Electron and Photon Energy Spectra Outside of 6 MV X-ray Small Radiotherapy Field Edges Produced by a Varian iX Linac

Abstract

Due to the increase in the survival probability for patients treated with modern radiotherapy techniques to live enough for experimenting the late radiation effect, low dose outside the treatment volume becomes a concern. However, besides the absorbed dose, the beam quality outside the field edge should be taken into account. This work aimed at investigating the photon and electron fluence spectra outside the field edges for several small radiotherapy fields for determining the quality of the beams in order to better evaluate the secondary effect after modern radiotherapy treatments. Phase-space files of a 6 MV X-ray beam produced by a Varian iX linac 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 at SSD = 100 cm were generated using the BEAMnrc code. The photon and electron fluences in each field were calculated at 0.15, 1.35, and 9.85 cm water depth and several off-axis distances using FLURZnrc. The number of low-energy electrons between 1 and 10 keV at 2 cm outside the field edge increases by 60% compared to the central axis. Due to the relatively high linear energy transfer (LET) of these electrons, the results of this work should help to better evaluate the possible late effect of secondary radiation on healthy organs close to the tumor volume after radiotherapy treatment. We also observed high-energy electrons outside the field edge that are attributed to the leakage of the primary electron beam from the head of the linac. From a standpoint of radiological protection, these electrons should be taken into account when evaluating the dose delivered to the patient’s skin.

Linear energy transfer (LET) distribution outside small radiotherapy field edges produced by 6 MV X-rays

“…As can be seen, independent of the eld size and the electron uence, L Δ,D is almost constant within the centre of the beam and increases as the distance from the eld's edge increases. This can be associated to the existence of low photon energy outside of the eld's edge which generated low-energy secondary electrons 22 . Note that in both Figs.…”

“…The calculations were made at 100 cm source-to-surface distance (SSD). Information about the generation of the phase space les and the benchmarking process are reported in our previous work 22 . In the simulations, 5 x 10 10 histories were followed.…”

A novel QA phantom based on scintillating fiber ribbons with implementation of 2D dose tomography for small‐field radiotherapy

“…4 It is also noted that there are fewer low‐energy photons scattered from LINAC head and phantom and reaching the field center in the case of small field. 5 This results in a hardening of photon energy spectrum (i.e., an increase in average photon energy) at any point on the beam axis. 2 …”

“…Penumbra is a region of dose falloff around the geometric beam edge due (i) to the transmission through the collimator jaws (transmission penumbra), (ii) to finite source size (geometric penumbra), and (iii) to in‐patient X ray scatter (scatter penumbra) 4 . It is also noted that there are fewer low‐energy photons scattered from LINAC head and phantom and reaching the field center in the case of small field 5 . This results in a hardening of photon energy spectrum (i.e., an increase in average photon energy) at any point on the beam axis 2 …”

“…4 It is also noted that there are fewer low-energy photons scattered from LINAC head and phantom and reaching the field center in the case of small field. 5 This results in a hardening of photon energy spectrum (i.e., an increase in average photon energy) at any point on the beam axis. 2 For the choice or design of detectors for small-field QA, there are small-field issues with significant penumbrae contribution to be considered: volume averaging effect, field size dependence of response related to energy dependence, absorbed dose rate dependence, water equivalence, and dose perturbation.…”