Brachy

Question 1

Doserates

Answer 1

LDR 0.4-2Gy/hr
MDR 2-12Gy/hr
HDR >12Gy/hr (in reality its much higher than this 90-100gy)

Question 2

Radioactive isotopes 1/2 lives

Answer 2

Radium 226 - 1600 years
Caesium 137 - 30 years
Cobalt 60 - 5.26 years
Iridium 192 - 74 days
Iodine 125 - 59.5 days

Question 3

Clinical advantages of HDR

Answer 3

Outpatient treatment
• Dose optimisation –adaptive B/T planning
• Reduced radiation exposure for staff under normal situations
• More stable positioning
• Smaller applicators
• High dose rate= short treatment time

Question 4

Clinical disadvantages of HDR

Answer 4

• More complex treatment and planning techniques
• Compressed time frame for planning
• Greater potential for error due to a much higher dose being delivered over a short timeframe
• Potential for high radiation dose to staff and patient with source failure

Question 5

Brachytherapy workflow

Answer 5

1. Implantation procedure: depends on tumour topography, size, OARs and pre planning
2. Image acquisition (US, CT, MRI)
3. CATHETERS 3D Digitization
4. Targets and OAR delineation
5. Treatment planning and optimisation
6. Quality control
7. Treatment delivery

Question 6

What is an after loader

Answer 6

This controls the flow of the radioactive source

Question 7

High risk CTV

Answer 7

major risk of local recurrence - residual macroscopic tumour at time of BT (smaller than at time of diagnosis)

Residual macroscopic tumour (Defined from MR)

Question 8

intermediate risk CTV

Answer 8

major risk of local recurrence - initial macroscopic tumour at time of diagnosis

Significant microscopic tumour

Question 9

Low risk CTV

Answer 9

potential microscopic tumour spread - treated with surgery and/or external beam radiotherapy but not brachytherapy

Question 10

Types of imaging

Answer 10

Pre treatment: PET/MRI/CT
Brachytherapy imaging for each insertion: MRI/CT/US
Post treatment imaging

Question 11

Pre treatment imaging

Answer 11

Evaluate tumour
• Determine treatment modality
• Determine optimum treatment volume & dose

Question 12

Imaging for each insertion

Answer 12

Evaluate tumour response,
• Verification of applicator position • Define HRCTV, IRCTV and OAR • Adaptive radiation therapy

Question 13

Post treatment imaging

Answer 13

Evaluate tumour response & toxicity

Question 14

MR images

Answer 14

Excellent soft tissue differentiation – CTV can be located
• Use to define target structures and OAR
• Smaller aperture size, which may not accomodate all patient sizes • Small FOV- external body of the patient is not visualised

Question 15

Ultrasound

Answer 15

Non invasive portable and cheap
Used during applicator insertion

Able to confirm applicator position and
check intrauterine tandem is centred in
uterine cavity
• Measurements can be made of uterine
dimensions for comparison with MRI
when planning
• Not able to reconstruct the applicators,
although some software has enabled
this
• Advantage of being able to image real
- time without moving the patient

Question 16

Intrafraction and interfraction variation

Answer 16

May be significant
• May cause deviation from prescribed dose
• More significant for HDR as the number of fractions is higher than for LDR
• High dose per fraction
• Steep dose gradient around the applicators
• Tumour shrinkage and normal tissue fibrosis may occur over the total length of time that HDR is delivered

Question 17

Prostate HDR advantages

Answer 17

Image guided needle placement
Optimised dose distribution
Organ motion minimised
Radiobiological advantage
Remote after loading
Single reusable source

Question 18

Needle placement

Answer 18

No standard arrangement
Specific to each implant due to the shape and position of the prostate, urethra and rectum contours
Often need more needles along post edge to allow sufficient modulation of dose around the rectum
Have to account for possible changes in contours after needle insertion

Question 19

Acute clinical issues for prostate HDR

Answer 19

Template/ Catheter movement
• Minimize movement of patient/ bed rest

Haematuria/ clot retention
• Continuous bladder irrigation
Perineal discomfort and back discomfort
• Analgesics
Infection risk
• Prophylactic antibiotics
DVT prophylaxis
• Stockings/Heparin
Defaecation
• Low residue diet prior to and during admission

Question 20

Clinical sites treated

Answer 20

Cervical cancer
Breast
Prostate

Question 21

Cervical treatment

Answer 21

Antiverted and retroverted uterus requires different OAR priority

Antiverted - uterus tilts towards abdomen
retroverted - uterus tilts towards the rectum

• HDR Brachytherapy is necessary for the adequate treatment of cervical cancer to ensure a tumouricidal dose is delivered whilst sparing OAR

EBRT cannot escalate dose due to proximity of OARs and limitation in visualisation.
Can utilise tandem or ring
Ultrasound images estimate length of uterus

Question 22

What are standard loading patterns

Answer 22

Tandem
Ring

Question 23

What structures are dose limiting and which are not

Answer 23

Rectum - not dose limiting due to use of an applicator with rectal retractor
Bladder - is dose limiting, due to acutely angled tandems

Question 24

Why does ICRU dose parameters not correlate

Answer 24

It underestimates dose to bladder
Dose to structures may only be to certain parts near to the GTV due to lack of low dose wash