Objectives:

The optimal management of localized prostate cancer has remained a subject of controversy despite recent advances in early detection and treatment of localized forms. Stereotactic Body Radiotherapy (SBRT) is a highly conformal extreme hypofractionated radiation therapy technique that can be safely delivered with excellent outcomes in patients. As the intrafraction prostate motion was increasingly influenced by the prolongation of treatment time, it is possible to shorten SBRT delivery by making adjustments at the planning stage when no organ motion management strategies are used. The aim of this study is to investigate the dosimetric quality and the treatment efficiency of single-arc (SA) versus dual-arc (DA) volumetric modulated arc therapy (VMAT) plans in Linac-based SBRT of localized prostate cancers.

Methods:

Twenty low to intermediate-risk prostate (36.25 Gy in 5 fractions) and twenty high-risk prostate (42.7 Gy in 7 fractions) SBRT plans treated during 2021 with dual-arc FFF-VMAT technique, were re-optimized by two medical physicists. The same PTV margin expansion (5 mm isotropic except 3 mm posterior) was used. A single-arc approach was adopted and new optimization parameters based on the increased planning and clinical experience were incorporated in a new template. Dosimetric parameters of newly optimized SA plans were evaluated and compared with those from original DA plans. The analysis included target coverage, organs at risk (OARs) sparing, treatment delivery time, and accuracy (perpendicular diode matrix tool, gamma analysis-passing ratio, PR). Paired t-test was used to assess the statistical significance level (alpha=0.05). The relationships of rectum and bladder volumes with the rectum and bladder dose metrics were also investigated. One senior radiation oncologist performed a blind choice between DA and SA plans for the clinical assessment.

Results:

In all cases, the SA optimization technique resulted in a better treatment plan than the original one. PTV Dmean was higher in SA plans (SA: 101.4%; DA: 101.1%; P< 0.001), while PTV D95% and PTV D2% were comparable between the two techniques (SA: 96.7% and 104.1%; DA: 96.5% and 103.9%; P>0.05). A significantly increased OARs sparing was observed in SA plans, especially in rectum and bladder mean doses. The mean absolute dose difference was -3.7 Gy [range, -7.6 – -0.6] for rectum Dmean (P< 0.001) and -1.2 Gy [range, -4.0 – 0.4] for bladder Dmean (P< 0.001). The only association was observed in the bladder volume that moderately correlated with bladder Dmean (R2 = 0.58) and bladder D40% (R2 = 0.61). The mean SA treatment delivery time was reduced by 22%, passing from 2.1 minutes [1.7 – 3.0] to 1.5 minutes [1.3 – 1.9] on average (P< 0.001). The mean monitor units rose from 1819 ± 332 to 1967 ± 301 (P< 0.001) due to higher plan complexity. Despite the increased fluence modulation, dose measurements reported an optimal agreement with dose calculations with a PR greater than 95% for 2%(local)-2 mm criteria (SA: 98.7; DA: 98.0%; P=0.004).

Conclusion(s):

SA VMAT planning technique, with newly optimized parameters, achieved clinically equivalent target coverage while significantly reducing the dose to the rectum and bladder compared to DA plans. Moreover, the treatment delivery time was substantially reduced, lowering the probability of prostate motion beyond the margins. These findings indicate a potential decrease in treatment-related toxicity and an improvement in actual target coverage during prostate SBRT treatments. Further investigations are warranted to assess the long-term clinical outcomes associated with this planning technique.