Effect of Lung Blocks Parameters on Lung and Cardiac Doses in Total Body Irradiation Using 3D Dosimetry.

Abstract

PURPOSE/OBJECTIVE(S)\nDespite being an integral part of stem cell transplant, the technical aspects of classical total body irradiation (TBI) remain relatively crude. In particular, the implementations of lung and cardiac shielding vary widely between institutions. We hypothesize that the block margins away from anatomical landmarks could substantially alter lung and cardiac dose parameters, and thus affecting the likelihood of TBI-related toxicities such as radiation pneumonitis and late cardiotoxicities. This work seeks to utilize a newly commissioned 3D TBI beam model to determine potential dosimetric effects caused by variations in the choice of shielding parameters to better understand the potential for such detrimental effects.\n\n\nMATERIALS/METHODS\nWe commissioned a TBI beam model in a treatment planning system. CT scans of 3 anonymized patients without gross disease are contoured following typical clinical practice. Patients are planned in the classic standing AP/PA at extended distance technique for a total dose of 1200 cGy in 6 fractions to midline at umbilicus level, with 3/6 fractions delivered with lung blocks. Lung blocks are modeled using automated script in the treatment planning system, with varying superior (from clavicles), inferior (from diaphragms), and lateral margins from 0 to 2 cm, as well as cases for which the cardiac silhouette was blocked or unblocked for the study. Dose to lungs and heart were calculated and compared for the following metrics: mean dose, volume receiving 10 Gy (V10 Gy) and 12 Gy (V12 Gy), and point dose behind the blocks.\n\n\nRESULTS\nThe results showed that the point dose behind the blocks is approximately 75% of prescription dose for all patients, with insignificant changes with varying block margins. The dose off the block center increases rapidly that much of the tissue under the blocks receives 80% to 90% of prescription dose. Similar results are observed for mean lung dose. In contrast, the lung V10 Gy increases by nearly two-fold, from approximately 30% to 60% over the 0 to 2 cm block margin range. The mean heart dose increased by approximately 7.5% when the cardiac silhouette is unblocked, while the cardiac V12 Gy increased by approximately 25%. The mean dose to left anterior descending artery (LAD) increases significantly in the unblocked case, with as much as 43% difference.\n\n\nCONCLUSION\nThis work has shown that margins of lung blocks could affect the dose to lungs and heart substantially, despite the indifference that the commonly used point dose parameter, point dose behind the blocks, indicates. A 3D TBI model could provide more dosimetry data to help future studies to correlate TBI-associated toxicity to the heart and lungs.