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Bulk Anatomical Density Maps for Quality Assurance in MRI-Only Prostate Treatment Planning

J Choi1*, D Lee1 , J Dowling2 , L OConnor3 , P Pichler3 , P Greer3 , (1) University of Newcastle, Waratah, NSW, (2) CSIRO, Brisbane, QLD, (3) Calvary Mater Newcastle, Waratah, NSW


(Tuesday, 7/16/2019) 10:00 AM - 10:30 AM

Room: Exhibit Hall | Forum 2

Purpose: Prostate cancer treatment planning can be performed using magnetic resonance imaging (MRI) only with synthetic CT scans. Bulk anatomical density (BAD) maps can be automatically generated based on anatomical contours for quality assurance of synthetic CT density maps. However, the dosimetric difference could be up to 3.3% when compared to conventional CT treatment plans. This study investigates optimization of the BAD map technique to improve dosimetric agreement with CT for MRI-only prostate planning.

Methods: 3D T2-weighted MRI and full density CT images of 54 Patients were used to create BAD maps with different tissue class combinations. Mean Hounsfield units (HU) of fat (below -30 HU) and tissue areas (Tissue1: excluding bone, Tissue2: excluding bone and fat) were calculated from the CT images and assigned to MRI images of 22 patients to create BAD maps; (1) BAD(BW) (density assignment to bone and water), (2) BAD(BT)â‚? (bone and Tissue1), (3) BAD(BTF) (bone, Tissue2, and fat). HU of Bone (288HU) was obtained from literature. Three treatment plans from each patient were developed and isocenter doses of each plan were compared to the corresponding CT doses.

Results: The mean HU values of fat, Tissue1, and Tissue2 areas were -109.5±12.9, -46.3±15.2, and 23.3±9.7 respectively. The isocenter doses of BAD(BW), BAD(BT)� and BAD(BTF) were -1.29±0.88%, 0.42±0.84%, and -0.08±0.70% different from the CT respectively. The interquartile range (between 25th and 75th quartiles) of BAD(BTF) were from -0.46% to 0.45% while BAD(BT)� was -0.03% to 0.93%. Mean gamma analysis (1%/1mm) result of BAD(BTF) of 22 patients was 90.34%.

Conclusion: BAD map with density assignment to bone, fat, and the rest of tissue provides closer dose agreement to the conventional CT plan with smaller dose variations and narrower interquartile range. Thus, this method can provide dosimetric assurance in MRI-only planning.

Funding Support, Disclosures, and Conflict of Interest: This study was funded in part by NHMRC and a University of Newcastle Scholarship.


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