For the case considered, the proposed(153)Gd-based I-RSBT system has the potential to lower the urethral dose relative to HDR-BT by 29%-44% if the clinician allows a urethral dose gradient volume of 0-5 mm around the urethra to receive a dose below the prescription. A multisource approach is necessary in order to deliver the proposed (153)Gd-based I-RSBT technique in reasonable treatment times.

RSBT can increase PTV EQD or decrease urethral D relative to HDR-BT at the cost of increased treatment time. Source aging reduces RSBT benefit, but RSBT remains theoretically superior to HDR-BT by >20% after 1 half-life has elapsed.

Purpose: To systematically review scientific literature on the use of intensity-modulated brachytherapy (IMBT), including static and dynamic shielding approaches, to enhance therapeutic ratio. Studies were evaluated for technique, disease site, dosimetry, applicators, dosimetric calculations, and planning algorithms. Comparisons with standard-of-care brachytherapy techniques, alternative IMBT methods, or both were performed for dose-to-target volumes, organs at risk (OARs), and treatment planning or delivery times. Methods and Materials: Inclusion criteria were any peer-reviewed journal articles on IMBT published from January 1, 1980, to January 1, 2019, on PubMed, Google Scholar, Cochrane Library, and EBSCO databases. Two independent investigators reviewed each article for inclusion and exclusion criteria and scope. Data collected on each study included technique, source or shield material, disease site, n of study (n Z number of simulated plans/treated patients), doseto-target/OARs, and planning or delivery times. This review adhered to the Preferred Reporting Items for Systemic reviews and Meta Analyses (PRISMA). Results: Database queries yielded 1734 results, which were reduced to 436 after exclusion criteria and 78 peer-reviewed journal articles after evaluation of scope. Studies per disease site were 31 for cervical; 16 for rectal; 10 for oculocutaneous; 7 for breast; 6 for prostate; and 8 for other, multiple, or no specific disease site. Eighteen studies demonstrated a significant decrease in dose to OARs (5.1%-68.2%), 11 improved treatment planning or delivery times (7.6%-99.7%), and 6 increased target coverage (18.6%-71.6%) relative to standard-of-care or alternative IMBT technique. IMBT consistently decreased dose to OAR compared with the standard of care at the cost of increased planning or delivery times. Innovations in dose calculation or planning algorithms and applicators were capable of ameliorating prolonged treatment intervals.

Purpose: To present and quantify the effectiveness of a method for the efficient production of 169 Yb high-dose-rate brachytherapy sources with 27 Ci activity upon clinical delivery, which have about the same dose rate in water at 1 cm from the source center as 10 Ci 192 Ir sources. Materials: A theoretical framework for 169 Yb source activation and reactivation using thermal neutrons in a research reactor and 168 Yb-Yb 2 O 3 precursor is derived and benchmarked against published data. The model is dependent primarily on precursor 168 Yb enrichment percentage, active source volume of the active element, and average thermal neutron flux within the active source. Results: Efficiency gains in 169 Yb source production are achievable through reactivation, and the gains increase with active source volume. For an average thermal neutron flux within the active source of 1 9 10 14 n cm À2 s À1 , increasing the active source volume from 1 to 3 mm 3 decreased reactor-days needed to generate one clinic-year of 169 Yb from 256 days yr À1 to 59 days yr À1 , and 82%-enriched precursor dropped from 80 mg yr À1 to 21 mg yr À1 . A resource reduction of 74%-77% is predicted for an active source volume increase from 1 to 3 mm 3 . Conclusions: Dramatic cost savings are achievable in 169 Yb source production costs through reactivation if active sources larger than 1 mm 3 are used.

Purpose: To assess the capability of an intracavitary 169 Yb-based helical multishield rotating shield brachytherapy (RSBT) delivery system to treat cervical cancer. The proposed RSBT delivery system contains a pair of 1.25 mm thick platinum partial shields with 45°and 180°emission angles, which travel in a helical pattern within the applicator. Methods: A helically threaded tandem applicator with a 45°tandem curvature containing a helically threaded catheter was designed. A 0.6 mm diameter 169 Yb source with a length of 10.5 mm was simulated. A 37-patient treatment planning study, based on Monte Carlo dose calculations using MCNP5, was conducted with high-risk clinical target volumes (HR-CTVs) of 41.2-192.8 cm 3 (average AE standard deviation of 79.9 AE 35.8 cm 3 ). All patients were assumed to receive 25 fractions of 1.8 Gy of external beam radiation therapy (EBRT) before receiving 5 fractions of high-dose-rate brachytherapy (HDR-BT). For each patient, 192 Ir-based intracavitary (IC) HDR- BT, 192 Ir-based intracavitary/interstitial (IC/IS) HDR-BT using a hybrid applicator with eight IS needles, and 169 Yb-based RSBT plans were generated. Results: For the IC, IC/IS, and RSBT treatment plans, 38%, 84%, and 86% of the plans, respectively, met the planning goal of an HR-CTV D 90 (minimum dose to hottest 90%) of 85 Gy EQD2 (a/ b = 10 Gy). Median (25th percentile, 75th percentile) treatment times for IC, IC/IS, and RSBT were 11.71 (6.62, 15.40) min, 68.00 (45.02, 80.02) min, and 25.30 (13.87, 35.39) min, respectively. 192 Ir activities ranging from 159.1-370 GBq (4.3-10 Ci) and 169 Yb activities ranging from 429.2-999 GBq (11.6-27 Ci) were used, which correspond to the same clinical ranges of dose rates at 1 cm off-source-axis in water. Extra needle insertion and planning time beyond that needed for intracavitary-only approaches was accounted for in the IC/IS treatment time calculations. Conclusion: 169 Yb-based RSBT for cervical cancer met the HR-CTV D 90 goal of 85 Gy in a greater percentage of the patients considered than IC/IS (86% vs 84%, respectively) and can reduce overall treatment time relative to IC/IS. 169 Yb-based RSBT could be used to replace IC/IS in instances where IC/IS treatment is not available, especially in instances when HR-CTV volumes are ≥30 cm 3 .

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Purpose: To demonstrate by measurement that using partially shielded Gd‐153 sources for rotating‐shield brachytherapy (RSBT) is feasible. RSBT is a potentially superior alternative to conventional high‐dose‐rate brachytherapy and provides the opportunity to dramatically improve tumor dose conformity for the treatment of, for example, prostate cancer. Methods: A custom‐built, stainless steel encapsulated 150 mCi Gd‐153 capsule with an outer length of 12.8 mm, outer diameter of 2.10 mm, active length of 9.98 mm, and active diameter of 1.53 mm was used. A partially shielded catheter was constructed with a 500 μm platinum shield and a 500 μm aluminum emission window, both with 180° azimuthal coverage. An acrylic phantom was constructed to measure the dose distributions from the shielded catheter in the transverse plane using Gafchromic EBT3 films. Film calibration curves were generated from 50, 70, and 100 kVp x‐ray beams with NIST‐traceable air kerma values to account for energy variation. Results: The transmission ratios of platinum to aluminum shielding at 1 cm off‐axis are 7.5% and 7.6% for Monte Carlo (MCNP5) predicted and experimental results, respectively. The predicted/measured relative dose rates at 1 cm, 2 cm and 3 cm off‐axis through the Al window were 100%/92.9%, 28.6%/27.0% and 13.8%/12.7%, respectively. Through the Pt shield, the predicted/measured relative dose rates were 7.5%/7.1%, 3.8%/3.0% and 2.4%/1.7%, respectively. Conclusion: Using partially‐shielded Gd‐153 sources for RSBT is a promising approach to improving brachytherapy dose distributions. The next step in making Gd‐153 based RSBT a reality is developing a Gd‐153 source that is small enough such that the source, shield, and catheter all fit within a 16 gauge needle, which has a 1.65 mm diameter. University of Iowa Research Foundation

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