Redesign of the Intracavitary Ring and Tandem Applicator for Improved Dosimetry in High Dose-Rate Brachytherapy

The commercial ring and tandem brachytherapy applicators (RTA) do not consistently match the specific anatomy of the endometrium and cervix of the patient. To conform variability in such situations we propose a redesign of the RTA used for cervical cancer treatments with the goal of improving patient-specific dosimetry. Unlike in the commercially available RTAs, the novel design of the RTA includes flexibility in positioning the ring with respect to the tandem. In particular, beside the possibility to place the tandem in the center of the ring, we propose an RTA capable of positioning the tandem distally and proximally with respect to the center of the ring (Fig.1). Therefore, the novel RTA has three possible variations in its final configuration. This RTA possesses additional flexibility to adapt different patient anatomies including daily variations in position and size of the organs at risk (OAR). For the evaluation of the benefit of the proposed methodology, 2 patients diagnosed with cervical cancer were treated with high dose-rate (HDR) brachytherapy using standard RTA and RTA in which the ring was placed distally towards the patient’s anterior surface during the same HDR brachytherapy course. A total of 10 brachytherapy treatment plans (TP) were analyzed; 5 per each patient. The HDR brachytherapy TPs were delivered as a boost after the patients had previously received 45 Gy in the initial external beam (EB) course. The prescription dose for the HDR treatments was 27.5 Gy in 5 fractions. The first patient was treated with 2 TPs generated using the standard RTA and 3 plans with the proposed RTA. The second patient was treated with 3 TPs generated using the standard RTA and 2 plans with the proposed RTA. The relevant dosimetric parameters such as D90 for the clinical target volumes (CTV) and D2cm3 for the OARs were compared to evaluate the patient dosimetry. An equivalent dose (EQD2) calculation was performed so that the total dose from 2 courses (EB and HDR) could be evaluated. In addition, we generated another 5 plans on the same computer tomography image sets to evaluate additional specific differences in dosimetry. When the modified RTA was used, the first patient received an average lower daily dose to the D2cm3 of the rectum, bladder and sigmoid of 54, 78 and 18 cGy, respectively, while the same CTV coverage (D90) was maintained. The average increase of the dose to the D2cm3 of the bowel was 29 cGy. This treatment resulted in a decrease of the total EQD2 of the rectum, bladder and sigmoid of 223, 488 and 105 cGy. The increase of the total EQD2 of the bowel was 182 cGy; however, the total dose to the bowel was significantly below the limits. The second patient received an average lower daily dose to the D2cm3 of the rectum and bladder of 52 and 44 cGy, which resulted in a decrease of the total EQD2 of 320 and 455 cGy, respectively. In this case, the increase of the total EQD2 for the CTV was 167 cGy. The total increase of the dose to the D2cm3 of the sigmoid was 156 cGy. The additional TPs generated for these patients revealed a possible decrease of the daily dose to the bladder, bowel, rectum and sigmoid of 33 (SD = 136) cGy, 1 (SD = 48) cGy, 34 (SD = 65) cGy and 67 (SD = 63) cGy, respectively. The novel design of the RTA has a strong potential for the complementary adjustment of the dose distributions. This RTA is capable of succeeding the daily variations of the patient’s anatomy so the daily dose to the critical OAR can be reduced. The increase of the total dose to the OARs (bowel in the first and sigmoid in the second case) can be regulated using the different position of the ring, so a greater decrease of the dose to the more affected OARs can be achieved. Therefore, the total equivalent dose can be modulated while the maximum dose constraints to the OARs are respected. Extended studies are required for additional conclusions.