Abstract
BackgroundThis study compared adaptive replanning and repositioning corrections based on soft-tissue matching for prostate cancer by using the magnetic resonance-guided radiation therapy (MRgRT) system.MethodsA total of 19 patients with prostate cancer were selected retrospectively. Weekly magnetic resonance image (MRI) scans were acquired for 5 weeks for each patient to observe the anatomic changes during the treatment course. Initial intensity-modulated radiation therapy (IMRT) plans (iIMRT) were generated for each patient with 13 coplanar 60Co beams on a ViewRay™ system. Two techniques were applied: patient repositioning and replanning. For patient repositioning, one plan was created: soft-tissue (prostate) matching (Soft). The dose distribution was calculated for each MRI with the beam delivery parameters from the initial IMRT plan. The replanning technique was used to generate the Adaptive plan, which was the reoptimized plan for the weekly MRI. The dose-volumetric parameters of the planning target volume (PTV), bladder, and rectum were calculated for all plans. During the treatment course, the PTV, bladder, and rectum were evaluated for changes in volume and the effect on dosimetric parameters. The differences between the dose-volumetric parameters of the plans were examined through the Wilcoxon test. The initial plan was used as a baseline to compare the differences.ResultsThe Adaptive plan showed better target coverage during the treatment period, but the change was not significant in the Soft plan. There were significant differences in D98%, D95%, and D2% in PTV between the Soft and Adaptive plans (p < 0.05) except for Dmean. There was no significant change in Dmax and Dmean as the treatment progressed with all plans. All indices for the Adaptive plan stayed the same compared to those of iIMRT during the treatment course. There were significant differences in D15%, D25%, D35%, and D50% in the bladder between the Soft and Adaptive plans. The Adaptive plan showed the worse dose sparing than the Soft plan for the bladder according to each dosimetric index. In contrast to the bladder, the Adaptive plan achieved better sparing than the Soft plan during the treatment course. The significant differences were only observed in D15% and D35% between the Soft and Adaptive plans (p < 0.05).ConclusionsPatient repositioning based on the target volume (Soft plan) can relatively retain the target coverage for patients and the OARs remain at a clinically tolerance level during the treatment course. The Adaptive plan did not clinically improve for the dose delivered to OARs, it kept the dose delivered to the target volume constant. However, the Adaptive plan is beneficial when the organ positions and volumes change considerable during treatment.
Highlights
This study compared adaptive replanning and repositioning corrections based on soft-tissue matching for prostate cancer by using the magnetic resonance-guided radiation therapy (MRgRT) system
cone beam computed tomography (CBCT) has insufficient image quality compared to planning computed tomography (CT) to provide an accurate delineation of structures and dose calculation owing to the higher noise and lower contrast [29, 32]
This study explored different MRgRT techniques to compensate for inter-fraction errors during prostate treatment
Summary
This study compared adaptive replanning and repositioning corrections based on soft-tissue matching for prostate cancer by using the magnetic resonance-guided radiation therapy (MRgRT) system. Clinical implementation of both offline and online adaptive techniques with a common imaging modality based on X-rays is still in the development phase because it is not sufficient to modify the treatment plan according to anatomic changes on inter- and intra-fraction bases. In 2009, Thongphiew et al compared three online IGRT techniques (bony-anatomy matching, soft-tissue matching, and online replanning) for prostate IMRT treatment using daily CBCT [18]. They demonstrated that CBCT can feasibly be used for reoptimizing the treatment plan online, which has significant benefits when a high degree of deformation or differential organ position displacement occurs. The inconsistency between the CT number to electron density curves of CT and CBCT images can result in a discrepancy in the dose calculations
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