Dose to Lungs and Kidneys during Total Body Irradiation: Are We Delivering the Expected Dose?

Abstract

Total Body irradiation (TBI)-based conditioning is key component for hemopoietic stem cell transplantation. TBI dose calculation is traditionally based on point-dose calculations based on estimates of body separation at different levels with the umbilicus as a reference point, an approach followed as standard protocol at most institutions. This homogenous dose calculation technique does not take into account tissue heterogeneity. Limited data exists for TPS-based TBI planning systems especially in the setting of lung and kidney partial transmission blocks, which is a void in our understanding of TBI dose delivery and is explored in this study. Primary objective of the study was to assess differences between radiation treatment planning software (TPS) estimated doses with heterogeneity correction and point calculation estimated doses when delivering TBI. Secondary objective was to validate accurate treatment delivery through optically stimulated luminescent dosimetry (OSLD) measurements at umbilicus obtained at first TBI fraction. Our retrospective study involved virtual dosimetric simulation of total body CT scan datasets in patients treated with a prescribed dose of 12 Gy in six fractions delivered twice daily over three days. Patients received TBI using a translating couch AP/PA technique with lung and kidney shielding (transmission blocks to achieve < 7 Gy point dose). The Varian Eclipse TPS was used to calculate the 3D dose distribution to the lungs, kidneys, and heart. Paired t-test with a significance value of 0.05 was used to compare means. 24 patients (Male-11, female-13; Age range: 23-60 years; Diagnosis: AML-7, ALL-8, HL-1, NHL-6, MDS-2) with total body CT scan datasets available were included in the analysis. Contrary to expected dose with shielding (< 7 Gy), the TPS simulated mean dose to lungs was 11.08 Gy (SD: 0.61, p= 9.66E-22) and to kidneys was 8.98 Gy (SD: 0.62, p: 2.31E-14). TPS mean dose to the unshielded heart was 11.38 Gy. Compared to the point dose calculation of 2 Gy at the midpoint of the umbilicus, the mean OSLD dose measured during the first fraction of treatment was 2.08 Gy (SD: 0.058, p: 2.07E-9). Using TPS of total body CT scan data, we demonstrated much higher than expected dose to lungs and kidneys than expected from point-based calculations. This study has triggered a comprehensive outcomes analysis of institutional data for assessing toxicities in patients receiving TBI 12 Gy-based conditioning prior to SCT with the goal to revisit the 7-Gy dose limit to the lungs and kidneys. Institutions are encouraged to pursue similar analyses of their TBI systems to refine dose to critical organs. (Supported by the Radiation Oncology Summer Research Fellowship and the Office of Student Research at the University of Maryland School of Medicine).