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Clinical Evaluation of Dosimetric Impact of Applicator Displacements On 3D Gynecological High Dose-Rate Brachytherapy Treatments

S Jayarathna1*, A Hoover1, J Clarke2,R Badkul1, (1) The University of Kansas Medical Center, Kansas City, KS, (2) Elekta Inc, Atlanta, GA

Presentations

PO-GePV-T-28 (Sunday, 7/10/2022)   [Eastern Time (GMT-4)]

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Purpose: To investigate the dosimetric impact on target dose coverage and organ at risk (OAR) doses due to tandem and ovoid applicator displacement between planning and the treatment delivery for 3-Dimensional high dose rate (HDR) cervical cancer brachytherapy.

Methods: Brachytherapy plans from five consecutively treated patients at a tertiary care academic medical center were evaluated. Twelve different applicator positions, representing potential displacement of the tandem and ovoid between placement and treatment delivery, were simulated on patient treatment planning CT or MRI scans. During virtual shifts, applicator reconstructions were moved to each of two cranial, four caudal, or three lateral (left/right) positions representing displacements between 2 and 7 mm along each primary axis. Dose changes at points A&B; D100, D90, and V90 of HR-CTV; D0.1cc, D1cc, D2cc for OAR’s; and ICRU rectum and bladder doses were analyzed for each virtual applicator shift.

Results: For the HR-CTV, greater than 5 mm displacement resulted in 5±2% change for D100/D90 along the caudal direction. Lateral shifts greater than 5 mm resulted in dosimetric differences of 18±2%. For 2mm displacements deviations were within ~2%. For rectum, 2mm shifts in the caudal direction resulted in D0.1cc/D1cc/D2cc variations from 6-46%, while lateral shifts resulted in less than 5% discrepancy up to7mm displacement. For bladder, D0.1cc/D1cc/D2cc, the changes were less than 4% up to 7mm displacement in cranial-caudal direction, but up to 10% with 7mm lateral displacement.

Conclusion: Brachytherapy applicator displacements along the cranial-caudal direction may result in clinically significant dosimetric changes for both the HR-CTV and OARs. Caution must be taken to avoid any applicator displacements of > 2mm to ensure proper delivery of HDR brachytherapy treatments. We suggest performing image verification immediately prior to treatment delivery.

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