Stereotactic radiotherapy of intrapulmonary lesions: comparison of different dose calculation algorithms for Oncentra MasterPlan®

BACKGROUND: The use of high accuracy dose calculation algorithms, such as Monte Carlo (MC) and Collapsed Cone (CC) determine dose in inhomogeneous tissue more accurately than pencil beam (PB) algorithms. However, prescription protocols based on clinical experience with PB are often used for treatment plans calculated with CC. This may lead to treatment plans with changes in field size (FS) and changes in dose to organs at risk (OAR), especially for small tumor volumes in lung tissue treated with SABR. METHODS: We re-evaluated 17 3D-conformal treatment plans for small intrapulmonary lesions with a prescription of 60 Gy in fractions of 7.5 Gy to the 80% isodose. All treatment plans were initially calculated in Oncentra MasterPlan® using a PB algorithm and recalculated with CC (CC(re-calc)). Furthermore, a CC-based plan with coverage similar to the PB plan (CC(cov)) and a CC plan with relaxed coverage criteria (CC(clin)), were created. The plans were analyzed in terms of D(mean), D(min), D(max) and coverage for GTV, PTV and ITV. Changes in mean lung dose (MLD), V(10Gy) and V(20Gy) were evaluated for the lungs. The re-planned CC plans were compared to the original PB plans regarding changes in total monitor units (MU) and average FS. RESULTS: When PB plans were recalculated with CC, the average V(60Gy) of GTV, ITV and PTV decreased by 13.2%, 19.9% and 41.4%, respectively. Average D(mean) decreased by 9% (GTV), 11.6% (ITV) and 14.2% (PTV). D(min) decreased by 18.5% (GTV), 21.3% (ITV) and 17.5% (PTV). D(max) declined by 7.5%. PTV coverage correlated with PTV volume (p < 0.001). MLD, V(10Gy), and V(20Gy) were significantly reduced in the CC plans. Both, CC(cov) and CC(clin) had significantly increased MUs and FS compared to PB. CONCLUSIONS: Recalculation of PB plans for small lung lesions with CC showed a strong decline in dose and coverage in GTV, ITV and PTV, and declined dose in the lung. Thus, switching from a PB algorithm to CC, while aiming to obtain similar target coverage, can be associated with application of more MU and extension of radiotherapy fields, causing greater OAR exposition.