C Lee*, E Kaza, T Harris, D O'Farrell, R Cormack, I Buzurovic, Department of Radiation Oncology, Dana-Farber/Brigham and Womens Cancer Center, Harvard Medical School, Boston, MA

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  C Lee

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WE-E-BRA-4 (Wednesday, 7/13/2022) 11:15 AM - 12:15 PM [Eastern Time (GMT-4)]

Ballroom A

Purpose: MRI-only treatment plan/planning (MRTP) for interstitial HDR brachytherapy may potentially eliminate CT, necessary for catheter digitization, from the current CT/MRI-based treatment planning process by using an optimized ‘Pointwise Encoding Time Reduction with Radial Acquisition (PETRA)’ MRI. The clinical translation potential of MRTP was evaluated by conducting dosimetric comparison between the MRTP and the clinical treatment plan (CTP) in gynecologic cancer patients.

Methods: An optimized PETRA MRI (TR\TE 3.3\0.7ms) was acquired on a 3T MAGNETOM Vida (Siemens) in eleven gynecological cancer patients with Syed-Neblett template and interstitial catheter placements. Interstitial digitization was conducted on PETRA images on Oncentra Brachy (Elekta Brachytherapy) Treatment Planning System. Catheters penetrating through the obturator lacked the contrast between catheters and obturator. The approach that uses Syed-Neblett template’s fixed shape, including pre-caved catheter paths, is proposed. Dwell coordinates from the clinical CT-based digitization were taken as the surrogate for this step. Excluding outliers, displacements in dwells and tip depth for each catheter were compared against those from the corresponding CTP. DVH and EQD₂ metrics for the CTV (D₉₀, D₅₀ and D₉₈) and the D2cc OARs (rectum, bladder and sigmoid) for were compared between CTP and MRTP with Wilcoxon signed-rank test.

Results: MRTP provided the average dwell and tip depth displacements of 1.72±0.48mm and 0.36±0.96mm, respectively, compared to the conventional CT-based approach. This corresponded to the absolute DVH differences of 1.49±1.93%, 2.08±2.12% and 2.31±4.16% for CTV D₉₀, D₅₀ and D₉₈, respectively, and -0.41±1.11%, 0.47±2.81% and -0.50±1.40% for rectum, bladder, and sigmoid, respectively. The mean relative differences in EQD₂ metrics were found to be 1.83±2.34% for CTV D₉₀ and 1.58±3.99%, 1.06±8.17%, -2.71±6.23% for rectum, bladder and sigmoid D2cc, respectively.

Conclusion: The feasibility of implementing MRTP for interstitial HDR brachytherapy in gynecologic cancer patients was validated with no significant differences in dosimetry between MRTP and the conventional CT/MRI-based approaches.

Keywords

MRI, Treatment Planning, Interstitial Brachytherapy

Taxonomy

IM/TH- MRI in Radiation Therapy: MRI guidance - Brachytherapy

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