Monte Carlo investigation of dosimetry in shielded region in total body irradiation treatment

<i>Purpose</i>. Total body irradiation (TBI) is commonly used to treat hematological diseases requiring bone marrow transplantation. Partial transmission blocks (PTB) are utilized to shield critical organs such as the lungs and kidneys. Previous phantom measurement and convolution algorithms confirmed that the percent depth dose (PDD) under PTB deviates significantly from those regions without the PTB. In this study, we investigated the dosimetry under the PTB using the Monte Carlo tool and validated with measurement.<i>Methods</i>. 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 point source photons (originating from the target) and extra-focal source photons (extra-focal source originating from flattening filter etc) components. It was hypothesized that they behave differently in the presence of PTB which is responsible for the uncommon dosimetry. A virtual filter module was developed to simulate the PTB of any transmission factors in EGSnrc. Further, a concept of Extra-focal Source Photon Enhancement Ratio (ESPER) was proposed to quantify how the extra-focal source photons' dose contribution changes with SSD, block size, block-to-patient distance, and transmission factor.<i>Results</i>. Extra-focal source photons accounts for 20% in 6X beam, but only 13% in 6XFFF. MC result of the virtual PTB filter agrees well with the measurement for PDD (<1.5%). For a clinical PTB of size 6 × 12 cm²at phantom surface, the ESPER at 5 cm depth increases from 1.34 to 2.38 when SSD 100 → 400 cm; decreases from 1.96 to 1.07 when block-surface-distance 149.7 → 10 cm; and decreases from 4.00 to 1.65 when PTB transmission factor 0 → 30%.<i>Conclusions</i>. The dosimetry under PTB for TBI can be explained by the different behavior of the primary point source and extra focal source photon components in phantom. This study provides valuable inputs on how to interpret the<i>in vivo</i>dose measurement under PTB.