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 further be reduced by 1-2 mm compared to online IG only strategy. The purpose of this study was to investigate the dosimetric benefits of such a hybrid strategy on the target and organs at risk. A total of 420 repeated helical computed tomography 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 shift of prostate only and prostate plus SV, were performed for IRP. The intensity-modulated radiotherapy was used in the simulation. Criteria on both cumulative 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 0-1 mm. The optimal number of fractions necessary for the ITV construction is 2 for LRPs and 3-4 for IRPs in a hypofractionation protocol. A new cumulative index of target volume 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.

Figure 1

Simulation of online image guidance on one axial helical CT slice for an intermediate-risk patient. (a) before online correction. Yellow contour is the CTV₀ in reference planning CT, Red contour is the treatment CTV_i. (b) after online correction based on COM. Yellow contour is the reference CTV₀, Blue is the treatment CTV_i after Type A registration, and Red is the treatment CTV after Type B registration. Red and Blue contours are slightly different from the CTV_i in (a) because the registration were in 3D.

Figure 2

Construction of ITV5 on an axial CT slice for intermediate-risk patient. Dark blue is the ITV₅, which is the composite of CTV from first five fractions. Red, purple, skyblue, green are subsequent CTV_i in treatment CTs after online correction.

Figure 3

Plan isodose distribution with zero ITV-to-PTV margin for one low-risk patient. Shaded areas are the 100% of prescription dose. Dark blue contour is the ITV₅, Red contours are subsequent CTV_i in treatment CTs after online correction.

Figure 4

Pass rate (in percent) as a function of required margin. (a) low-risk patients, (b) intermediate-risk patients with Type A registration, and (c) intermediate-risk patients with Type B registration.

Figure 5

Dose indices (gEUD) of critical organs for online IG and hybrid strategies. The gEUD values are expressed as a ratio of cumulative dose to plan dose at optimized margin for online IG. (a) bladder; (b) rectal wall.

Figure 6

The percentage of fractions that failed the criteria of 2% PTV dose reduction of CTV₀ and ITV₅ as a function of the margin for low-risk and intermediate-risk patients.

Figure 7

The average volumes and the margin added volumes of CTV₀, ITV_n (n=2,3,4,5) versus number of fractions to construct the ITV_n. (a) Low-risk patients; (b) Intermediate-risk patients with Type A registration; (c) Intermediate-risk patients with Type B registration. The symbols on the left are for online IG and the rest for hybrid.

Figure 8

Cumulative index of target volume (CITV) for online IG and hybrid strategies. The symbols on the left are for online IG and the rest for hybrid.