Radiobiological aspects of alternative dose delivery systems Flashcards
Which of the following statements about carbon ion therapy is FALSE?
A. For a given dose to the tumor in the Bragg peak, carbon ions produce better sparing of normal tissues in the entrance region of the beam than either protons or photons
B. Carbon ions have a high RBE in the Bragg peak region
C. There is reduced scattering in both the lateral and longitudinal directions for carbon ions compared to protons
D. There is a greater variation in radiosensitivity between oxygenated and hypoxic tumor cells using carbon ions compared with photons
E. PET verification can be used for carbon ion treatment
D
Carbon ions represent a high LET form of radiation and, as such, display less dependence upon oxygen for cell killing (and therefore have a lower OER). Hence, there should be fewer hypoxic tumor cells surviving carbon ion therapy than following treatments using either X-rays or protons.
Basic research with light ions established that carbon ions suitable for radiotherapy (~400 MeV/amu) have superior depth-dose profiles from the entrance region of the beam up through the Bragg peak (Answer Choice A).
Two centers, one at the HIMAC in Chiba, Japan, and the other at the HIT in Heidelberg, Germany, have been treating with carbon ions using a gantry for over a decade and a number of other centers have come online since then. Carbon ions show an increased RBE for both cells irradiated in vitro and tissues exposed in vivo. The exact RBE depends on the energy of the beam and the characteristics of the cells at risk (Answer Choice B).
An additional advantage of treatment with carbon ions is the reduction in lateral and longitudinal scatter (Answer Choice C).
It is possible to verify the carbon ion treatment plan using PET since a small fraction of the ions undergo nuclear fragmentation when a beam of carbon ions penetrates a thick absorber. Often, one or two neutrons are stripped, converting the stable 12-C to the positron emitting isotopes 11-C
and 10-C. These isotopes travel with almost the same velocity as the main beam and stop in nearly the same location. They have short half-lives and as the emitted positron combines with an electron in an annihilation reaction, two 0.51 MeV photons are produced that can be detected by a PET scanner. As a consequence, the high dose treatment volume can be visualized (Answer Choice E).
Which of the following statements concerning intensity-modulated radiation therapy (IMRT) is CORRECT?
A. IMRT employs significantly higher energy photon beams than unmodulated radiation dose-delivery techniques.
B. IMRT results in fewer radiation therapy-induced second cancers in the pediatric population as compared to adults.
C. IMRT is most conformal if used in the conventional 1.8-2.0 Gy/fraction format
D. IMRT allows for higher doses to acutely responding normal tissues while decreasing dose to late responding normal tissues.
E. The whole-body patient dose is increased with IMRT, compared to treatment plans involving unmodulated beams due to leakage from the head and scatter from the collimator.
E
The whole-body patient dose is higher with intensity modulated radiation therapy (IMRT) technique because, in addition to leakage from the head, there is scatter from the collimator.
IMRT usually employs a linear accelerator at mega-voltage energies, which are similar to or lower than energies used to deliver treatment doses with an unmodulated field (Answer Choice A).
The higher risk of IMRT radiotherapy-induced second cancers in pediatric patients than in adult patients is a direct consequence of the smaller size of the body of a child compared with an adult. As originally discussed by Hall (2006), radiogenic organs are closer to the treatment site in a child and thus receive larger radiation doses than when a comparable treatment is delivered to an adult (Answer Choice B).
IMRT is most conformal if all target volumes are treated simultaneously using different fraction sizes (Answer Choice C). This permits graded dose levels to the gross tumor with embedded normal tissues and tissues at risk for tumor spread (normal tissues surrounding the gross tumor and lymph nodes). Such a treatment strategy is called the simultaneous integrated boost (SIB). The SIB strategy uses the same plan for the entire course of treatment to deliver prescribed doses to treated
volumes.
The effect of modified fractionation on acute and late toxicity of normal tissue is taken into account during treatment planning (Answer Choice D). The SIB-IMRT fraction sizes are estimated using an isoeffect relationship based on the linear-quadratic (LQ) equation using the values of LQ model parameters (such as α/β ratios and tumor doubling time) for the isodose calculations for various tissues components in the treatment volume.
Which one of the following statements concerning radiolabeled immunoglobin therapy is FALSE?
A. One disadvantage associated with the use of 90Y-labeled antibodies is that the relatively low energy (<100keV) and short range of the B-particles emitted limit the so-called “crossfire effect.”
B. Both ibritumomab tiuxetan (Zevalin) and tositumomab (Bexxar) target CD20.
C. Radiation safety is an important issue regarding the use of 131-I labeled compounds because this isotope emits y-rays that may pass through the patient.
D. The dose-limiting organ associated with the use of tositumomab (Bexxar) is the bone marrow.
E. The dose-limiting organ associated the use of tositumomab (Bexxar) is the thyroid gland
A
90Y emits B-particles with a relatively high energy (0.9 MeV) and long range that can penetrate several millimeters into the tissue. Thus, there is a significant crossfire effect, i.e., cells adjacent to those that have taken up the radioisotope are also irradiated.
Radioimmunotherapy (RIT) involves treatment with a targeted radiopharmaceutical that combines a tumor-selective monoclonal antibody conjugated to a radionuclide, typically a medium-range B-
emitter. Two radiopharmaceuticals have been approved by the FDA for the management of relapsed and refractory CD20-positive low-grade Bcell non-Hodgkin’s lymphoma (NHL): 90Y-ibritumomab tiuxetan (Zevalin) and 131I-tositumomab (Bexxar; Answer Choice B). Both drugs are composed of a murine antibody selective for the CD20 surface antigen found on over 95% of NHL B-cells (in addition to all normal mature B cells).
90Y is a pure B-emitter with a short effective half-life; therefore, very little of the radioactivity produced by Zevalin escapes the patient, minimizing the radiation safety hazard. However, a surrogate imaging isotope, such as 111In, must be incorporated into the Zevalin framework to allow
positional localization. Bexxar incorporates 131I, which is a medium-energy, mixed-spectrum B- and y-emitter with a y emission at 364 keV that can be detected using a gamma camera. Because of the penetrating y-rays of 131I and eight-day half-life, more rigorous radiation safety precautions must be used with Bexxar (Answer Choice C).
Hematologic toxicity is the major dose-limiting toxicity for RIT (Answer Choice D). Thyroid uptake of 131I is not uncommon but is not dose-limiting. To prevent thyroid uptake of 131I, oral iodine is given a day before Bexxar administration and continued for 14 days after the therapeutic dose.
Which of the following 5Rs of radiobiology likely has a negative impact on severely hypofractionated schedules (1-5 fractions) used in stereotactic body radiotherapy?
A. Radiosensitivity
B. Repair/recovery
C. Redistribution/reassortment
D. Repopulation
E. Reoxygenation
E
Most human tumors except for very small ones have radioresistant hypoxic cells. The negative influence of hypoxic cells against local tumor control is greater in hypo-fractionated radiotherapy compared to conventional therapy. SBRT treatments are usually completed within <1 to 2 weeks and re-oxygenation during the course of SBRT therapy is very limited to negligible. Laboratory and clinical data suggest an intra-fraction interval of at least 3 days to increase possibility for reoxygenation of tumor cells between fractions.
Which of the following are TRUE about particle therapy?
A. Protons have a Bragg peak and are considered high LET radiation
B. C-ions have no Bragg peak and are considered high LET radiation
C. Boron-neutron capture therapy delivers high LET radiation to the tumor
D. Si-ions have a Bragg peak and are considered low LET radiation.
E. Photons are considered as low LET radiation and do not have a Bragg peak associated with them.
C
BNCT uses neutrons to activate Boron-containing compounds to release alpha particles in the tumor. These alpha particles are high LET. Protons have a Bragg peak but are considered to be low LET radiation with an RBE of near 1.1. C-ions have a Bragg peak and are considered high LET radiation. Si-ions have a Bragg peak and deliver high LET radiation. Photons are low LET but have no Bragg peak associated with them.
