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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.

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