Dosimetric and radiobiological evaluation of manually and inversely optimized brachytherapy treatment planning

Background: The purpose of the study was to compare five inverse treatment planning techniques with the conventional manually optimized planning approach for brachytherapy of cervical cancer with respect to dosimetric and radiobiological parameters. Materials and Methods: Eighteen cervical cancer patients treated with external radiotherapy (2Gy x 25) and MRI guided HDR brachytherapy (5Gy x 4) were included in this study. For each patient six plans were created: one manually optimized and five inversely planned for each of the four HDR brachytherapy fractions. Three inverse planning simulated annealing (IPSA 1-3) approaches, using different dose objectives not only for the target but also for the organs at risk, were investigated together with plans created with equal dwell times and prescription to target points, respectively. The dose was prescribed to the surface of the target volume. The resulting dose volume histograms were analyzed and compared from the dosimetric and the radiobiological point of view. The dosimetric parameters that were quantified and compared were D90 and D100 for the target and the dose to the most exposed 2cm3 volume, D2ccm for rectum, bladder and the sigmoid colon. The conformal index, COIN, were also assessed for the target volume. The probability of complication free cure, P+ was calculated Results: IPSA led to superior target coverage and similar COIN values compared to the manual optimization. The manual optimization led to the better results with respect to D2ccm for the organs at risk. Nevertheless, the radiobiological evaluation of the plan rendered comparable probabilities of complication free cure within the uncertainties originating from the radiobiological parameters. Overall, the best results were obtained with manual optimization and IPSA3 with volumetric constrains. Conclusions: Inverse planning simulated annealing with proper dose objectives is a good choice when it is necessary to minimize staff workload and treatment planning time. However, the dose to organ at risk must be inspected thoroughly and manually adjusted if exceeding the recommended organ dose limits. Nevertheless, dosimetric and radiobiological criteria are similarly fulfilled with manual or inverse optimization approaches indicating the potential of inverse planning simulated annealing for brachytherapy.