Gy EQD2 Research Articles

SU‐GG‐T‐02: Inverse Optimization for Cervix Cancer Brachytherapy Including Automatic Loading, DVH Optimization and Modification Restriction Adapted From Manual Planning

Purpose: This study tests the difference in dosimetric values and performance between a new Hybrid Inverse Planning Optimization (HIPO) tool for cervix cancer brachytherapy and conventional manual optimization. Method and Materials: The clinically used treatment plans of ten tandem/ring (T/R) and ten cases with additional needles (T/R+N) planed with PLATO (v14.3, Nucletron) were included. Standard loading patterns were manually optimized to reach an optimal coverage with 7 Gy per fraction to the High Risk‐CTV and dose constraints for organs at risk. A second plan was retrospectively created with OncentraGYN (v0.9.14, Nucletron). Anatomy based automatic dwell position loading was used to produce loading patterns similar to standard Fletcher loadings, but adapted to the topography of bladder, rectum, and sigmoid. The optimization algorithm included individual dwell time gradient restrictions and modification restriction for the T/R and needle optimization to preserve the spatial high dose and dwell time distribution as accepted for manual plans. Results: HIPO could achieve a better target coverage (V100) for all T/R and 7 T/R+N patients. The D2cc per fraction for bladder, rectum and sigmoid was on average lower by 0.2 Gy, 0.4 Gy, 0.2 Gy respectively for T/R patients and 0.6 Gy, 0.3 Gy, 0.3 Gy for T/R+N patients (e.g.: a decrease from 4.5 to 4 Gy per fraction means a total dose reduction of 5 Gy EQD2 for a 4 fraction schedule). In 7/10 cases the dwell time in the additional needles was lower as with manual planning. The optimization procedure including handling, evaluation and calculation time (<3s per run) was lower compared to manual treatment planning. Conclusion: HIPO is a feasible and quick method for treatment planning in cervix cancer brachytherapy. Essential is anatomy based loading, anatomy based optimization and a dedicated concept to control spatial dwell time distribution. Conflict of Interest: Research sponsored by Nucletron.

A dose planning study on applicator guided stereotactic IMRT boost in combination with 3D MRI based brachytherapy in locally advanced cervical cancer

Purpose. Locally advanced cervical cancer is usually treated with external beam radiotherapy followed by brachytherapy (BT). However, if response or tumour topography is unfavourable it may be difficult to reach a sufficient BT dose. The purpose of this study was to explore whether an applicator guided stereotactic IMRT boost could be combined with brachytherapy to improve dose volume parameters. Material and methods. Dose plans of 6 patients with HR CTV volumes of 31–100cc at the time of BT were analysed. MRI was performed with a combined intracavitary (IC)-interstitial (IS) ring applicator in situ. A radiotherapy schedule consisting of 45Gy (1.8Gy×25) IMRT followed by boost of 28Gy (7Gy×4fx) was modelled. Four different boost techniques were evaluated: IC-BT, IC/IS-BT, IC-BT+IMRT and IMRT. Dose plans were optimised for maximal tumour dose (D90) and coverage (V85Gy) while respecting DVH constraints in organs at risk: D2cc <75Gy in rectum and sigmoid and <90Gy in bladder (EQD2). In combined BT+IMRT dose plans, the IMRT plan was optimised on top of the BT dose distribution. Volumes irradiated to more than 60 Gy EQD2 (V60Gy) were evaluated. Results. Median dose coverage in IC plans was 74% [66–93%]. By using IC/IS or IC-BT+IMRT boost, the median coverage was improved to 95% [78–99%], and to 96% [69–99%] respectively. For IMRT alone, a median coverage of 98% [90–100%] was achieved, but V60Gy volumes were significantly increased by a median factor of 2.0 [1.4–2.3] as compared to IC/IS. It depended on the individual tumour topography whether IC/IS-BT or IC-BT+IMRT boost was the most favourable technique. Conclusion. It is technically possible to create dose plans that combine image guided BT and IMRT. In this study the dose coverage could be significantly increased by adding IS-BT or IMRT boost to the intracavitary dose. Using IMRT alone for boost cannot be advocated since this results in a significant increase of the volume irradiated to 60Gy.