Exhibit Hall | Forum 3
Purpose: Patients treated with modern radiotherapy techniques have a relatively high survival probability to live enough for experimenting late radiation effects. Thus, low dose outside the treatment volume has been a concern and many groups have performed studies related to low-dose outside radiotherapy fields. However, besides the absorbed-dose, the linear energy transfer (LET) should be taken into account since it is considered as an ideal parameter to evaluate biological effects. This work aim at investigating the track-average (LΔ,t) and dose-average (LΔ,D) LET of electrons produced outside of 6 MV X-ray small radiotherapy field-edges.
Methods: We have studied separately the ‘primary-electrons’ generated directly by photons from the ‘secondary-electrons’ (SE) produced by electron–electron interactions outside of eight small radiotherapy fields. The importance of the SE is because they have been reported as the main participants in the processes of ionization and are responsible for radiation damage. The electron fluences were calculated at 0.15, 1.35, and 9.85 cm water depth and several off-axis distances using FLURZnrc for eight small fields of 0.7×0.7 cm², 0.9×0.9 cm², 1.8×1.8 cm², 2.2×2.2 cm², 2.7×2.7 cm², 3.1×3.1 cm², 3.6×3.6 cm², and 4.5×4.5 cm² and for the reference 10×10 cm² field produced by a 6 MV X-ray Varian iX linac. The LET was calculated following ICRU 16.
Results: Both LΔ,t and LΔ,D are almost constant within the field size and increase as the distance from the field-edge increases. For the total fluence, LΔ,t increases by ~30% from the center of the beam to 20 mm from the field-edge, while LΔ,D the increment depends on depth. For SE, similar behavior is observed.
Conclusion: LΔ,D of SE, considered as the most reliable parameter to describe the biological effects, are up to 7keV/μm outside the field-edge, which should allow for a better evaluation of late radiation effects in normal tissue.
Funding Support, Disclosures, and Conflict of Interest: This work is partially supported by PAPIIT-UNAM grant IN118120
Not Applicable / None Entered.