Monte Carlo Investigation of Dosimetry Under Partial Transmission Blocks in Total Body Irradiation Treatment

Purpose: Total body irradiation (TBI) is commonly performed using opposing photon beams of maximum field size at extended SSD. Partial transmission blocks (PTB) are utilized to shield critical organs such as the lungs and kidneys. Both phantom measurement and convolution algorithms confirmed that PDD under PTB deviates significantly from those regions without the blocks. The relationship is complex and depends on many factors. In this study, we investigated the dosimetry under the PTB using the Monte Carlo tool.Methods: The photon phase space (PSP) for Truebeam linac from MyVarian was used as input in the EGSnrc package. The PSP was analyzed and separated into primary (originating from the target) and scatter ( extra-focal source originating from flattening filter) components. It was hypothesized that they behave differently in the presence of PTB which is responsible for the uncommon dosimetry. In this study, a virtual filter was developed to simulate the PTB of any transmission factors in EGSnrc. Further, a concept of scatter photon enhancement ratio (SPER=〖scatter〗_PTB/(〖primary〗_PTB+〖scatter〗_PTB )/〖scatter〗_open/(〖primary〗_open+〖scatter〗_open ) ) was proposed to quantify how the scatter photons’ dose contribution changes with SSD, block size, block-to-patient distance, and transmission factor. Results: Scatter accounts for 12% for 6X, but only 5% for 6XFFF. MC result of the virtual PTB filter agrees well with the measurement for PDD (<1.5%). For a clinical PTB of size 6x12 cm2 at the surface, the SPER at 5cm depth increases from 2.01 to 3.27 when SSD 100 to 400cm; decreases from 3.38 to 1.09 when block-surface-distance 15010cm; and decreases from 8.38 to 2.60 when PTB transmission factor 0 to 30%. Conclusion: The dosimetry under PTB for TBI can be explained by the different behavior of the primary and scatter photon components. MC allows the separation and independent investigation of each component. For in vivo dose measurement under PTB, the measurement needs to be interpreted carefully using the correct dosimetry.