Purpose: We evaluated the feasibility of using a novel treatment planning technique for total body irradiation (TBI) using modern VMAT fields on conventional linacs with the aim of improving planning efficiency and minimizing dose variation in field junction regions.
Methods: Two clinical TBI cases were used in this study. The head-in upper body CT simulation and feet-in lower-extremity CT simulation for each case were registered and concatenated to form a whole-body simulation from the top of skull to toes. A treatment plan was optimized to treat the whole-body target volume for 13.2 Gy total in 8 fractions, in which nine arc fields were used to treat the upper body and three wide aperture arc fields were used to treat lower extremities from the mid-thigh level. The optimized plan was separated into an upper-body and a lower-extremity plan. An in-house software program converted the lower-extremity plan into a new plan with the feet-in position by re-arranging control points and rotating field apertures at every control point. The converted lower extremity plan was re-imported into the TPS for recalculation and validation and the total dose was verified by an independent calculation algorithm.
Results: In both the whole-body TBI plans the mean lung dose was < 8 Gy with the upper-body and lower-extremity targets receiving the prescribed dose to at least 85% volume. With the feet-in patient position, the re-calculated lower-extremity plans showed identical dose distribution to the original plans. Both the upper-body and lower-extremity plans showed a linear dose gradient in the mid-thigh junction region with an average drop-off of 30 cGy/mm.
Conclusion: Using this planning technique, VMAT TBI plans can be generated efficiently with homogeneous dose in the junction region while minimizing dose variation from setup uncertainties. Our clinical experience showed that this takes similar time and resources as conventional TBI.