Spatially-Fractionated (LATTICE) Radiotherapy for Metastatic Sarcoma

LATTICE radiotherapy (LRT) is a form of spatially-fractionated radiotherapy that delivers a high dose of radiation to small volumes within a tumor target while simultaneously reducing dose to normal tissue. Single fractions of 10-20 Gy LRT have been used to treat radioresistant lesions with a 72-81% response rate and no acute or late grade 3 toxicities. Single-fraction LRT presents an attractive approach for implementation of ablative therapy for oligo-metastatic disease in radioresistant soft tissue sarcoma (STS) histotypes, such as chondrosarcoma. The purpose of this study was to develop a dosimetry workflow for creating LRT plans for soft tissue and lung metastases of STS. LRT plans were created using volumetric arc therapy (VMAT) in a commercially available treatment planning system on a simulation scan from a metastatic STS patient. A neck (624 cc) and a lung (619cc) gross tumor volume (GTV) were outlined and symmetrically expanded to include a 1 cm planning tumor volume (PTV). The dosimetric characteristics of plans delivering 20 Gy, 15 Gy, and 10 Gy LRT were assessed. 1.5 cm diameter target spheres were used with a 4.5 cm spacing from each sphere edge in each axial plane. Each axial plane with a sphere centroid was 2 cm apart, and spheres were offset by 3 cm to avoid stacking. Sphere packing was optimized and constrained within the PTV. Planning attempted to achieve a gradient of 100% to the spheres to 35% at the mid-point between adjacent spheres. The LRT spheres and intersphere volume were well covered at each dose are (Table 1). The max dose for the neck plan was 130.5%, 130.7%, and 122%, for the 20 Gy, 15 Gy, and 10 Gy plans, respectively. The max dose for the lung plan was 136%, 135.3%, and 128%, for the 20 Gy, 15 Gy, and 10 Gy plans, respectively. All neck and lung plans were able to achieve negligible dose to the spinal cord, esophagus, untreated lung, and heart. The treated lung had V8 Gy of 1.9%, 0.8%, and 0.1% for 20 Gy, 15 Gy, and 10 Gy doses, respectively.The lung plans required a higher number of MUs (3512-12,327) than the neck plans (2280-9764). This study demonstrates that it is feasible to create LRT plans for neck and lung tumors using VMAT. These plans are currently under quality assurance testing. We plan to use this planning process in an upcoming institutional trial to examine the safety, efficacy, and immunogenicity of ablative LRT for oligometastatic conventional chondrosarcoma, a radioresistant, “cold” tumor with few promising treatment options.Abstract 3637; Table 1achievable coverage of target volumes at different dose levels20 Gy VMAT15 Gy VMAT10 Gy VMATNeckV100% Spheres88.7%97.6%70.6%V30% PTV Target96%88.9%98.9%LungV100% Spheres84.5%89.7%75.1%V30% PTV Target84.8%87.3%89.6% Open table in a new tab