Current prostate high-dose-rate (HDR) brachytherapy treats the whole prostate gland with one prescribed dose level since prostate cancer is known to be multifocal. 1-5 However, dominant intraprostatic lesions (DILs) are commonly identified in prostate cancer histopathologic studies. A single or few DILs provide the large majority of the tumor burden despite typically representing less than 10% of the total gland volume. Importantly, the DIL is the most common sites of recurrence after radiation therapy. 6-8 Studies show that selectively escalating the dose to DILs has the potential to increase tumor control probability with low toxicity. 9,10 Thus, it is desirable to dose escalate DILs during whole prostate HDR brachytherapy. Routine DIL boost, however, is not performed due to several challenges. First, it requires accurate definition of DIL location and size. Recent advances in multiparametric MRI (mp-MRI) have shown efficacy in identifying DILs, 11-14 of which the reliability, accuracy, and reproducibility are validated by pathology gold standards. 15-18 Thus, it is promising to provide image guidance for treatment planning in external-beam radiation therapy (EBRT) or HDR brachytherapy for DIL boost. 19,20 Secondly, accurate image registration is needed between the mp-MRI images and CT simulation images to provide DIL contours for plan optimization and dose evaluation for CT-based brachytherapy. Registration of the mp-MRI with live transrectal ultrasound (TRUS) images at the time of needle placement would also be valuable in order to optimize the

Purpose: With many plan variables to determine, manual forward planning for Gamma Knife (GK) radiosurgery is very challenging. Inverse planning eases GK planning by determining the variables via solving an optimization problem. However, due to the vast search space, most inverse planning algorithms, including the one provided in Leksell GammaPlan (LGP) treatment planning system, have to predetermine the isocenter locations using some geometric methods and then optimize the shot shapes and durations at these preselected isocenters. This sequential planning scheme does not necessarily lead to optimal isocenter locations and hence globally optimal plans. In this study, we proposed a multiresolution-level (MRL) inverse planning approach, attempting to approach this large-scale GK optimization problem via an iterative method. Methods: In our MRL approach, several rounds of optimizations were performed with a progressively increased resolution used for isocenter candidates. At each round, an optimization problem was solved to optimize the beam-on time for each collimator and sector at each isocenter candidate. The isocenters that obtained nonzero beam-on times at the previous round and their neighbors on a finer resolution were used as new isocenter candidates for the next round of optimization. After plan optimization, shot sequencing was performed to group the optimized sectors to deliverable composite shots. Results: We have tested our MRL approach on six GK cases previously treated in our institution. For the five cases that have a single target, with similar target coverage obtained, our MRL inverse planning approach achieved better plan quality compared to manual forward planning and LGP inverse planning, with higher selectivity (0.73 AE 0.07 vs 0.72 AE 0.08 and 0.62 AE 0.10), lower gradient index (2.71 AE 0.25 vs 2.78 AE 0.24 and 3.00 AE 0.29), lower brainstem D 0.1cc dose (6.10 AE 4.46 Gy vs 8.87 AE 4.82 Gy and 9.17 AE 3.80 Gy), and shorter total beam-on time (62.1 AE 22.9 min vs 83.6 AE 28.2 min and 70.7 AE 16.7 min). For the case that have six targets, compared with manual planning and LGP inverse planning, our MRL approach achieved higher selectivity (0.68 vs 0.57 and 0.47) and lower gradient index (3.77 vs 4.51 and 5.11). The beam-on time of our plan was slightly longer than manual planning and LGP inverse planning (206.4 min vs 204.7 min and 199.3 min). We have also performed sector duration optimization at the isocenters determined by manual planning or the LGP inverse planning, and the resulting plan qualities were found to be inferior to our MRL approach for all the six cases. Conclusions: This preliminary study has demonstrated the efficacy and feasibility of our MRL inverse planning approach for GK radiosurgery.

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To quantify the potential of autoplanning (AP) for standardizing mono-isocenter radiosurgery (SRS) of multiple brain metastases. Materials/Methods: Twenty-three SRS institutions from six regions

PURPOSE: Safe delivery of brachytherapy and establishing a safety culture are critical in highquality brachytherapy. The American Brachytherapy Society (ABS) Quality and Safety Committee surveyed members regarding brachytherapy services offered, safety practices during treatment, quality assurance procedures, and needs to develop safety and training materials. METHODS AND MATERIALS: A 22-item survey was sent to ABS membership in early 2019 to physicians, physicists, therapists, nurses, and administrators. Participation was voluntary. Responses were summarized with descriptive statistics and relative frequency distributions. RESULTS: There were 103 unique responses. Approximately one in three was attending physicians and one in three attending physicists. Most were in practice O10 years. A total of 94% and 50% performed gynecologic and prostate brachytherapy, respectively. Ninety-one percent performed two-identification patient verification before treatment. Eighty-six percent performed a time-out. Ninety-five percent had an incident reporting or learning system, but only 71% regularly reviewed incidents. Half reviewed safety practices within the last year. Twenty percent reported they were somewhat or not satisfied with department safety culture, but 92% of respondents were interested in improving safety culture. Most reported time, communication, and staffing as barriers to improving safety. Most respondents desired safety-oriented webinars, self-assessment modules, learning modules, or checklists endorsed by the ABS to improve safety practice. CONCLUSIONS: Most but not all practices use standards and quality assurance procedures in line with society recommendations. There is a need to heighten safety culture at many departments and to shift resources (e.g., time or staffing) to improve safety practice. There is a desire for society guidance to improve brachytherapy safety practices. This is the first survey to assess safety practice patterns among a national sample of radiation oncologists with expertise in brachytherapy.