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Significant improvements in plan quality using automated planning have been previously demonstrated.  The aim of this study was to develop an optimal automated class solution for SBRT planning of prostate cancer using the new Feasibility module implemented in the Pinnacle Evolution. 

Twelve patients were retrospectively enrolled in this planning study. Five plans were designed for each patient. Four plans were automatically generated using the four proposed templates for SBRT optimization implemented in the new Pinnacle Evolution TPS, differing for different settings of dose-fallout (Low, Medium, High and VeryHigh). Based on the obtained results, the  fifth plan (Feas) was generated customizing the template with the optimal criteria obtained from the previous step and integrating in the template the “a-priori” knowledge of OARs sparing based on the Feasibility module, able to estimate the best possible DVHs of OARs before starting optimization.  Prescribed dose was 35 Gy to the prostate in 5 fractions. All plans were generated with a full VMAT arc and 6MV FFF beams, and optimized to ensure the same target coverage (95% of the prescription dose to 98% of the target). OARs constraints were as follow: rectum, V18Gy<35%, V28Gy<10% and V35Gy<5%; bladder, V18Gy<40%, V30Gy<15% and Dmax<38 Gy; femoral heads, V25Gy<10%. Plans were assessed according to dosimetric parameters, monitor units, and efficiency planning time. Conformity indexes at 95% and 50% dose levels and cConformation number (CN) were used for comparison.  Differences among the plans were evaluated using a Kruskal-Wallis one-way analysis of variance.

No significant differences were found among the five techniques for prostate coverage in terms of all dosimetric metrics (D95%, D50% and D2%). Feas plans showed consistently better dose conformity with respect to other solutions providing a mean CN value of 0.88 (p<0.05) (Figure1). Feas plans showed significant reduction of rectum  irradiation; Dmean and V18 decreased by 19-21% (p<0.05) and 6-7% (p<0.05), respectively. No statistically significant differences were found in bladder, femoral heads and penile bulb irradiation for all dosimetric metrics. Feas plans showed a significant increased of MU/Gy (mean: 214; p<0.05), reflecting an increased level of fluence modulation. Thanks to the new efficient optimization engines implemented in Pinnacle Evolution (as L-BFGS and Layered Graph), mean planning time was decreased to only 9.5±2.4 minutes for all plans.

The integration of DVHs a-priori knowledge provided by the Feasibility module in the automated planning process for SBRT planning has shown to significantly improve plan quality compared to generic protocol values as inputs.