2276 - A Feasibility Study for Pediatric Total Marrow Irradiation on a Novel Ring Gantry-Based Radiotherapy System
Presenter(s)
Q. Xu, J. Y. C. Wong, A. Liu, and C. Han; Department of Radiation Oncology, City of Hope National Medical Center, Duarte, CA
Purpose/Objective(s): Total marrow irradiation (TMI) has been used for patients undergoing hematopoietic cell transplant to minimize radiation-related toxicities. TMI is typically delivered with intensity modulated radiotherapy (IMRT) techniques on helical tomotherapy or C-arm medical linear accelerators (linacs). A new radiotherapy system, the RefleXion X1, is currently available for both image-guided radiotherapy (IGRT) and biology-guided radiotherapy (BgRT). We aim to assess the clinical feasibility of using the X1 machine for TMI delivery and report dosimetric quality for pediatric TMI plans on the RefleXion system.
Materials/Methods: The RefleXion X1 machine delivers radiation using a compact linac mounted on a slip-ring gantry that rotates at 60 rotations/minute. The linac generates a 6MV-FFF beam with a nominal dose rate of 1,000 MU/min, a field size of 40 cm in the lateral direction and either 1 or 2 cm in the longitudinal direction. Beam modulation is achieved by a 64-leaf binary multileaf collimator (MLC). 3 pediatric patients who previously received TMI or intensity-modulated total body irradiation treatments were retrospectively selected. A planning target volume (PTV) was delineated on the CT simulation image to include all the skeletal bones (excluding the mandible) from the top of skull to upper thigh plus setup margins. The PTV was then divided equally to a superior PTV (PTVsup) and an inferior PTV (PTVinf) as separate treatment areas. Over 20 organs at risk (OARs) were delineated. A TMI plan was created in the RefleXion treatment planning system (TPS) with a prescription dose of 12 Gy in 8 fractions to both PTVs. The doses to OARs were kept low during plan optimization.
Results: A RefleXion TMI plan with 2 treatment areas was created for each pediatric patient. With the PTV length ranging from 60.5 to 70.0 cm, the average total treatment time was 22.3 min (range 17.9 – 25.6 min). The average treatment time for each target area was 11.2 min (range 8.3 – 12.8 min). The average coverage by the prescription dose was 83.53% and 81.17% for PTVsup and PTVinf, respectively. The mean dose for OARs were less than 7.5 Gy for all 3 cases. The dosimetric results for the PTVs and major OARs are listed in the table below.
Conclusion: The RefleXion TMI plans achieved adequate PTV coverage and OAR dose sparing with clinically feasible treatment delivery time for pediatric patients. The same technique of using multiple treatment areas in one plan can be used to treat long targets, such as in craiospinal irradiation, on the RefleXion machine.
Abstract 2276 - Table 1| Dose (Gy) (mean and range) | ||||
| D 85% | D 50% | Mean | Max | |
| PTVsup | 11.9 (11.7 – 12.1) | 13.1 (12.9 – 13.3) | 13.1 (12.8 – 13.4) | 17.8 (16.2 – 19.0) |
| PTVinf | 11.8 (11.7 – 11.9) | 13.0 (12.9 – 13.2) | 12.9 (12.7 – 13.2) | 17.5 (15.9 – 19.4) |
| Brain | 1.8 (1.0 – 2.7) | 4.0 (3.7 – 4.4) | 5.4 (5.0 – 5.8) | 15.8 (15.3 – 16.6) |
| Lt Lung | 3.1 (2.7 – 3.6) | 5.3 (5.0 – 6.6) | 5.9 (5.5 – 6.4) | 12.8 (12.4 – 13.4) |
| Rt Lung | 3.3 (3.1 – 3.7) | 5.0 (4.6 – 5.7) | 5.7 (5.4 – 6.3) | 13.0 (12.5 – 13.3) |
| Liver | 3.3 (2.1 - 4.1).6) | 4.4 (3.1 - 5.1) | 4.9 (3.6 - 5.6) | 13.9 (13.6 - 14.3) |