TU‐B‐BRA‐01: Total Body Irradiation (TBI): Looking from Differrent Angles

Purpose: To explore the new degree of freedom of VMAT to design a novel technique for TBI, in which patient is positioned supine/prone on the floor and an arc field is delivered with variable dose rate and/or dynamic collimators. Method and Materials: A formalism of designing an arc field with variable dose rate and/or dynamic jaw/MLC to deliver an arbitrary one‐dimensional dose profile is developed. The dose rate and/or jaw/MLC position are obtained by minimizing the difference between prescribed and calculated dose at each spatial point. The formalism is applied to design TBI treatment plan to achieve (1) a uniform dose over the entire body, and (2) a reduced dose in the lung region. MATLab codes were developed to solve the optimization problem using gradient decent method. Three types of treatment plans were generated: (1) variable dose rate (VDR) only, (2) VDR with dynamic MLC (DMLC), and (3) DMLC only. The optimized treatment plans were converted into DICOM‐RT format and delivered in a Varian 21‐EX machine capable of Rapidarc in DICOM‐RT mode. Measurements with farmer chamber in air with buildup cap were performed and compared with the optimization results. Results: For a patient with 200cm height placed at 100cm below the linac iso‐center, the differences between measured profile from plans with VDR only for uniform/lung sparing case are (−1.4±1.9)% and (−0.2±2.4)%, respectively. They improved to (−0.4±1.0)% and (−0.5±2.2)% with VDR+DMLC. The latter also had 40% higher efficiency. The results for DMLC only were less desirable with reduced range of (−85cm, 85cm) and slightly higher discrepancies. Conclusion: It is feasible to deliver TBI using VMAT, thereby eliminating the need for large treatment room size or hand‐made compensators. Regular linac operating in arc therapy with dynamic MLC alone can be also used, although with less efficiency.