Author: Yukio Fujita, Satoshi Ishikura, Fumiki Ito, Yujiro Nakajima, Takumi Sakamoto, Naoki Tohyama, Ryohei Yamauchi 👨🔬
Affiliation: Komazawa University Graduate School, St. Luke's International Hospital 🌍
Purpose: The aperture shape controller (ASC) is an Eclipse feature that increases muti-leaf collimator (MLC) aperture size. This study aimed to evaluate the effects of ASC settings on the robustness of volumetric modulated arc therapy (VMAT) plans in postmastectomy radiotherapy (PMRT).
Methods: A retrospective study was conducted on 10 patients who received PMRT-VMAT for left breast cancer. All patients were treated with 50 Gy in 25 fractions with four arcs. The virtual bolus (VB) method was applied during IMRT optimization to account for respiratory motion and deformation. The settings were VB density of 0.7 g/cm³, VB thickness of 13 mm, and planning target volume (PTV) expansion of 10 mm. ASC was set respectively to off, low, moderate, and high. Dose distributions were recalculated with the isocenter (IC) displaced for all conditions. The displacement amounts of the IC were set to 5, 7, and 10 mm to simulate the respiratory motion of the breast in the right and posterior directions. Evaluation indices included D95% and D2% of the PTV (PTV 3 mm off the body surface), Dmean of the heart, and V20Gy of the lung. Post-shift doses were evaluated based on the reference value of 0 mm shift.
Results: Without IC shift, PTV D95% differences among the ASC settings were within 0.4 Gy (range, -0.1 – 0.4 Gy). A 10 mm posterior IC displacement resulted in PTV D95% dose changes of -1.50 Gy without ASC, -1.03 Gy with low, -1.01 Gy with moderate, and -1.07 Gy with high ASC. The most significant increase in lung V20Gy (3.6%) was observed with high ASC.
Conclusion: Using ASC in PMRT-VMAT offers the possibility of greater robustness against isocenter shifts in the respiratory direction compared to the VB method alone.