Dosimetric Considerations for VMAT Total Body Irradiation In Adult Patients
Abstract
Purpose
Total body irradiation (TBI) is a key component of myeloablative conditioning before hematopoietic stem cell transplantation. Modern TBI techniques are increasingly moving towards multi-isocentric VMAT approaches to improve dose homogeneity and organ sparing while reducing bunker space constraints. Published implementations have focused predominantly on pediatric patient populations. This study describes the clinical implementation of VMAT-TBI in an adult population and evaluates size-related dosimetric challenges and limiting factors.
Methods
From December 2022 to September 2025, 50 adult patients underwent VMAT-TBI. Prescriptions were 200 cGy given in 1 (n=27), 2 (n=18), or 6 (n=5) fractions. Mean patient height was 169 cm (range 148-195 cm) and mean weight was 85 kg (range 46-148 kg). Automated script-based plans were created using 5-7 isocenters, 2-4 arcs per isocenter, to deliver homogenous dose to the entire body (PTV), excluding lungs and kidneys. Treatment plans were evaluated based on dose coverage and high-dose volumes.
Results
All planned treatments met guidelines for TBI target coverage, however, substantial variability in high-dose volumes was observed. Average(±SD) maximum dose to 1 cubic centimeter (D1cc) was 128.7%±7.6%, and average volume of PTV receiving 110% prescription dose (V110%) was 17.4±12.1%. PTV V110% and D1cc demonstrated positive correlations with patient weight (R2 = 0.70, 0.68) and maximum patient separation (R2=0.69, 0.60), while minimal correlations were observed relating to patient height (R2 = 0.29, 0.27). Doses to lungs and kidneys showed no correlations to patient size.
Conclusion
VMAT-TBI is feasible in adults with a wide range of body sizes. Patient weight and separation, rather than height, appear to be the primary factors for dose inhomogeneity. Unlike pediatrics, larger adult bodies may limit achievable high-dose control despite optimization and additional isocentres. These findings suggest that patient size–based planning considerations and acceptance criteria may be necessary when extending VMAT TBI techniques from pediatric to adult populations.