Author: Chunhui Han, An Liu, Anthony Magliari, Lesley Rosa 👨🔬
Affiliation: Office of Medical Affairs, Varian, A Siemens Healthineers Company, Department of Radiation Oncology, City of Hope National Medical Center 🌍
Purpose: To evaluate the performance of a knowledge-based planning (KBP) optimization model for total marrow and lymphoid irradiation (TMLI) on a ring-gantry linac with a dual-layer MLC and to evaluate optimal isocenter arrangements.
Methods: Treatment planning images and contours were retrieved for sixteen TMLI patients treated on helical tomotherapy (HT). The patient lateral width ranged from 43 to 62 cm. TMLI plans using volumetric modulated arc therapy (VMAT) fields on a ring-gantry linac were created with a prescription dose of 12 Gy to skeletal bones, lymphoid volumes, and spleen. Three isocenter arrangements were evaluated: 1) at an equal 14-cm distance along the patient midline, 2) at alternating distances of 14 cm and 10.5 cm along the midline, and 3) at equal distance but with lateral offsets except the most superior isocenter. TMLI plan optimization was performed using a previously trained KBP model. Plan delivery efficiency was evaluated with delivering on an anthropomorphic phantom.
Results: For patients with a width < 52 cm, adequate coverage to lateral aspects of PTVs was achieved with isocenters along patient midline; for patients with a lateral width > 52 cm, optimal target coverage was achieved with isocenters placed at a 7-cm lateral offset from the midline. Delivery was more efficient with alternating 10.5-cm and 14-cm isocenter distances, as two isocenters can be set up in one sub-plan on the linac. Compared with historical HT plans, the VMAT plans achieved significantly improved mean dose to the rib PTV and comparable mean dose to other PTVs, comparable mean dose to 4 OARs, and significantly lower mean dose to 16 OARs.
Conclusion: TMLI plans generated by the KBP model showed comparable or better target coverage and OAR sparing compared to historical HT plans. Optimal isocenter arrangements were identified. The KBP model is now publicly available for clinical use.