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Dosimetric Investigations of Using Population-Based Hounsfield Units for Bulk Density Assignment Toward MRI-Based Treatment Planning in Abdomen

I Zawisza

I Zawisza*, S Hsu , Q Peng , W Tome , Montefiore Medical Center, Bronx, NY


TU-D-FS4-5 (Tuesday, August 1, 2017) 11:00 AM - 12:15 PM Room: Four Seasons 4

Purpose: This study investigated the dosimetric accuracy of using population-based Hounsfield Units (HUs) and ICRU-based HUs in electron density assignment for dose calculations in abdominal SBRT for pseudo-CT generation from MRI-alone.

Methods: A retrospective study was performed on 10 patients who received abdominal SBRT. HU values from the CT were collected for several OARs and targets. The OAR HU values were classified into 6 groups to be averaged and assigned for calculations. HU values were also acquired for these tissues based on ICRU-46. Doses were recalculated using clinical plans and fluence with (1) population-based HU assignment, (2) ICRU-based HU assignment, and (3) density correction off. Percentage difference was calculated for data points between each of the three calculations and clinical treatment plan.

Results: The PTV D99.9% for ICRU-based, population-based, and heterogeneity correction off compared to the clinical CT showed -1.8±2.0, -1.0±1.5 and -4.2±4.6 percent difference, respectively. The bowel D0.5cc percent difference were 1.8±5.2, 2.0±5.2, and 2.2±5.1, respectively. In general, no significant difference in dose calculations were found in PTV coverage using either population- or ICRU-based corrections compared to density correction off, but a difference was found of up to 12% for D99.9% when targets were in the upper abdomen. Up to 40% difference was found in dose calculations without density correction in the esophagus and duodenum. While the targets and OARs between the population- and ICRU-based corrections did not demonstrate a significant difference, the PTV D99.9% was 4.3% versus 5.3%, respectively, and the heart minimum (D0.03cc) was 3.9% versus 24.7%, respectively.

Conclusion: Our study found that applying HU values is of particular importance for cases where the treatment target is in the upper abdomen close to the lung. The population- and ICRU-based corrections yielded dosimetrically similar values, such that either can be appropriate for pseudo-CT creation.

Funding Support, Disclosures, and Conflict of Interest: This work was supported by a research grant from Varian Medical Systems, Inc.

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