ESTRO 2020 Abstract Book

S1089 ESTRO 2020

Six postimplant prostate cancer patients were imaged under a HIPAA compliant and IRB approved protocol. Informed consent was acquired prior to imaging. Patients were imaged on a 3T Siemens Prisma scanner with two 18- channel external array coils. An ERC was not used for imaging. Typical scan parameters were similar to a previous publication. Moderate, high, and very high spatial resolution scans were acquired. 2-3 signal averages (NEX) were acquired at an acceleration factor (R) of 2 for each scan, with the goal of keeping the scan time between 4-6 minutes to minimize motion artifacts. The k-space data of the accelerated acquisitions were reconstructed with PICS algorithms (ESPIRiT and L1-ESPIRiT), and subsequently processed in the Siemens image calculation environment. Images were analyzed for radioactive seed and seed marker visualization, anatomy delineation, signal-to-noise ratio (SNR), and artifacts. Results A single-phase acceleration GRAPPA sampling pattern yielded the highest seed marker conspicuity (Figure 1). Including an L1 wavelet penalty into the k-space reconstruction reduced the noise amplification caused by the g-factor effect (Figure 1, ESPIRiT vs. L1-ESPIRiT). Partial volume artifacts caused some of the seed markers to be unrecognizable when imaging at the lower spatial resolution. Increasing the spatial resolution mitigated these artifacts and improved overall seed marker detection and visualization at the expense of reduced SNR (Figure 2A). Additionally, imaging without the ERC produced inherently lower SNR than imaging with the ERC. However, the reduced SNR due to imaging at higher spatial resolution and without an ERC was sufficiently compensated by the multi-NEX acquisitions that were enabled by PICS (Figure 2B).

Conclusion The ability to acquire high-quality postimplant MRIs without an ERC has several advantages including reduced costs, improved patient tolerance, higher clinical throughput, and higher accessibility to community practices. We previously demonstrated the feasibility of using PICS to accelerate postimplant prostate brachytherapy MRIs retrospectively. This study confirmed the feasibility of this approach in the context of prospective clinical patient imaging. High spatial resolution images were acquired using a total scan time between 4-6 minutes, indicating the practicality of this technique for integration into routine clinical workflow. Multi-NEX acquisitions enabled by PICS provided the highest quality images for postimplant dosimetry and were superior to the clinical standard. OC-1035 Impact of choices in dosimetric calculation method for high dose rate brachytherapy of breast cancer V. TurgeoN 1 , B. Bahoric 1 , M. Morcos 2 , S.A. Enger 3 1 Jewish General Hospital, Radiation Oncology, Montreal, Canada ; 2 McGill University, Medical Physics Unit, Montreal, Canada ; 3 McGill University, Oncology, Montreal, Canada Purpose or Objective To investigate the impact of dosimetric calculation method and segmentation techniques for high dose rate (HDR) breast brachytherapy by evaluating dosimetric indices in the clinical target volume (CTV), planning target volume (PTV) and organs at risk (OARs). Material and Methods Treatment plans for 8 breast cancer patients treated at Jewish General Hospital, Montreal, Canada, were recalculated with a Monte Carlo (MC) based treatment planning software (TPS) called RapidBrachyMCTPS. The median follow-up time after brachytherapy was 5.8 years. The dwell times and positions were taken from the Oncentra TPS and the 192Ir microSelectronV2 (Elekta AB, Stockholm, Sweden) source was used. Patient tissue was segmented: (1) According to TG-43 formalism where everything is considered water with unit density, (2) voxel by voxel assignment of tissue and mass density derived