Dosimetric and geometric evaluation of a hybrid strategy of offline adaptive planning and online image guidance for prostate cancer radiotherapy

Abstract

For prostate cancer patients, online image-guided (IG) radiotherapy has been widely used in clinic to correct the translational inter-fractional motion at each treatment fraction. For uncertainties that cannot be corrected online, such as rotation and deformation of the target volume, margins are still required to be added to the clinical target volume (CTV) for the treatment planning. Offline adaptive radiotherapy has been implemented to optimize the treatment for each individual patient based on the measurements at early stages of treatment process. It has been shown that offline adaptive radiotherapy can effectively reduce the required margin. Recently a hybrid strategy of offline adaptive replanning and online IG was proposed and the geometric evaluation was performed. It was found that the planning margins can be further reduced by 1–2 mm compared to online IG only strategy. The purpose of this study was to investigate the dosimetric benefits of such hybrid strategy on the target and organs at risk (OARs). A total of 420 repeated helical computed tomography (HCT) scans from 28 patients were included in the study. Both low-risk patients (LRP, CTV = prostate) and intermediate-risk patients (IRP, CTV = prostate + seminal vesicles, SV) were included in the simulation. Two registration methods, based on center-of-mass (COM) shift of prostate only and prostate plus SV, were performed for IRP. The intensity modulated radiotherapy (IMRT) was used in the simulation. Criteria on both cumulative dose and fractional doses were evaluated. Furthermore, the geometric evaluation was extended to investigate the optimal number of fractions necessary to construct the internal target volume (ITV) for the hybrid strategy. The dosimetric margin improvement was smaller than its geometric counterpart and was in the range of 0 mm to 1 mm. The optimal number of fractions necessary for the ITV construction is 2 for LRP and 3–4 for IRP in a hypofractionation protocol. A new cumulative index of target volume (CITV) was proposed for the evaluation of adaptive radiotherapy strategies, and it was found that it had the advantages over other indices in evaluating different adaptive radiotherapy strategies.

Adaptive radiotherapy strategies for pelvic tumors – a systematic review of clinical implementations

“…In addition to one in silico implementation of online compensation [58], these were the only studies where feedback from delivered dose triggered adaptations. Contributing to differences in applied strategies and workflows could be the additional manual contouring, which was a pre-requisite in over 70% of the simulated ART workflows [22,59,60,[62][63][64][65][66][68][69][70]72,74,78,79,[81][82][83]85,86,88,89,91,93,[95][96][97][98][100][101][102][103][104][105][106][107]. To reduce the need of manual contours, the segmentation could be propagated using deformable image registration.…”

“…Deformable image registration was more frequently applied in the in silico studies, mostly for online re-planning/re-optimization [59,61,67,71,86,87,94] and offline re-planning/re-optimization strategies [60,63,66,69,75,107] but also applied for online MLC/field adjustments [77,89]. To facilitate adaptations, especially for online execution, modifications of the planning system or automatic activation/selection of plans was applied in 50% of the in silico studies [58][59][60][61][67][68][69]71,72,[74][75][76][77][78][79][80][82][83][84][85][86][87][88][92][93][94]. In view of the increasing speed of optimization algorithms, limited in-room imaging quality together with manual (re-)contouring remains the biggest bottlenecks for clinical implementation of many of the ART strategies and certainly limiting online applications.…”

Evaluations of an adaptive planning technique incorporating dose feedback in image‐guided radiotherapy of prostate cancer

“…For example, the CTV-to-PTV margin accounting for inter-fraction motion can be reduced to 3 mm with online IG alone, 25 and $1 mm with a hybrid strategy combining online and offline adaptive re-planning. 9,10 Both strategies are geometric in nature. From this study, the margin can be even further reduced if dose compensation is performed in combination with online correction.…”

“…Due to the prevalence of the online IG for prostate cancer in the clinic, naturally a hybrid strategy was recently proposed which combine online IG and offline geometric ART for further margin reduction. 9,10 An alternative technique to account for the residuals from online correction is the dosimetric ART or dose compensation technique, which was first proposed for image-guided radiotherapy (IGRT) of prostate cancer, 11,12 and the cumulative dose distribution from early fractions was incorporated into adaptive IMRT inverse re-planning for future fractions. In addition, the dose compensation is the only known technique that can correct mistakes occurred in the online IG.…”

Dosimetric evaluation of three adaptive strategies for prostate cancer treatment including pelvic lymph nodes irradiation

“…Several groups have proposed solutions for offline-ART, such as dose-feedback integration, 18,19 patient specific mar-gins 20 and set-up error corrections. 21 Online corrections allow daily adjustments: algorithms to adjust the multileaf collimator apertures based on the prostate position have also been discussed 22,23 and maintain almost the same dose coverage as it was initially planned. 16 However, all those methods involve plan validation or patient-specific additional verification during the treatment course.…”