Monte-Carlo Based Total-Body Irradiation Using Linac-Based Volumetric Modulated Arc Therapy

Abstract

To assess the accuracy, feasibility, and reliability of a total-body irradiation (TBI) technique with linac-based volumetric modulated arc therapy (VMAT) in patients with acute myeloid leukemia (AML) or acute lymphoblastic leukemia (ALL). From 2015-2017, 26 patients (15 men, 11 women) with AML or ALL underwent allogenic stem-cell transplantation following TBI with linac-based VMAT technique, which consisted of three isocenters and three overlapping arcs for head first CT sets. We added one or two more overlapping arcs, depending on the height of the patient for feet first CT sets. Patient age ranged from 6-63 years with a mean and median age of 20.5 and 17.5 years, respectively. TBI dose was prescribed to 90% of the clinical target volume receiving 12 Gy in six fractions, at two fractions per day. Mean lung and kidney doses were restricted less than 10 Gy, and maximum lens dose less than 6 Gy. Dose rate was variable, and mean dose rate was 250 MU/min. Calculation parameters were grid spacing 0.4 cm, minimum segment width 0.5 cm, maximum 180 control points per arc, fluence-smoothing medium, statistical uncertainty 1.0% per calculation, gantry increment of 30°, and calculate dose deposition to medium. Patients were immobilized in supine position using customized thermoplastic masks for head support and a vacuum cushion for body support on an adjustable combined immobilization board. Laser-based optical surface-guided devices were used for patient positioning and intra-fraction motion detection. Quality assurance (QA) comprised the verification of the radiation plans via dose-volume histograms (DVH) based 3D patient QA system. Average mean lung dose was 9.7 ± 0.2 Gy, mean kidney dose 9.6 ± 0.2 Gy, maximum lens dose 4.5 ± 0.4 Gy, mean planning treatment volume (PTV) dose 12.7 ± 0.1 Gy, and heterogenity index of PTV was 1.16 ± 0.02 in all patients. Overall acute toxicity was low with only grade 1 and 2 morbidity: patchy mucositis in 16 (62%), nausea not requiring antiemetics in 12 (46%), headache in 11 (42%), fatigue in 9 (35%), neck pain in 8 (31%), xerostomia in 7 (27%), loss of appetite in 6 (23%), and faint or dull erythema in 4 (15%) patients. Grade 3 and more acute radiation toxicity was not observed. All patients are stil alive. When comparing plan and DVH-based 3D patient QA results, average differences of 3.4% ± 1.3 in mean kidney doses, 1.0% ± 0.7 in mean lung doses, and 0.9% ± 0.4 in mean target doses were observed. Linac-based VMAT simplifies the process of TBI, and is accurate, feasible, and reliable. Partial cone-beam CT and optical surface-guided system assure patient positioning. Moreover, up to 6-mm longitudinal misalignment does not cause an average more than 3.5% mean lung dose enhancement. DVH-based 3D patient dose verification QA was possible with linac-based VMAT showing small differences between planned and delivered doses.