Room: ePoster Forums
Purpose: To evaluate the dosimetric effect of utilizing the aperture shape controller (ASC) photon optimization parameter in Volumetric Arc Therapy (VMAT) planning.
Methods: A set of 25 patients previously treated on a Varian Trilogy unit were selected for treatment planning utilizing ASC. ASC imposes a constraint on the optimizer that favors Multileaf Collimator (MLC) patterns that are smooth and open. This is compared to the erratic, small apertures present in overmodulated plans, which can increase both the beam-on time and head scatter dose, as well as decrease the accuracy of the patientâ€™s calculated dose. For each of the 25 patients selected in this retrospective study, four treatment plans were created with varying levels of ASC strength: None, â€œVery Lowâ€?, â€œMediumâ€?, and â€œVery Highâ€?. As the strength of the ASC increased, the changes in OAR mean doses, maximum patient dose, dose gradient, and monitor units (MU) were analyzed. Patient-specific quality assurance (QA) was also run on each of these plans using a Sun Nuclear Mapcheck2 diode array.
Results: Dosimetric analysis showed that the use of ASC across various treatment sites resulted in a tradeoff between dose sparing and MU, where a mean OAR dose increase of ~0.5% yielded an MU decrease of ~6% when the â€œMediumâ€? ASC strength setting was used, as compared to a fully modulated plan. As expected, this tradeoff was shown to be more pronounced the higher the ASC strength setting. Furthermore, the patient-specific QA passing rate was shown to increase slightly with the use of ASC.
Conclusion: This study showed that ASC can make mild, yet consistent treatment delivery improvements for most VMAT cases at the cost of small OAR dose increases by directly influencing the modulation of the MLC aperture.