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<p>Purpose: To develop an automatic collimator setting optimization algorithm to improve
dosimetric quality of pancreas Volumetric Modulated Arc Therapy (VMAT) plans for Stereotactic
Body Radiation Therapy (SBRT). </p><p>Methods: Fifty-five pancreas SBRT cases were
retrospectively studied. Different from the conventional practice of initializing
collimator settings manually, the proposed algorithm simultaneously optimizes the
collimator angles and jaw positions which are customized to the patient geometry.
This algorithm includes two key steps: an iterative optimization algorithm via simulated
annealing that generates a set of collimator settings candidates, and a scoring system
that choose the final collimator settings based on organs-at-risk (OARs) sparing criteria
and dose prescription. The scoring system penalizes 3 factors: 1) jaw opening ratio
on Y direction to X direction; 2) unmodulated MLC area within the jaw aperture in
a dynamic MLC sequence; 3) OAR shielding capability by MLC with MLC aperture control
constraints. For validation, the other 16 pancreas SBRT cases were analyzed. Two dual-arc
plans were generated for each validation case, an optimized plan (Planopt) and a conventional
plan (Planconv). Each plan was generated by a same set of auxiliary planning structures
and dose-volume-histogram (DVH) constraints in inverse optimization. Dosimetric results
were analyzed and compared. All results were tested by Wilcoxon signed-rank tests.</p><p>Results:
Both plan groups had no statistical differences in target dose coverage V95% (p=0.84)
and Root Conformity Index (p=0.30). Mean doses of OARs were improved or comparable.
In comparison with Planconv, Planopt reduced maximum dose (D0.03cc) to stomach (-49.5cGy,
p=0.03), duodenum (-63.5cGy, p<0.01), and bowel (-62.5cGy, p=0.01). Planopt also showed
lower modulation complexity score (p=0.02), which implies its higher modulation complexity
of the dynamic MLC sequence. </p><p>Conclusions: The proposed collimator settings
optimization algorithm successfully improved dosimetric performance for dual-arc VMAT
plans in pancreas SBRT. The proposed algorithm was demonstrated with great clinical
feasibility and readiness.</p>
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