Dose accumulation during vaginal cuff brachytherapy based on rigid/deformable registration vs. single plan addition
Introduction
Endometrial carcinoma (EC) is the third most common cancer diagnosed in women and is the most common female genital tract malignancy worldwide (1). Early-stage EC treatment involves surgery (total abdominal hysterectomy and bilateral salpingo-oophorectomy with or without pelvic lymph node dissection) followed by adjuvant radiotherapy in selected cases. Prospective randomized studies have shown that radiotherapy (RT) reduces the risk of pelvic relapse but does not improve the overall survival in patients with early EC [2], [3]. The PORTEC-2 trial demonstrated that patients with intermediate-risk EC can be safely treated with postoperative brachytherapy in the absence of whole pelvis external beam radiotherapy, decreasing toxicity (4). The vaginal cuff remains the most common site of relapse. Subsequently, there has been a shift away from external beam irradiation and an increase in the use of vaginal cuff brachytherapy (VBT) in these patients.
With the adoption of three-dimensional (3D) image-based planning, there has been concern regarding the movement of pelvic organs, as well as interfraction variation between implants. The cervical cancer literature suggests the benefit of individualized plans for each brachytherapy fraction [5], [6], [7]. Evidence for image-based planning with each VBT fraction is not as clear, with some reports suggesting an absence of benefit (8). Although their study aimed to evaluate the usefulness of repeated organs at risk (OARs) dose–volume histogram (DVH) calculations in multi-fractionated treatments, ours involved analyzing the different dose accumulation methods.
Deformable registration and dose accumulation is an important field of research in radiotherapy [9], [10]. Dose summation with fractionated VBT should be considered to ascertain the dose distribution around the implant and OARs as accurately as possible. Variations in organ position, shape, and volume can cause discrepancies between planned and delivered doses. These anatomic changes could be heightened by brachytherapy applicators. Applying standard procedures to calculate the total does not factor in anatomic deformations, given that only the first fraction doses are taken into account, or at best, treatment fractions are rigidly registered. Deformable registration may overcome these limitations, improving both anatomic alignment and dose accumulation.
We analyzed the variation in dose to the rectum and bladder according to three accumulation strategies: single plan (SP), rigid registration, and b-spline deformable registration (Fig. 1). As there is no current consensus on the optimal number of fractions to be delivered, each type of accumulation strategy was calculated, mimicking a 3 or 5 fractions treatment.
Section snippets
Patient data
Brachytherapy CT scans of 19 consecutive patients, who had received postoperative 3D HDR-VBT, with single-channel vaginal cylinders for EC, and who had at least five brachytherapy CT scans available for review, were retrospectively studied. Eighteen patients had exclusive postoperative VBT for early stage EC (Ia, one patient; Ib, 13 patients; Ic, four patients) and one patient had salvage VBT. The median age was 68 years. Brachytherapy was performed with the largest diameter cylinder that
Rectum parameters
No statistically significant differences were observed when RDWS and DDWS were compared with the SP approach. Results were similar in both the 3 and 5 dose fractions. Table 1 shows the raw dose values expressed as a median and an interquartile range based on each dose summation scenario. Variation in the DDWS values was lower than the RDWS values compared with the SP doses. All the same, the median registered metrics differed from the SP summation procedure between a −5.54% and 10.72%. On
Discussion
The results shown for dose deformation and dose accumulation do not support a customized dosimetric plan for each fraction. Under optimal circumstances, VBT is delivered using a customized plan with each fraction, although the American Brachytherapy Society stated that this may not be necessary assuming a fixed geometry of the implant for every insertion based on the first fraction (17). The results presented here demonstrate small dosimetric differences between the simple SP approach and the
Conclusions
Despite the current uncertainties in the field of dose-deformation and dose-accumulation, we believe that our results support the use of a SP approach rather than a customized CT-based plan for each VBT fraction, given the slight differences observed between the SP approach and the two dose summation methods used. Our data are consistent with previously published results produced with different experimental methodologies.
We are of the opinion that the SP approach should become the standard
Acknowledgment
We would like to express our gratitude to the reviewers and editor for the extremely helpful comments and guidance in the revision.
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Conflict of interest: None.
Part of the study was presented in Chicago, USA (December 1–6, 2013) at the 99th Scientific Assembly and Annual Meeting of the Radiological Society of North America (RSNA).