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Reducing Dose to Uninvolved Rectum with Dynamic Shield Intensity Modulated Brachytherapy

M Morcos1,2*, T Vuong3,5, S Abbasinejad Enger1,3,4,5, (1) Medical Physics Unit, McGill University, Montreal, QC, Canada, (2) Johns Hopkins Medicine, Baltimore, MD, USA, (3) Department of Oncology, McGill University, Montreal, QC, Canada, (4) Research Institute of the McGill University Health Centre, Montreal, QC, Canada, (5) Lady Davis Institute for Medical Research, Jewish General Hospital, Montreal, QC, Canada


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Pre-operative neoadjuvant chemoradiation combined with total mesorectal excision is the mainstay of treatment in patients with locally advanced rectal cancer as it results in high rates of local control, sphincter preservation and patient survival. Radiation-induced fibrosis increases the technical difficulty of the operation and the risk of surgical complications and loco-regional recurrence. In this work we assess the uninvolved rectum sparing capacity of a novel MRI-compatible dynamic shield intensity modulated brachytherapy (IMBT) delivery system on the uninvolved rectum.

The developed IMBT applicator is compatible with a previously presented delivery system. The rotating, single-grooved, tungsten IMBT shield has a diameter of 15 mm and is designed to fit inside an endorectal intracavitary mold applicator (Elekta Brachytherapy, Veenendaal, Netherlands) with a custom 15 mm-wide central bore. The simulated source was modeled after the Flexisource (Elekta Brachytherapy, Veenendaal, The Netherlands) with 192Ir as the active core material. Three endorectal HDR-BT brachytherapy cases with high-risk-CTV (HR-CTV) ranging between 5.1-13.7 cm³, were planned and optimized in RapidBrachyMC-TPS, a MC-based research TPS. Shield rotations were limited to 10-degree increments. The contralateral, uninvolved rectal D50 was compared by normalizing the clinical and IMBT plans such that CTV D90 received 6.5 Gy per fraction.

Mean treatment times were 2.1±0.6 min and 15.1±3.3 min for clinical and IMBT, respectively. IMBT reduced the uninvolved rectum D50 by 55.5±3.9% relative to conventional HDR-BT. Similarly, IMBT improved CTV D98 by 12.4±5.5%.

Dynamic shield IMBT applicator for endorectal brachytherapy is capable of improving healthy tissue sparing while improve target coverage.


HDR, Brachytherapy, Monte Carlo


TH- Brachytherapy: HDR Brachytherapy

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