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 (CCre-calc). Furthermore, a CC-based plan with
coverage similar to the PB plan (CCcov) and a CC plan with relaxed
coverage criteria (CCclin), were created. The plans were analyzed
in terms of Dmean, Dmin, Dmax and coverage for GTV, PTV and ITV.
Changes in mean lung dose (MLD), V10Gy and V20Gy 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 V60Gy
of GTV, ITV and PTV decreased by 13.2%, 19.9% and 41.4%,
respectively. Average Dmean decreased by 9% (GTV), 11.6% (ITV) and
14.2% (PTV). Dmin decreased by 18.5% (GTV), 21.3% (ITV) and 17.5%
(PTV). Dmax declined by 7.5%. PTV coverage correlated with PTV
volume (p < 0.001). MLD, V10Gy, and V20Gy were significantly
reduced in the CC plans. Both, CCcov and CCclin 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.