Evaluation of treatment planning feasibility of a positron emission tomography (PET)–Linac system for spine stereotactic body radiotherapy (SBRT)

BACKGROUND AND PURPOSE: The RefleXion X1 (RefleXion Medical, Inc., Hayward, CA) is a unique radiotherapy platform integrating kVCT and PET as on-board imaging guidance. This study aims to evaluate the feasibility and treatment planning quality of RefleXion X1 treatment planning system (TPS) for spinal stereotactic body radiation therapy (SBRT). METHODS: This retrospective study analyzed treatment plans of 10 different patient cases, including five single-dose level SBRT cases and five simultaneously integrated boost (SIB) SBRT cases. All plans were created using the X1 TPS with a 10 mm jaw size and evaluated against institutional planning guidelines to ensure clinical acceptability. To assess planning quality, the X1-generated plans were compared to clinically treated plans. Key metrics such as planning target volume (PTV) coverage, maximum point dose (measured as maximal dose to any volume of 0.035cc), conformity index (CI), and organs-at-risk (OARs) constraints were evaluated. Statistical analysis was performed using the Wilcoxon signed-rank test, with significance defined as p < 0.05. RESULTS: All RefleXion X1 plans met our institutional planning guidelines for spinal SBRT and were deemed clinically acceptable. No statistically significant differences were observed in all PTV metrics, including $D_{\max }$ ( $p=0.70$ ), $D_{\mathrm{mean}}$ ( $p=0.77$ ), and $D_{\min }$ ( $p=0.23$ ). Meanwhile, X1 plans demonstrated slightly higher average maximum cauda equina doses (X1: $12.87$  Gy, clinically treated: $11.57$  Gy, $p=0.05$ ), while maintaining comparable dose sparing for other OARs. The average CI values for X1 plans and clinically treated plans were $1.19$ and $1.18$ , respectively ( $p=0.67$ ), demonstrating similar dose conformity. Additionally, homogeneity index (HI) and gradient index (GI) values also showed no significant differences, further supporting the comparable performance of X1 plans. CONCLUSIONS: The RefleXion X1 system can generate clinically acceptable treatment plans for complex spinal SBRT, with plan quality comparable to that of clinically treated plans at our institution.