Extended SSD VMAT treatment for total body irradiation.

In this work, we develop a total body irradiation technique that utilizes arc delivery, a buildup spoiler, and inverse optimized multileaf collimator (MLC) motion to shield organs at risk. The current treatment beam model is verified to confirm its applicability at extended source-to-surface distance (SSD). The delivery involves 7 - 8 volumetric modulated arc therapy arcs delivered to the patient in the supine and prone positions. The patient is positioned at a 90 degrees couch angle on a custom bed with a 1 cm acrylic spoiler to increase surface dose. Single-step optimization using a patient CT scan provides enhanced dose homogeneity and limits organ at risk dose. Dosimetric data of 109 TBI patients treated with this technique is presented along with the clinical workflow. Treatment planning system (TPS) verification measurements were performed at an extended SSD of 175 cm. Measurements included: a 4-point absolute depth-dose curve, profiles at 1.5, 5, and 10 cm depth, absolute point-dose measurements of an treatment field, 2D Gafchromie (R) films at four locations, and measurements of surface dose at multiple locations of a Alderson phantom. The results of the patient DVH parameters were: Body-5 mm D98 95.3 +/- 1.5%, Body-5 mm D2 114.0 +/- 3.6%, MLD 102.8 +/- 2.1%. Differences between measured and calculated absolute depth-dose values were all <2%. Profiles at extended SSD had a maximum point difference of 1.3%. Gamma pass rates of 2D films were greater than 90% at 5%/1 mm. Surface dose measurements with film confirmed surface dose values of >90% of the prescription dose. In conclusion, the inverse optimized delivery method presented in the paper has been used to deliver homogenous dose to over 100 patients. The method provides superior patient comfort utilizing a commercial TPS. In addition, the ability to easily shield organs at risk is available through the use of MLCs.