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Calculating Size Specific Dose Estimates (SSDE): The Effect of Using Water Equivalent Diameter (WED) Vs. Effective Diameter (ED) On Organ Dose Estimates When Applying the Conversion Coefficients of TG 204

M Khatonabadi

M Khatonabadi1*, D Oria2, K Mok2, C Cagnon1, J DeMarco1, M McNitt-Gray1, (1) UCLA School of Medicine, Los Angeles, CA, (2) UCLA, Los Angeles, CA

MO-D-134-4 Monday 2:00PM - 3:50PM Room: 134

Purpose: AAPM Task Group (TG) 204 described the Size-Specific Dose Estimate (SSDE) using effective diameter (ED) as the size parameter in the conversion tables. The purpose of this investigation was to assess the effect on SSDE when a different size metric, water equivalent diameter (WED), was used with the conversion tables; and to compare these values to results from detailed Monte Carlo (MC) simulations.

Methods: For a set of 101 thoracic and 82 abdomen/pelvis scans, including adult and pediatric patients, both ED and WED were calculated at the middle of the scan length. Each size metric was used as the input value to the SSDE conversion tables in TG204 to obtain two sets of SSDE values (SSDEED and SSDEWED). Additionally, voxelized patient models were generated from original scans and used in MC simulations to estimate dose to liver, spleen, and kidneys in the abdomen/pelvis, as well as lung and glandular breast tissue in the thoracic examinations under fixed tube current mode. SSDEED, SSDEWED were compared to organ doses by calculating percent differences and Root Mean Square errors.

Results: For the abdomen, SSDEED and SSDEWED resulted in different RMSE from organ dose (for kidneys 8.8% and 6.4%, respectively) while SSDEWED demonstrated higher correlation with kidney dose than SSDEED (0.89 and 0.78, respectively). Similarly, for thorax, SSDEED and SSDEWED showed different RMSE (for lung 17.0% and 13.1%, respectively), with higher correlation for SSDEWED than SSDEED (0.92 and 0.78, respectively).

Conclusion: Generated conversion factors based on ED appear to be applicable to WED and as originally assumed using WED not only improves dose estimates in the thorax but it also improves estimates in the abdomen. It is worth mentioning that these conclusions only apply to fixed tube current examinations while for tube current modulated (TCM) exams new set of conversion factors are recommended.

Funding Support, Disclosures, and Conflict of Interest: Dr. Michael McNitt-Gray Institutional research agreement, Siemens AG Recipient research support Siemens AG Consultant, Flaherty Sensabaugh Bonasso PLLC Consultant, Fulbright and Jaworski, LLC Maryam Khatonabadi: ipient research support Siemens AG

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