Modeling photon output caused by backscattered radiation into the monitor chamber from collimator jaws using a Monte Carlo technique

Abstract

Dose per monitor unit in photon fields generated by clinical linear accelerators can be affected by the backscattered radiation into the monitor chamber from collimator jaws. Thus, it is necessary to account for the backscattered radiation in computing monitor unit setting for a treatment field. In this work, we investigated effects of the backscatter from collimator jaws based on Monte Carlo simulations of a clinical linear accelerator. The backscattered radiation scored within the monitor chamber was identified as originating either from the upper jaws ͑Y jaws͒, or from the lower jaws ͑X jaws͒. From the results of Monte Carlo simulations, ratios of the monitor-chamber-scored dose caused by the backscatter to the dose caused by the forward radiation, R(x,y), were modeled as functions of the individual X and Y jaw positions. The amount of the backscattered radiation for any field setting was then computed as a compound contribution from both the X and Y jaws. The dose ratios of R(x,y) were then used to calculate the change in photon output caused by the backscatter, S cb (x,y). Results of these calculations were compared with available measured data based on counting the electron pulses or charge from the electron target of an accelerator. Data from this study showed that the backscattered radiation contributes approximately 3% to the monitorchamber-scored dose. A majority of the backscattered radiation comes from the upper jaws, which are located closer to the monitor chamber. The amount of the backscatter decreases approximately in a linear fashion with the jaw opening. This results in about a 2% increase of photon output from a 10 cmϫ10 cm field to a 40 cmϫ40 cm field. The off-axis location of the jaw opening does not have a significant effect on the magnitude of the backscatter. The backscatter effect is significant for monitor chambers using kapton windows, particularly for treatment fields using moving jaws. Applying the backscatter correction improves the accuracy of monitor-unit calculation using a model-based dose calculation algorithm such as the convolution method. © 2000 American Association of Physicists in Medicine. ͓S0094-2405͑00͒00904-4͔

Verification of monitor unit calculations for non‐IMRT clinical radiotherapy: Report of AAPM Task Group 114

“…It is typically a complex function of the field size projected to the isocenter and depends on the design of the treatment unit collimation system. [38][39][40][41] The phantom scatter factor, S p , and the TPR together correct for differences from the reference conditions in scatter and attenuation, with S p primarily accounting for changes in scatter and TPR accounting for the effects of tissue attenuation as well as changes in scattering conditions with depth. These factors depend on the effective equivalent square [42][43][44] of the irradiated area ͑at SPD͒, which is typically estimated from the field shape defined by the MLC or poured blocks.…”

Deriving detector‐specific correction factors for rectangular small fields using a scintillator detector

“…Depending on the size defined by the upper Y jaw, different amounts of backscattered photons are generated 33 , 34 . A smaller upper jaw (Y) opening induces more backscatter into the monitor chamber, which subsequently results in fewer MUs delivered, producing a smaller OF.…”

An MLC-based version for the ecliptic method for the determination of backscatter into the beam monitor chambers in photon beams of medical accelerators

“…Backscatter into the beam monitor chambers is known to contribute to the field size dependence of the photon output of medical linear accelerators [1][2][3][4][5][6].…”