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. 2011 Dec;38(12):6362-70.
doi: 10.1118/1.3658567.

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

Han Liu  1 , Qiuwen Wu
Affiliations

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

Han Liu et al. Med Phys. 2011 Dec.

Abstract

Purpose: Online image guidance (IG) has been used to effectively correct the setup error and inter-fraction rigid organ motion for prostate cancer. However, planning margins are still necessary to account for uncertainties such as deformation and intra-fraction motion. The purpose of this study is to investigate the effectiveness of an adaptive planning technique incorporating offline dose feedback to manage inter-fraction motion and residuals from online correction.

Methods: Repeated helical CT scans from 28 patients were included in the study. The contours of prostate and organs-at-risk (OARs) were delineated on each CT, and online IG was simulated by matching center-of-mass of prostate between treatment CTs and planning CT. A seven beam intensity modulated radiation therapy (IMRT) plan was designed for each patient on planning CT for a total of 15 fractions. Dose distribution at each fraction was evaluated based on actual contours of the target and OARs from that fraction. Cumulative dose up to each fraction was calculated by tracking each voxel based on a deformable registration algorithm. The cumulative dose was compared with the dose from initial plan. If the deviation exceeded the pre-defined threshold, such as 2% of the D₉₉ to the prostate, an adaptive planning technique called dose compensation was invoked, in which the cumulative dose distribution was fed back to the treatment planning system and the dose deficit was made up through boost radiation in future treatment fractions. The dose compensation was achieved by IMRT inverse planning. Two weekly compensation delivery strategies were simulated: one intended to deliver the boost dose in all future fractions (schedule A) and the other in the following week only (schedule B). The D₉₉ to prostate and generalized equivalent uniform dose (gEUD) to rectal wall and bladder were computed and compared with those without the dose compensation.

Results: If only 2% underdose is allowed at the end of the treatment course, then 11 patients fail. If the same criteria is assessed at the end of each week (every five fractions), then 14 patients fail, with three patients failing the 1st or 2nd week but passing at the end. The average dose deficit from these 14 patients was 4.4%. They improved to 2% after the weekly compensation. Out of these 14 patients who needed dose compensation, ten passed the dose criterion after weekly dose compensation, three patients failed marginally, and one patient still failed the criterion significantly (10% deficit), representing 3.6% of the patient population. A more aggressive compensation frequency (every three fractions) could successfully reduce the dose deficit to the acceptable level for this patient. The average number of required dose compensation re-planning per patient was 0.82 (0.79) per patient for schedule A (B) delivery strategy. The doses to OARs were not significantly different from the online IG only plans without dose compensation.

Conclusions: We have demonstrated the effectiveness of offline dose compensation technique in image-guided radiotherapy for prostate cancer. It can effectively account for residual uncertainties which cannot be corrected through online IG. Dose compensation allows further margin reduction and critical organs sparing.

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Figures

Figure 1
Figure 1
Prostate D99 differences [Eq. 1] between the normalized cumulative dose and the initial plan dose as a function of treatment fraction for each patient. The pre-defined criterion (solid horizontal lines) chosen was −2%. Black star dashed curve: patients passed the criterion at the end of treatment course. Open circle solid curve: patients failed. The “#” in each figure indicates the patient ID.
Figure 2
Figure 2
Prostate D99 differences as a function of treatment fraction after application of weekly dose compensation. The pre-defined criterion (solid horizontal lines) chosen was −2%. Black star dashed curve: patients passed the criterion at the end of treatment. Open circle curve: patients failed. Blank space: patients do not need dose compensation. Only results from schedule A strategy are shown.
Figure 3
Figure 3
Comparison of two weekly compensation delivery strategies for two typical patients. (a) Schedule A had better dose improvement than schedule B; (b) schedule B had better dose improvement than schedule A.
Figure 4
Figure 4
Pass rate at the end of each treatment week with and without dose compensation.
Figure 5
Figure 5
Dose-volume histograms (DVHs) comparison of prostate, bladder, and rectal wall for one patient at the end of treatment. Notice that the dose axis for prostate does not start from 0 in order to show the differences among different curves. “Planned” was the initial plan; “Cumulative” was the cumulative dose distribution delivered to the patient based on the initial plan with online IG only; “Compensation” was the cumulative dose after performing weekly compensation.
Figure 6
Figure 6
Dose indices (gEUD) of critical organs. The gEUD values are expressed as a ratio of plan doses at zero margins.
Figure 7
Figure 7
Comparison of different dose compensation schedules for patient 24.
Figure 8
Figure 8
Daily and cumulative D99 differences as a function of treatment fraction for patient 5.
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