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Quantification of the Increase in Risk of Pneumonitis and Ischemic Heart Disease Among Left Breast Cancer Patients Due to Megavoltage Imaging Beam in TomoTherapy

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H Choi

Herve HF Choi1*, Yick Wing Ho1 , Maria YY Law1,2 , Po Man Wu1 , Kin Yin Cheung1 , Siu Ki Yu1 , (1) Hong Kong Sanatorium & Hospital, Hong Kong, (2) School of Medical and Health Sciences, Tung Wah College, Hong Kong


SU-I-GPD-T-96 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Megavoltage computerized tomography (MVCT) is used for verification of patient setup prior to treatment in TomoTherapy; however, the MVCT dose is not taken into account in planning. This study aims to quantify the increase in risk of radiation pneumonitis (RP) and ischemic heart disease (IH) among left breast cases due to the MVCT dose.

Methods: A Rando anthromorphic phantom preloaded with thermoluminescent dosimeter (TLD) chips was scanned with MVCT. The measured MVCT doses to the lungs and heart were then added to the doses calculated in twenty-five actual patient plans. Doses were measured twice for each of the three scan slice thicknesses – fine, normal, and coarse. The risks of RP and IH were calculated and compared with risks estimated without the MVCT dose. In this study, only left-sided breast plans were considered, with cases with supraclavicular fossa treatment and post-mastectomy plans excluded. The risk increase was then compared with the uncertainties propagated from the parameters in the sigmoidal mean-lung-dose model for RP and the relative seriality model for IH given in the QUANTEC reports.

Results: The MVCT dose is approximately inversely proportional to the scanning thickness. The nominal risks of RP before the addition of MVCT dose range from 0.041 to 0.064 while the uncertainties range from 0.023 to 0.038. The maximum increase in risk of RP among the cases studied is 0.003. The nominal risks of IH in the treatment plans range from 6.7e-6 to 1.9e-2 while the uncertainties range from 1.8e-4 to 3.1e-2. All increases of IH risk due to MVCT in fine mode are between 2.2e-6 and 1.6e-3, less than the modeled risk uncertainties in the respective plans.

Conclusion: The increase in risk of RP and IH due to the MVCT dose is less than the uncertainties in the biological models in left breast cases.

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