Dosimetric Comparison of Volumetric Modulated Arc Therapy with Tomotherapy Based Total Body Irradiation for Patients Undergoing Conditioning Prior to Hematopoietic Stem Cell Transplantation for Acute Lymphocytic Leukemia

Abstract

Total body irradiation (TBI) as a component of myeloablative condition prior to hematopoietic stem cell transplantation (HSCT) for acute leukemia has been associated with high rates of pulmonary toxicity. The purpose of this study is to compare the dosimetry of volumetric modulated arc radiotherapy (VMAT) with tomotherapy (tomo) for TBI in patients undergoing myeloablative conditioning prior to HSCT for acute lymphocytic leukemia (ALL), and to report acute toxicity associated with these regimens.Sixteen patients with ALL undergoing high dose TBI at our institution were retrospectively identified. Prescription dose was to 13.2 Gy for 15 patients and 12 Gy for one patient, delivered in 8 twice daily fractions, at least 6 hours apart. The planning target volume was defined as the external body contour minus the lung, heart, and kidneys. Six or seven isocenters were utilized with VMAT plans with two arcs delivered at each isocenter. Immobilization was achieved with a vacuum cradle for tomo patients and with a stereotactic body radiotherapy (SBRT) board with rigid thermoplastic immobilization for VMAT patients. Daily cone beam CT was acquired at each isocenter for VMAT patients. Mean dose to organs at risk (OARs) were compared with T-tests. Results are presented with 95% confidence intervals (CI).Sixteen patients with ALL planned for high dose TBI were identified for analysis. Ten patients were planned for VMAT-based TBI while six patients were planned for tomo-based treatment. There was no significant difference between mean heart dose for VMAT plans (5.71 Gy, 95% CI 5.16-6.27) vs tomo plans (6.26 Gy, 95% CI 5.51-7.01), P = 0.18. Mean lung dose was lower for VMAT (7.20 Gy, 95% CI 7.01-7.40) compared to tomo (7.75 Gy, 95% CI 7.24-8.26), P = 0.04. The mean kidney dose was lower for tomo-based plans (5.55 Gy, 95% CI 4.91-6.19) vs VMAT (6.31 Gy, 95% CI 5.87-6.75), P = 0.03. The median follow-up from completion of TBI was 198.5 days and the median time to HSCT after completion of TBI was four days. No acute or late pulmonary toxicity was observed. Seven of the 14 patients (50%, 95% CI 23-77) who completed TBI developed acute grade 3 or grade 4 mucositis, including 2 of the 5 tomo patients (40%, 95% CI 5-85) and 5 of the 9 VMAT patients (56%, 95% CI 21-86). One patient treated with VMAT developed acute grade 3 diarrhea while one patient treated with Tomo experienced acute grade 3 enteritis and dermatitis. One patient treated with tomo developed late grade 2 xerophthalmia, and there were no other late radiotherapy related toxicities observed. Four patients expired during the study follow-up period at a median of 21 days following TBI. One patient had relapsed disease approximately 8 months following completion of TBI.High dose TBI planned with VMAT and delivered with SBRT board and thermoplastic immobilization is feasible and decreases mean lung dose compared to tomotherapy based plans. While mucositis was a common adverse event, no pulmonary toxicity was observed.