Quality assessment of automatically planned o-ring linac SBRT plans for pelvic lymph node and lung metastases, evaluating the optimal minimum target size.

The purpose of this study is to assess the influence of Planning Target Volume (PTV) on the quality of automatic planned O-Ring Halcyon linac stereotactic body radiation therapy (SBRT) plans of pelvic lymph nodes (LN) and lung metastases and to evaluate an absolute PTV volume threshold as a plan quality prediction criterion. A total of 21 pelvic LN and 18 lung clinical treatment plans were replanned for Halcyon with unattended autoplanning. The prescription dose range was 26-40 Gy for LN and between 39-54 Gy for the lung in the mean 3 fractions. The mean/median PTV was 4.0/ 3.6 cm3 for LN and 4.9/ 4.3 cm3 for the lung. The criteria for the plan quality evaluation consisted of using dose metrics for conformity, spillage, and coverage and dose limits on healthy tissue assessment. A statistical study was performed based on systematic Mann-Whitney U test and cluster analysis to evaluate a PTV volume predictor threshold of plan quality. 95% (n = 20) LN and 100% (n = 18) lung plans met all tolerance criteria. For both cohorts of plans, a PTV threshold was determined, indicating a reduction of particular dose indices when below this threshold. Low risk of toxicity in healthy tissues was predicted. A PTV threshold of 4.0 cm3 was estimated as quality criteria in both cohorts of plans. The results of our study demonstrated the promising performance of Halcyon for pelvic and lung SBRT for small tumors, although plan-specific QA is required to verify machine performance during plan delivery.