Dosimetric and robustness comparison of single- and double-arc volumetric-modulated arc therapy incorporating avoidance structures for prostate cancer with simulated bilateral hip prostheses.

This study compared the dosimetric performance and robustness of single-arc (SA) versus double-arc (DA) volumetric-modulated arc therapy (VMAT) incorporating avoidance structures (AS) for prostate cancer with simulated bilateral hip prostheses. A total of 15 prostate cancer cases without hip prostheses were retrospectively analyzed. Bilateral hip prostheses were simulated by assigning titanium-equivalent Hounsfield unit values to both femoral heads. Identical dose constraints and full-arc geometry with AS configured to suppress beam entry through prosthetic regions were used to generate SA and DA plans. Dosimetric indices for target volumes and organs at risk were compared. Plan robustness was evaluated by simulating image-guided position correction with 5-mm and 10-mm shifts of the clinical target volume (CTV) in the anterior-posterior and superior-inferior directions; CTV D98% served as the primary robustness metric. Under identical planning conditions, DA demonstrated statistically superior performance for several planning target volume and rectal dose parameters; however, absolute differences were small. SA plans showed greater asymmetry in the intermediate-dose region. With posterior and inferior position corrections, DA demonstrated significantly better robustness than SA, whereas superior position correction showed only borderline significance. Absolute differences in CTV D98% between SA and DA under posterior and inferior position corrections were ≤0.6 Gy, indicating limited clinical relevance. No significant difference in CTV D98% was observed under anterior position correction. Both approaches showed reduced CTV coverage at a 10-mm anterior shift, with mean decreases of 1.7% for SA and 1.6% for DA. DA VMAT showed marginally superior robustness under posterior and inferior position corrections, whereas SA achieved comparable overall plan quality with theoretically shorter delivery times. Careful consideration of anterior internal error, including treatment planning optimization strategies accounting for hip prostheses and strict patient preparation protocols, remains crucial for both approaches.