Knowledge-Based Planning for Identifying High-Risk Stereotactic Ablative Radiation Therapy Treatment Plans for Lung Tumors Larger Than 5 cm

Purpose: Stereotactic ablative body radiation therapy (SABR) for lung tumors ≥5 cm can be associated with more toxicity than that for smaller tumors. We investigated the relationship between dosimetry and toxicity and used a knowledge-based planning solution to retrospectively perform individualized treatment plan quality assurance (QA) with the aim of identifying where planning could have been improved. Methods and Materials: Previous retrospective analysis of 53 patients with primary or recurrent non-small cell lung cancer ≥5 cm, treated with 5- or 8-fraction volumetric modulated arc therapy SABR between 2008 and 2014, showed 30% with grade ≥3 toxicity. During this period, several improvements were made to departmental planning protocols. RapidPlan was used to compare dosimetry of patients with or without grade ≥3 toxicity. A model comprising plans from patients without toxicity and compliant with the current planning protocol was used to provide QA for the plans from patients who had toxicity. Results: Sixteen of 53 patients had grade ≥3 toxicity, including 10 with radiation pneumonitis (RP), 3 with lung hemorrhage (1 of these also had RP), and 1 with airway stenosis/atelectasis. RP was again shown to be significantly correlated with contralateral and total-lung V5 and mean lung dose. The 4 highest contralateral-lung doses belonged to patients with RP. Five of 10 clinical plans in patients with RP had a contralateral-lung mean dose up to 2.5 times higher than that of the knowledge-based plan. For 2 of 3 patients with lung hemorrhage and 1 with airway stenosis/atelectasis, the clinical plans had the highest proximal bronchial tree doses, which was also higher than in plans from the model. In 8 patients with grade ≥3 toxicity, clinical plans had dosimetry similar to that in the predictions from the model. Conclusions: A “no-toxicity” RapidPlan model identified the potential for dosimetric improvement in nearly 50% of historical treatment plans from patients with grade ≥3 toxicity after SABR for lung tumors ≥5 cm. Model-based QA may be useful for benchmarking treatment planning protocols in routine practice and in clinical studies.