Does help structures play a role in reducing the variation of dwell time in IPSA planning for gynaecological brachytherapy application?

Purpose: To report our experience of dosimetric comparison of IPSA and manual plans, with a focus on the use ofhelp structures (HS) during optimization. Material and methods: 33 patients who underwent MR image-based HDR intracavitary-brachytherapy for cervixcancer based on GYN-ESTRO recommendations were selected for evaluation. Tandem/ovoid (T/O) and Vienna applicatorswere used. HS of diameter of 5 mm were drawn around the tandem/needles/ovoid and ring. Three plans weregenerated: manual optimized plan (MOPT), IPSA without help structures (IPSA_woHS) and IPSA with help structures(IPSA_wHS). Dose-volume parameters and the loading pattern were evaluated. Results: For T/O, the use of HS did not make significant impact in the dose-volume parameters and in the loadingof tandem and ovoids, however steep variation was found in the individual dwell time. In case of Vienna applicator,inclusion of HS in the optimization made a significant impact in loading of needles. The percentage ratio oftotal time of needles to the tandem (TN/T%) was found to be 14 ± 2.5, 53 ± 9, 22 ± 6 for MOPT, IPSA_woHS andIPSA_wHS, respectively, which implies that in IPSA_woHS the dwell time in needles were half of the dwell time inthe tandem, while in MOPT the needles were loaded only in 14%, and in IPSA_wHS it was 22% of the dwell timeof tandem. Inclusion of HS in the optimization has reduced the contribution of dwell time of needle in IPSA_wHS.The individual variation of dwell time was also reduced in IPSA_wHS, however drawing of HS is a time consumingprocedure and may not be practical for a routine practice. Conclusion: The role of HS was evaluated for IPSA for T/O and Vienna-applicator, the use of HS may be beneficialin case of combined intracavitary – interstitial approach.