question 19

Discuss the potential effects of significant ionising radiation exposure on the human embryo and foetus.

Answer 19

Preimplantation
0-9 days (1st week)
* All or nothing response, either survives or dosesn’t after IR exposure
* Radiation causes embryonic death above 0.1Gy

Embryonic stage / organogenesis
10days – 6 weeks
* Theshold dose of 0.1Gy
* Radiation causes major structural abnormalities at this stage as it is organoogenesis
* Can also cause Severe intrauterine growth restriction from cellular depletion
* Microcephaly
Early Foetal stage
6 weeks – ?end of 2nd trimester (26 weeks) * Mental retardation, most severe if exposed between 8-15 weeks. Result from failure of cells to migrate from proliferative zones.
* Threshold dose 0.1Gy
* IQ decrease of 25 points per Gy
* Eye, skeletal and genital abnormalities

Late foetal stage ?third trimester
27 weeks – 40 weeks
* Microcephaly, most marked in 3rd trimester
* Cancer following irradiation in utero
* Increases risk of childhood cancer by 40% above spontaneous level
* Excess absolute risk is 6% per Gy

Question 20

Define the term “doubling dose”.
What is the estimated doubling dose for humans according to The United Nations Scientific Committee on the Effects of Atomic Radiation (UNSCEAR)?

Answer 20

Doubling dose is the dose required to double the incidence of spontaneous anomalies within a population.
Esitmated doubling dose for humans is about 2 Sv of absorbed radiation dose from atomic bomb data.

Question 21

Regarding combinations of radiation therapy and systemic therapies:
- Define spatial co-operation and give one clinical example

Answer 21

Define spatial co-operation. (1)
i. Spatial co-operation refers to when the targets of radiation and chemotherapy are located at different anatomical sites, each modality acts independently of the other.

ii. Give one clinical example. (1)
i. Radiation used to treat primary site, and chemotherapy used for micrometastatic disease. Eg. Breast cancer
ii. Seclusion sites: brain, testes – where chemotherapy is the primary modality, and radiation is used for those sanctuary sites with inadequate chemotherapy penetration, such as PCI for small cell lung cancer. Or RT to testes and chemo for systemic treatment in testicular treatment

Question 22

List classes of immunotherapy agents and give an example of an individual target in each

Answer 22

1. Checkpoint Inhibitors:
   * Example: Pembrolizumab (Keytruda)
   * Target: Programmed Death 1 (PD-1) receptor
2. Checkpoint Ligands:
   * Example: Atezolizumab (Tecentriq)
   * Target: Programmed Death Ligand 1 (PD-L1)
3. CAR-T Cell Therapy:
   * Example: Kymriah (tisagenlecleucel)
   * Target: CD19 (Chimeric Antigen Receptor-T Cell Therapy targeting CD19)
4. Cytokines:
   * Example: Interferon-alpha (various brand names)
   * Target: Various immune cells, including interferon receptors
5. Oncolytic Viruses:
   * Example: Talimogene laherparepvec (T-VEC or Imlygic)
   * Target: Replication in cancer cells, leading to cell lysis and immune activation
6. Monoclonal Antibodies:
   * Example: Rituximab (Rituxan)
   * Target: CD20 antigen on B cells in certain types of lymphoma and leukemia
   * Example: Trastuzumab (Herceptin)
   * Target: Human Epidermal Growth Factor Receptor 2 (HER2) in HER2-positive breast cancer
   * Example: Bevacizumab (Avastin)
   * Target: Vascular Endothelial Growth Factor (VEGF) to inhibit angiogenesis in various cancers
7. Immune Checkpoint Ligand Blockers:
   * Example: Nivolumab (Opdivo)
   * Target: Programmed Death 1 (PD-1) receptor
   * Example: Ipilimumab (Yervoy)
   * Target: Cytotoxic T-lymphocyte-associated protein 4 (CTLA-4)
8. Vaccines:
   * Example: Sipuleucel-T (Provenge)
   * Target: Prostatic Acid Phosphatase (PAP) in metastatic prostate cancer

Question 23

Describe the four main steps in the ‘abscopal’ effect. For each step, outline a key activity that takes place.

Answer 23

The ‘abscopal effect’ is a phenomenon in cancer treatment where localized radiation therapy not only affects the targeted tumor but also triggers an immune response that leads to the regression of non-irradiated tumors in distant parts of the body. This effect occurs due to the release of tumor-specific antigens and damage-associated molecular patterns (DAMPs) from the irradiated tumor, which activate the immune system to attack tumors outside the radiation field. The four main steps in the abscopal effect are as follows:

1. Local Tumor Irradiation:
   Key Activity: The primary step involves delivering radiation therapy to the localized tumor. The radiation causes DNA damage and cell death in the targeted tumor cells.

2.Release of Tumor Antigens and DAMPs:
Key Activity: As a result of radiation-induced cell death, the irradiated tumor releases tumor-specific antigens and damage-associated molecular patterns (DAMPs). DAMPs are endogenous molecules released from dying cells that signal tissue damage to the immune system.

3. Antigen Presentation and Immune Activation:
   Key Activity: Dendritic cells, which are specialized antigen-presenting cells, take up the released tumor antigens and present them to T cells. This process activates tumor-specific cytotoxic T cells, a type of immune cell capable of recognizing and attacking cancer cells.
4. Systemic Immune Response:
   Key Activity: The activated cytotoxic T cells, along with other immune cells, enter the bloodstream and travel to distant sites in the body. These T cells recognize and attack cancer cells in non-irradiated tumors located far away from the original irradiated tumor. This systemic immune response is responsible for the regression of the non-irradiated tumors, resulting in the abscopal effect.

The abscopal effect is an essential mechanism in immunogenic cell death induced by radiation therapy. It highlights the potential of radiation therapy to not only directly kill tumor cells but also engage the immune system to target cancer cells throughout the body. While the abscopal effect is an exciting concept, it may not occur in all cases, and ongoing research is focused on understanding the factors that influence its occurrence and ways to enhance its effectiveness in cancer treatment.

Question 24

What is the rationale for combining radiation therapy and immune system blockade? What are two mechanisms by which radiation can cause immune modulation?

Answer 24

• The rationale for combining radiation therapy and immune system blockade lies in the potential synergistic effects of these treatments. Both radiation therapy and immune system blockade (e.g., checkpoint inhibitors) have shown promise in cancer treatment, and combining them can enhance their effectiveness in several ways:

1. Abscopal Effect: Radiation therapy can cause localized tumor cell death within the treatment field. However, it can also stimulate the release of tumor-specific antigens and damage-associated molecular patterns (DAMPs) from dying cancer cells. This process, known as the abscopal effect, can trigger an immune response outside the irradiated area, leading to the recognition and destruction of distant, non-irradiated tumor cells by the immune system. Immune system blockade can further boost this systemic immune response, enhancing the abscopal effect and potentially improving overall tumor control.
2. Immunogenic Cell Death: Radiation-induced cell death can be immunogenic, meaning it releases tumor antigens and inflammatory signals that activate the immune system. This can lead to the recruitment of immune cells, such as dendritic cells and cytotoxic T cells, to the tumor site, promoting a more robust anti-tumor immune response. Immune system blockade can prevent immune checkpoints from inhibiting these activated immune cells, allowing them to better recognize and attack cancer cells.

• Two mechanisms by which radiation can cause immune modulation are:
  1. Inflammatory Cytokine Release: Radiation can induce the release of various inflammatory cytokines, such as tumor necrosis factor-alpha (TNF-α), interleukin-1 (IL-1), and interleukin-6 (IL-6). These cytokines can recruit immune cells, promote antigen presentation, and enhance the immune response against cancer cells.
  2. Dendritic Cell Activation: Dendritic cells play a critical role in presenting tumor antigens to T cells, initiating an adaptive immune response. Radiation can cause the release of tumor antigens, which are then taken up by dendritic cells and presented to T cells, leading to the activation of tumor-specific cytotoxic T cells that can target cancer cells.
• By combining radiation therapy and immune system blockade, clinicians aim to achieve better tumor control, improved response rates, and potential long-term benefits for patients.

Question 25

An irradiated tumour has been described as an ‘immunogenic hub’, leading to local antigen formation as well as distant ‘abscopal’ effects.
Briefly describe the four main steps in the ‘abscopal’ effect. For each step, outline a key activity that take place. (4)

Answer 25

The ‘abscopal effect’ is a phenomenon in cancer treatment where localized radiation therapy not only affects the targeted tumor but also triggers an immune response that leads to the regression of non-irradiated tumors in distant parts of the body. This effect occurs due to the release of tumor-specific antigens and damage-associated molecular patterns (DAMPs) from the irradiated tumor, which activate the immune system to attack tumors outside the radiation field. The four main steps in the abscopal effect are as follows:
1. Local Tumor Irradiation:
* Key Activity: The primary step involves delivering radiation therapy to the localized tumor. The radiation causes DNA damage and cell death in the targeted tumor cells.
2. Release of Tumor Antigens and DAMPs:
* Key Activity: As a result of radiation-induced cell death, the irradiated tumor releases tumor-specific antigens and damage-associated molecular patterns (DAMPs). DAMPs are endogenous molecules released from dying cells that signal tissue damage to the immune system.
3. Antigen Presentation and Immune Activation:
* Key Activity: Dendritic cells, which are specialized antigen-presenting cells, take up the released tumor antigens and present them to T cells. This process activates tumor-specific cytotoxic T cells, a type of immune cell capable of recognizing and attacking cancer cells.
4. Systemic Immune Response:
* Key Activity: The activated cytotoxic T cells, along with other immune cells, enter the bloodstream and travel to distant sites in the body. These T cells recognize and attack cancer cells in non-irradiated tumors located far away from the original irradiated tumor. This systemic immune response is responsible for the regression of the non-irradiated tumors, resulting in the abscopal effect.
* The abscopal effect is an essential mechanism in immunogenic cell death induced by radiation therapy. It highlights the potential of radiation therapy to not only directly kill tumor cells but also engage the immune system to target cancer cells throughout the body. While the abscopal effect is an exciting concept, it may not occur in all cases, and ongoing research is focused on understanding the factors that influence its occurrence and ways to enhance its effectiveness in cancer treatment.

Question 26

1. Describe the steps involved in the activation of a “signal transduction pathway” and provide the name of a recognised pathway as an example. (2 marks)

Answer 26

1. Receptor Activation
2. Signal Amplification
3. Intracellular Signal Transduction
4. Activation of Effector Proteins
5. Cellular Response

The Ras-MAPK Pathway

Question 27

2. Briefly describe the structure of the epidermal growth factor receptor (EGFR), how it is activated and its normal function.

Answer 27

Structure:
* EGFR is a single-chain glycoprotein that spans the cell membrane.
* It consists of an extracellular ligand-binding domain, a single hydrophobic transmembrane domain, and an intracellular domain with tyrosine kinase activity.

Activation:
* EGFR activation typically begins when a specific ligand, such as epidermal growth factor (EGF), binds to the extracellular domain of the receptor.
* Ligand binding induces a conformational change in EGFR, leading to the formation of receptor dimers (pairs of EGFR molecules).
* These dimers allow the intracellular tyrosine kinase domains to phosphorylate each other on specific tyrosine residues (autophosphorylation).
* Autophosphorylation activates the tyrosine kinase activity of EGFR, creating docking sites for downstream signaling molecules.

Normal Function:
* EGFR activation triggers several downstream signaling pathways, including the Ras-MAPK pathway
* The Ras-MAPK pathway regulates gene expression, leading to cell proliferation and differentiation.

Question 28

3. Describe three differences between a somatic and germline mutation. Is EGFR mutation somatic or germline? (2 marks)

Answer 28

1. Timing of Mutation Occurrence:
   * Somatic Mutation: Somatic mutations occur in the non-reproductive (somatic) cells of an organism after conception.
   * Germline Mutation: Germline mutations occur in the reproductive (germline) cells, specifically in the sperm or egg cells, or in the early stages of embryonic development.
2. Inheritance:
   * Somatic Mutation: Somatic mutations are not inherited.
   * Germline Mutation: Germline mutations are inherited.
3. Role in Disease:
   * Somatic Mutation: Somatic mutations are often associated with the development of diseases like cancer. These mutations can lead to uncontrolled cell growth and the formation of tumors in specific tissues or organs.ie they will usually develop later in life
   * Germline Mutation: Germline mutations, when inherited, can lead to genetic disorders or predispositions to certain conditions that affect an individual throughout their life. And can result in conditions being present at birth or very early in their life These mutations are present in every cell of the individual’s body.

Be aware that germline mutations can also lead to cancer – Knudson’s hypothesis – and if there is a germline mutation predisposing to a certain cancer then it will occur earlier in life than would be expected for that cancer and can lead to cancer in all organs associated with that cancer risk e.g both breasts and both ovaries if an inherited BRCA mutation

Question 29

4. The DNA sequence of a gene can be altered in a number of ways. Describe five types of genetic mutations. (3 marks)

Answer 29

1. Point Mutation: (sense and non-sense)
   * A point mutation involves the substitution of a single nucleotide (base) with another nucleotide in the DNA sequence.
2. Insertion Mutation:
   * An insertion mutation involves the addition of one or more extra nucleotides into the DNA sequence. This can disrupt the reading frame, leading to significant changes in the protein’s amino acid sequence downstream of the insertion.
3. Deletion Mutation:
   * Deletion mutations involve the removal of one or more nucleotides from the DNA sequence. Like insertions, deletions can shift the reading frame and result in a completely different amino acid sequence in the protein.
4. Frameshift Mutation:
   * Frameshift mutations occur when the number of inserted or deleted nucleotides is not a multiple of three (the number of nucleotides in a codon). This leads to a shift in the reading frame of the gene, causing significant alterations in the amino acid sequence of the resulting protein.
5. Duplication Mutation:
   * Duplication mutations involve the replication of a segment of DNA, leading to an extra copy (or copies) of that segment in the gene. This can result in the production of a longer-than-normal protein or changes in gene regulation.
6. Inversion mutations:
   * Where a segment of DNA is flipped in orientation
7. Translocation mutations:
   * Where segments of DNA from one chromosome move to another

Question 29

5. Name six hallmarks of cancer. Which of these concepts apply to the role of EGFR mutation in the development of human cancer? (5 marks)

Answer 29

1. Sustaining Proliferative Signaling
2. Evading Growth Suppressors
3. Resisting Cell Death (Apoptosis
4. Enabling Replicative Immortality
5. Inducing Angiogenesis
6. Activating Invasion and Metastasis
7. Avoiding Immune Destruction
8. Tumor-Promoting Inflammation
9. Deregulating Cellular Energetics.
10. Genome Instability and Mutation

EGFR mutations are primarily associated with the first hallmark, Sustaining Proliferative Signaling. When EGFR is mutated it can cause continuous signaling for cell growth and division. This uncontrolled signaling can lead to the development and progression of various types of cancer, particularly non-small cell lung cancer (NSCLC).
While EGFR mutations primarily relate to the first hallmark, EGFR mutations can indirectly lead to resistance to cell death, and genomic instability, by promoting cell survival through sustained growth signaling.

Question 30

1. Define the term “tolerance dose”. (1 mark)

Answer 30

The maximum radiation dose or intensity of fractionated radiotherapy that is associated with an acceptable probability of developing clinically relevant signs and symptoms of late normal tissue damage.(or at a specific time interval in the future)

Question 31

List six factors that may not be accounted for when determining the tolerance dose of an organ according to the QUANTEC papers. (2 marks)

Answer 31

• Fractionation (and time treatment delivered in)
• Total irradiated volume
• Age of patient
• Concomitant therapies
• Health status of organ/patient
• Previous radiation treatment to that area

Question 32

c. Explain the concept of tissue architecture as described by Withers and how it is structured in a serial and parallel organ.
When considering the maximum tolerance dose of an organ at risk, how would this differ if you were irradiating a parallel organ as compared to a serial organ? ( 2 marks)

Answer 32

Serial Organs:
- In a serial organ, the functional subunits are arranged in a series or sequential fashion, meaning they are connected in a way that if one subunit is damaged or irradiated, the entire organ’s function may be compromised.
- Examples of serial organs include the spinal cord and small bowel

Parallel Organs:
- In a parallel organ, the functional subunits are organized in parallel, meaning they function independently of each other. Damage to one subunit does not necessarily affect the function of the remaining subunits.
- Damage to a portion of a parallel organ may lead to a reduction in overall organ function, but it doesn’t result in complete loss of function. Examples of parallel organs include the lungs and kidneys.

Differences in Irradiating Parallel and Serial Organs:
1. Parallel Organ:
* More susceptible to volume effect rather than a threshold dose. With increasing volume of FSUs irradiated in the organ, there is an increase of organ dysfunction.
* Threshold volume of organ irradiation before dysfunction is seen.

2. Serial Organ:
   * In contrast, for a serial organ, the tolerance dose is relatively low because damage to any part of the organ can lead to complete loss of function, and this is considered unacceptable in clinical practice. Therefore there is a threshold dose.
   * Not significantly affected by volume of organ irradiated as long as under threshold dose.

Question 33

d. How does the “volume effect” impact on clinical tolerance when irradiating a skin cancer and why does this happen? (2 marks)

Answer 33

Skin is mostly a “parallel FSU”. Therefore it’s clinical tolerance is more dependent on its total volume irradiated rather than total dose.
The smaller the volume of skin irradiated the more potential for un-irradiated clonogenic/ stem cells to migrate into the irradiated volume to help heal the irradiated volume. The larger the volume the harder it is for migration of outer stem cells.- this results in slow healing of the irradiated area which can lead to severe pain and infection and would be poorly tolerated by the patient and may lead to consequential late effects

Question 34

What is the equation for EQD2

Answer 34

EQD2 = D x (a/b +d)/(a/b+2)

Question 35

6. Briefly discuss at least six factors that need to be considered when considering retreatment to an organ. (3 marks)

Answer 35

1. Patient factors
2. Expected life span of function of tissue/organ being retreated, i.e. is the underlying organ function good or does the patient have underlying medical conditions such as emphysema and needing to retreat lung
3. patient or QOL if left untreated: are they at risk of developing late toxicities, or do the benefits of retreating outweigh late toxicity risks
4. Patient age and ability to tolerate RTx – and what does the patient want!
5. Tumour factors
6. Alpha/beta ratio of tumour – might also consider tumour’s radiosensitivity
7. Alternative treatment options – consider if surgery or supportive care would be better options
8. Are the organs in the area being treated serial or parallel organs
9. Treatment factors
10. Proposed treatment dose and fractionation
11. Time interval between the initial treatment and the planned retreatment, is there a potential for recovery of normal tissue tolerance
12. Concurrent treatments (chemotherapy or immunotherapy) delivered at the time of initial RT which could affect tolerance of OARs
13. Time interval between the initial treatment and the planned retreatment, is there a potential for recovery of normal tissue tolerance – this is a repeat of number 2, but is very important!
14. Previous treatment dose, fractionation and type
15. Volume of spinal cord previously treated – the question was generic rather than about the spinal cord specifically but what you are stating is correct and relates to the additional point I put in tumour factors

Question 36

7. Outline the concept of “forgotten dose” in relation to the tolerance dose of organs at risk. (1 mark)

Answer 36

Forgotten dose is the concept that organs at risk may have some recovery of their tolerance dose following radiation exposure, such that a proportion of the radiation dose that had been previously given is “forgotten”

Question 37

What is the tolerance dose for spinal cord

Answer 37

tolerance dose for spinal cord as 50Gy (0.02% risk of myelopathy)

Question 38

i. List at least 4 types of DNA damage that can be induced by ionising radiation. (1 mark)

Answer 38

• Double-strand breaks
• Single strand breaks
• Base damage
• Cross-linking

Question 39

Use a table to compare and contrast the two double-strand break DNA repair pathways in relation to: (8marks)
- repair mechanism
need for a template
repair accuracy
time required for repair
predominance in phases of a cell cycle tissue type predominance
- used proteins / protein complexes
- associated genetic conditions

Answer 39

Question 39

i. Define the terms sublethal damage and potentially lethal damage. (1 mark)

Answer 39

Sublethal damage = DNA damage which is insufficient to cause cell death, assuming intact DNA repair pathways.

Potentially lethal damage = DNA damage that can cause cell death unless favourable conditions are in place to allow repair such as cell environment or cell cycle position

Question 40

Outline the purpose of pulsed-field gel electrophoresis (PFGE) in regards to DNA damage and the key steps in this assay.

Answer 40

PFGE is useful for studying DNA damage as it allows for separation and visualization of large DNA fragments.

Key Steps in PFGE:
1. DNA Sample Preparation:
* Isolate the DNA of interest, which may be from cells, tissues, or other sources.
* Treat the DNA, if necessary, to induce damage or generate specific fragments. This treatment might involve restriction enzymes to create specific cleavage points or other methods to induce DSBs.

2. Embedding in Agarose Gel:
   * Mix the DNA sample with a molten agarose gel, which will immobilize the DNA within the gel matrix.
   * Pour the mixture into a mold to create a gel block containing the DNA.
3. Electrophoresis Setup:
   * Place the agarose gel block in a specialized PFGE chamber that can generate a pulsed electric field.
   * Submerge the gel in an electrolyte buffer that facilitates DNA movement during electrophoresis.
4. Electrophoresis with Pulsed Electric Field:
   * Apply an electric field across the gel, but unlike traditional gel electrophoresis, PFGE alternates the direction of the field in a pulsing manner.
   * The pulsing of the electric field changes the direction of DNA migration within the gel, allowing for the separation of large DNA fragments based on size.
5. Visualization:
   * After electrophoresis, the gel is stained with a DNA-binding dye, typically ethidium bromide.
   * The DNA fragments are visualized under UV light, revealing a distinctive banding pattern of separated fragments.
6. Analysis:
   * Analyze the PFGE gel to determine the size and distribution of DNA fragments.
   * The resulting pattern can provide information about the extent and nature of DNA damage.

Question 41

Identify the main advantage of comet assay versus PFGE? (1 mark)

Answer 41

Higher sensitivity to subtle DNA damage and relatively simple and cost effective

Question 42

List at least two lethal and two non-lethal chromosome aberrations

Answer 42

Lethal:
Aneuploidy
Euploidy

Non-lethal:
Symmetric translocation
Small deletions

Question 43

Which chromosome aberration assay is used to assess total body irradiation dose, and what is the lowest dose it can detect? (1 mark)

Answer 43

The dicentric chromosome assay (DCA). It is a biodosimetry technique that quantifies the number of dicentric chromosomes in peripheral blood lymphocytes to estimate the radiation dose received by an individual. Dicentric chromosomes are abnormal structures formed when two separate chromosomes become fused at their centromeres due to radiation-induced DNA damage.

0.05Gy is lowest dose it can detect

Question 44

1. State the formula for the linear quadratic model of cell kill and individually define each of the terms of the equation. (3 marks)

Answer 44

S= e^(-aD-BD^2)
S= surviving fraction
A and B are the co-efficiants of radiation sensitivity
D = dose delivered as single fraction

Question 45

1. Below are cell survival curves for low and high α/β ratio cell lines. (1 mark)

Answer 45

Question 46

ii. Define the term α/β value and describe how it can be found from a cell survival curve. (1 mark)

Answer 46

a/B describes the bendiness of the curve and assumes 2 components of cell killing, one proportional to dose and one proportional to square of dose. Initial slope determined by a, B component causes the curve to bend at higher doses
a/B is found at the point of the curve when the a component = B component

Question 47

1. State two limitations of the linear quadratic model.

Answer 47

• Treatment time not taken into account
• Less reliable at extremes of dose
• A/B are an estimation
• May be heterogeneity of a/b in tumour and normal tissues
• Does not account for other factors: concurrent chemo, radiosensitisers, oxygen etc

Question 48

For both acute and late responding normal tissues, provide α/β value ranges. (1 mark)

Answer 48

High 8-10Gy
Low 2-3Gy

Question 49

For both acute and late responding normal tissues, state the relative effects on normal tissue complication probability for the following four scenarios: (4 marks)
* increasing total dose
* hyperfractionation
* hypofractionation
* reducing overall treatment time (keeping total dose constant)

Answer 49

increasing total dose
- Acute – increased effect correct
- Late – increased effect not always as dose per fraction rather than total dose is important but there is a still a tolerance dose which might be exceeded if total dose increased

Hyperfractionation
- Acute – ?no change – if pure hyperfractionation then no change or may be slightly reduced as less dose delivered in a time period as less than 1.8 Gy per per day ie the time is not shortened – although the total dose is potentially still the same so ultimately it may be no change
- Late – less late effects or no change

Hypofractionation
- Acute – if more total dose given in a shorter period which is the usual effect of hypofractionation then acute effects could increase
- Late – More late effects yes as sensitive to dose per fraction

reducing overall treatment time (keeping total dose constant)
- Acute likely to increase as delivering a higher total dose in a time period compared to standard fractionation – so acute effects will develop earlier and likely to be more intense
- Late – no change as long as time between fractions ok, late effects impacted by dose per fraction not time

Question 50

Define the term hyperfractionation. (0.5 marks)

Answer 50

<1.8Gy – write as a sentence ie hyperfractionation is the delivery of radiation at a dose of less than 1.8 – 2 Gray per fraction

Question 51

Discuss the impact of hyperfractionated treatment schedules on tumour control and normal tissue effects.

Answer 51

Hyperfractionation if delivered as a once daily fractionation will extend treatment time and therefore has the potential to enable accelerated repopulation in the tumour which can worsen tumour control. As such hyperfractionation is usually delivered in combination with acceleration so that more than one fraction is delivered per day. This prevents an extended treatment course and may even result in a shortened treatment course compared to standard fractionation – this reduces the risk of repopulation and can improve tumour control.

Hyperfractionation on normal tissues has the potential to reduce effects in late responding tissues as the tissues are sensitive to dose per fraction. Acute responding tissues may have no change in effects if total dose the same and treatment deilivered as a once per day fractionation. If hyperfractionation is combined with acceleration (ie more than one fraction per day) then there should be no increase in the effects on late responding tissues as long as there is adequate time between fractions for repair. This may make it possible to give a higher total dose which would not change the effects on late responding tissues but would improve tumour control. If hyperfractionation is combined with acceleration this could increase the effects on early responding tissues as there will be more dose delivered in a shorter time period and will also increase effects in early responding tissue if the total dose is increased.

Question 52

What are the hallmarks of cancer

Answer 52

1. evading growth suppressors
2. avoiding immune destruction
3. sustained proliferative signalling
4. replicative immortality
5. resisting cell death
6. inducing angiogenesis
7. activating invasion and metastasis
8. tumour-promoting inflammation
9. genomic instability and mutation
10. deregulating cellular energetics

Question 53

List five types of cell death following ionizing radiation.

Answer 53

1. Mitotic catastrophe
2. Apoptosis (accept programmed cell death or interphase death) 3. Radiation induced senescence
3. Necrotic cell death
4. Autophagy

Question 54

Describe the mechanism of cell death for mitotic catastrophe and apoptosis, following ionizing radiation.

Answer 54

Mitotic catastrophe
* Occurs during or shortly after a failed mitosis.
* Cell attempts to divide without proper repair of DNA damage, there are stops in
metaphase with aberrant mitosis and multinucleated giant cells.
* Mitotic catastrophe can also serve as a trigger for other cell death pathways.

Apoptosis (accept programmed cell death or interphase death)
* Occurs via intrinsic and extrinsic pathways. Radiation activates the intrinsic pathway and relies on p53.
* Apoptosis can be a result of both early or late cell death. Late apoptosis may be activated by mitotic catastrophe.
* DNA damage elicits downstream signaling to either block cell cycle progression to allow DNA repair, or progression to cell death when DNA damage overwhelming.
* Defined by morphological criteria (rounding up of the cells, nuclear pyknosis, karyohexis and phagocytosis of the apoptotic body by adjacent cells), the requirement for active participation of the dying cell and DNA laddering on gel electrophoresis.

Question 56

Hh

Answer 56

Ji