Dosimetric Advantages of VMAT TBI Plans: A Direct Comparison to Conventional TBI Plans.

Abstract

Total body irradiation (TBI) continues to play an integral role in the conditioning of patients undergoing bone marrow transplantation. Historically, conventional TBI (C-TBI) has been delivered based on a clinical plan without CT simulation. However, volumetric modulated arc therapy TBI (V-TBI) is emerging as an alternative method to deliver TBI in a contemporary fashion. We aimed to compare these two methods of TBI delivery. Patients undergoing treatment with V-TBI were identified. C-TBI plans were created using their existing simulation CT images. Patient thickness was measured on the scan and compensators such as lead sheets to attenuate dose in areas with less separation and lung blocks drawn on digitally reconstructed radiographs (DRR) were added when necessary. A 3D dose distribution was then calculated allowing for the direct comparison between C-TBI and V-TBI on the same patient using the same CT image set. Dosimetric data from each plan including target volume coverage, dose homogeneity, absolute max dose, and dose to lungs were recorded. V-TBI and C-TBI plans for a total of four patients were preliminarily analyzed. Two patients were prescribed 200 cGy in a single fraction, while the other two were prescribed 1200 cGy in eight fractions. V-TBI resulted in a more favorable dosimetry for all four patients in most evaluated metrics including dose coverage, dose homogeneity, and lung dose (Table 1). V-TBI did result in an increased absolute maximum dose to all four patients compared to c-TBI, but still met the desired constraint of D0.03cc<125%. V-TBI resulted in more favorable dosimetry for all four patients compared to C-TBI. To our knowledge, this is the first direct dosimetric comparison between the two methods. Analysis of 8 more V-TBI cases is currently underway. In the future, we plan to design a prospective study to evaluate the clinical outcomes of patients undergoing V-TBI vs C-TBI.