Deformable image registration–based contour propagation yields clinically acceptable plans for MRI-based cervical cancer brachytherapy planning

Purpose: To study the dosimetric impact of deformable image registration–based contour propagation on MRI-based cervical cancer brachytherapy planning. Methods and Materials: High-risk clinical target volume (HRCTV) and organ-at-risk (OAR) contours were delineated on MR images of 10 patients who underwent ring and tandem brachytherapy. A second set of contours were propagated using a commercially available deformable registration algorithm. “Manual-contour” and “propagated-contour” plans were optimized to achieve a maximum dose to the most minimally exposed 90% of the volume (D₉₀) (%) of 6 Gy/fraction, respecting minimum dose to the most exposed 2cc of the volume (D_2cc) OAR constraints of 5.25 Gy and 4.2 Gy/fraction for bladder and rectum/sigmoid (86.5 and 73.4 Gy equivalent dose in 2 Gy fractions [EQD₂] for external beam radiotherapy [EBRT] + brachytherapy, respectively). Plans were compared using geometric and dosimetric (total dose [EQD₂] EBRT + brachytherapy) parameters. Results: The differences between the manual- and propagated-contour plans with respect to the HRCTV D₉₀ and bladder, rectum, and sigmoid D_2cc were not statistically significant (per-fraction basis). For the EBRT + brachytherapy course, the D_2cc delivered to the manually contoured OARs by the propagated-contour plans ranging 98–107%, 95–105%, and 92–108% of the dose delivered by the manual-contour plans (max 90.4, 70.3, and 75.4 Gy for the bladder, rectum, and sigmoid, respectively). The HRCTV dose in the propagated-contour plans was 97–103% of the dose in the manual-contour plans (maximum difference 2.92 Gy). Increased bladder filling resulted in increased bladder dose in manual- and propagated-contour plans. Conclusions: When deformable image registration–propagated contours are used for cervical brachytherapy planning, the HRCTV dose is similar to the dose delivered by manual-contour plans and the doses delivered to the OARs are clinically acceptable, suggesting that our algorithm can replace manual contouring for appropriately selected cases that lack major interfractional anatomical changes.