Evaluating the biological impact of increased scattered radiation in single and composite field radiation beams

Abstract

Low energy scattered particles cause the average energy at the treatment field edge to be lower relative to the center of the beam. These particles have a higher LET, making them more biologically damaging. With a single beam (large field-size (FS)), the amount of these highly damaging electrons is low relative to within the treatment beam. However, when multiple beam angles and smaller FS are used (IMRT/VMAT), the relative contribution of scattered radiation increases and biological consequences may vary. The purpose of this study was to evaluate the theoretical impact of increased scattered radiation in single and composite field beams. Monte Carlo Geant4.10.3 was used to score the electron energy spectra at different depths in water with a 6 MV photon beam incident. The energy spectra were then used to calculate the maximum RBE, RBE M . Beams examined were phase-spaces of a Varian Clinac 600C 6 MV linac (10×10 cm 2 beam, 1×1 cm 2 beam), and a composite 10×10 cm 2 beam. The composite 10×10 cm 2 treatment field (simple IMRT) was created by summating one hundred 1×1 cm 2 beams to form a relatively uniform 10×10 cm 2 field. For smaller FS (1.5 cm depth), an increase in RBE M was seen 5.2 cm outside the beam (17%). The 10×10 cm 2 beam showed an increase of 14%, 9.2 cm away from beam's edge (1.5 cm depth). The composite 10×10 cm 2 beam exhibited similar RBE M enhancement to the 10×10 cm 2 phase-space, however, the region of increased damage occurred closer to the beam (5.6 cm away). The results indicate that although the region inside the primary beam is not affected, the contribution of damaging particles happens much closer to the beam's edge in the composite field case relative to an open field. This may have potential implications regarding the effective dose to organs at risk during radiotherapy.

Quantifying Radiobiological Variation in Cancer Radiotherapy Using Monte Carlo Simulation and Doped Plastic Scintillators

“…With the advent of new technologies in radiation therapy, such as IMRT, the highly damaging component may be causing more damage than previously thought. [18][19][20][21][22] For particle therapies such as protons, a generic weighting factor of 1.1 is used. This means that in patients, it is assumed that protons are 10% more damaging than photons emitted by cobalt-60.…”

“…Since RBE variation under low-dose conditions was examined, the maximum RBE, RBE M was the quantity of interest. In this study, the approach used by Nusrat et al 22 was used to generate RBE M . According to the theory of dual radiation action, the monoenergetic electron α/β can be obtained through:…”

“…It has been demonstrated that maximum RBE can change by up to 16% when complex beam cases are used due to the presence of increased scattered (low energy) radiation. 22 In brachytherapy, the radioactive source(s) are placed inside the patient to minimize entrance and exit doses relative to EBRT. Brachytherapy may be more susceptible to RBE variations given that the radioactive sources used are lower in energy.…”

“…This, in turn has been shown to cause more cell death than predicted by absorbed dose alone, resulting in increased out-of-field RBE. 19,22,59,60,62,171 Typically, RBE is used to quantify the efficacy of a given type of radiation at killing cells. It is defined as the amount of absorbed dose required to achieve a biological endpoint (e.g.…”

Maximum RBE change in 192Ir, 125I, and 169Yb brachytherapy and the corresponding effect on treatment planning

“…Since the photon energies used in EBRT are relatively high, the variation of relative biological effectiveness (RBE) is usually minimal. It has been demonstrated that maximum RBE can change by up to 16% when complex beam cases are used due to the presence of increased scattered (low energy) radiation [3].…”