Session Item

IMRT with fixed beams is a much applied technique for treatment of locally advanced non-small cell lung cancer (LA-NSCLC), to avoid large lung volumes receiving low dose. However, manually selecting the optimal beam configuration for each patient is a challenging task due to large anatomical variations between patients. The aims of this study were to use automated IMRT planning with integrated beam angle optimization (BAO) to 1) enhance plan quality compared to manual planning, 2) investigate patterns in selected beam configurations and 3) investigate the impact of the applied number of beams on plan quality.

26 LA-NSCLC patients were prospectively included. The prescribed dose was 60-70 Gy in 2 Gy fractions. All patients had a clinical 6-beam IMRT plan (CLIN), manually created by an expert planner. Using a novel in-house developed system for automated multi-criterial IMRT planning with integrated BAO, clinically deliverable plans with 4, 6 and 8 optimized beams were created for each patient (AUTO). Candidate beam angles were 140°-40° for right-sided tumors and 320°-220° for left-sided tumors (5° spacing). Dose-volume parameters related to toxicity and patterns of selected beam angles were compared for 6-beam CLIN and AUTO plans. 4-, 6- and 8-beam AUTO plans were mutually compared. The Wilcoxon signed-rank test (p ≤ 0.05) was used for statistical testing.

In the CLIN plans, there were mainly small variations from the planning beam template and all patients had opposing beams in the AP direction. In contrast, the angles in the 6-beam AUTO plans were spread out across the candidate beam space, demonstrating more patient-specific selection (Fig. 1). The resulting AUTO plans were dosimetrically clearly superior to the CLIN plans (Fig. 2). While PTV coverage and lung dose were similar, the median heart D_mean was reduced from 9.0 Gy to 8.1 Gy (p = 0.02), median esophagus D_mean from 20.3 Gy to 18.5 Gy (p = 0.02), median heart V_30Gy from 11.0% to 6.2% (p = 0.002) and median esophagus V_20Gy from 38.4% to 36.8% (p = 0.008). Fig. 1b shows large differences in selected beams between 6-beam CLIN and AUTO plans for an example patient, with clear impact on heart and esophagus sparing. Dosimetric QA at the linac proved deliverability of the 6-beam AUTO plans. Overall, increasing the number of optimized beams improved OAR sparing, with a larger impact of going from 4 to 6 than from 6 to 8 (Fig. 2).

Selected beam angles have a significant impact on IMRT plan quality for LA-NSCLC patients. Automated planning with integrated computerized BAO showed a great potential for enhanced OAR sparing compared to planning with manually selected angles. Dosimetric implications of increasing or reducing the number of beams were patient-specific, but overall, increasing the number of beams improved OAR sparing. Autoplanning allows for generation of several plans with different numbers of optimized beams to select the optimal number for each individual patient.