2298 - Pre-Clinical Evaluation of a Ring Gantry-Based Linac with a Dual-Layer Mlc for Total Marrow and Lymphoid Irradiation and Intensity-Modulated Total Body Irradiation for Pediatric Patients

C. Han¹, A. Magliari², L. Rosa², B. Liu¹, J. Y. C. Wong¹, T. M. Williams¹, and A. Liu¹; ¹Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA, ²Varian Medical Systems Inc, Palo Alto, CA

Purpose/Objective(s): A new ring gantry-based linac with a dual-layer multi-leaf collimator offers fast imaging and delivery, and adaptive radiotherapy capability. We aimed to perform pre-clinical evaluation of treatment planning and delivery workflow for total marrow and lymphoid irradiation (TMLI) and total body irradiation (TBI) using volumetric modulated arc therapy (VMAT) fields on this linac. Materials/

Methods: We retrospectively retrieved four pediatric/adolescent patients (age: 7 – 11) who previously received intensity modulated TBI at our institution. In-house developed software (TXIHelper, publicly available in online Github repository) concatenated upper body and lower extremity CT scans into one CT image set. In the TMLI plans, a prescription dose of 12 Gy in 8 fractions was prescribed to the skeletal bones, spleen, spinal canal, and lymphoid volume. In the TBI plans, the prescribed dose was 12 Gy in 8 fractions for the body volume excluding the lungs. Each VMAT field has a field size of 28 cm × 28 cm with the collimator at 90° and a full gantry rotation. Isocenters were placed with equal spacing of 14 cm along the patient’s longitudinal direction. Two VMAT fields were placed at each isocenter for the upper body up to mid-thigh, while one VMAT field was placed at each isocenter for the lower extremities. In-house software rotated lower extremity fields for feet-in delivery. The nominal dose rate was 800 MU/minute, and the maximum gantry rotation speed was 24°/sec. Institutional dosimetric constraints were used for optimization. Treatment plans were delivered on an anthropomorphic phantom on the machine.
Results: The average body length in the concatenated CT scans was 137.3 cm (range: 126.0 – 152.3 cm). Five isocenters were used for the upper body from the top of skull to mid-thigh, while four isocenters were used for the lower extremities for all patients. In the VMAT TBI plans, the average mean lung dose (MLD) was 6.3±0.3 Gy (range: 5.9 – 6.5 Gy); while in the VMAT TMLI plans, it was 6.1±0.5 Gy (range: 5.8 – 6.9 Gy). The average OAR dose in the TMLI plans ranged from 15.8% to 78.4% relative to the prescription dose. The average net beam-on time in the TMLI plans was 254.8±12.1 sec (range: 239.6 – 269.1 sec), while it was 222.5±2.8 sec (range: 218.4 – 224.3 sec) in the TBI plans. The average dose gradient was 1.1 Gy/cm in the longitudinal direction in field overlapping regions. At each isocenter, the CBCT scan plus image registration took less than 60 seconds, while up to 2 minutes were needed to set up the patient from one isocenter to another as the couch position had to be changed inside the linac vault.
Conclusion: Arrangement of VMAT fields in this study allowed simple clinical setup and adequate plan robustness against setup uncertainty in TBI and TMLI treatments. The CBCT-based image setup time and net beam-on time in the TMLI and TBI plans were significantly shorter compared to other linac models. However, overhead time cannot be ignored as the current system mandates entering the linac vault for each couch movement.