Cyberknife as a novel platform for targeted mouse tumor radiation studies

Abstract

Cancer remains to be a leading cause of death worldwide, accounting for almost 10 million deaths in 2018. Radiation therapy (RT) is a common nonsurgical treatment in the management of patients with cancer that reduces disease recurrence and improves overall survival. However, preclinical RT modeling for accelerated bench‐to‐bedside transition of combination therapies is largely missing. While genetically engineered orthotopic murine tumor models can mimic human disease, conventional linear particle accelerator systems are not suited for targeted radiation of orthotopic tumors in mice. Although select institutions provide specialized mouse radiation systems overcoming these obstacles, most researchers are forced to use transplantable flank xenograft tumor models for targeted RT. We here report image‐guided, targeted radiation to mouse tumors using the Cyberknife (Accuray) radiation platform. The Cyberknife is a stereotactic radiosurgery system that utilizes real‐time image‐guidance to deliver high doses of radiation specifically to the tumor with minimal damage to the surrounding tissues using circular collimators as small as 5 mm. We applied targeted radiation of 2, 4, 6, and 8 Gy using field sizes of 7.5 to 12.5 mm to average gross tumor volumes (GTV) of volumes 0.87 cm3 (minimal 0.19 cm3) using Xsight (Accuray) spine‐based tumor localization. Treatment time lasted between 14 min (2 Gy), and 31 min (8 Gy), averaging at 20 min across all used doses. Doses were prescribed to 70% isodose, leading to a mean relative GTV dose of 92.1% (SD 1.6%), with a minimum dose of 80.3% (SD 3.8%). Coverage (volume of tumor that receive the prescription dose) was 99.4% (SD 0.8%) with a conformity (ratio of total tissue volume that receives the prescription isodose or more to tumor volume that receives the prescription isodose or more) of 1.10 (SD 0.03). To optimize the planning target volume (PTV), we examined 0, 1, 2, and 3 mm PTVs and analyzed the tumor tissue by immunohistochemistry staining of y‐H2AX. We found that 1 mm PTV is sufficient for target radiation of mouse tumors using the Cyberknife. Furthermore, we analyzed the impact of radiation on tumor infiltrating lymphocytes in a model of non‐small cell lung cancer (mutated for Kras and Trp35) by flow cytometry. We observed that higher radiation doses (6–8 Gy) decreased absolute cell numbers of lymphocytes and myeloid cells by approximately half compared to low doses (2–4 Gy) or non‐radiated. In summary, we here demonstrate that the Cyberknife system allows image‐guided targeted radiation of murine tumors with high precision and thus constitutes a novel promising platform for mouse radiation studies, and especially combination treatment studies. Support or Funding Information This study was supported by the DFG (HE 6810/3‐1 to J.M.H., HE 6897/2‐1 to G.H.S.) and the CMMC (CAP‐13 to J.M.H. and CAP‐16 to G.H.S. as well as B02 to G.H.S. and J.M.H.).