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Actual Dose of Breast TARGIT A Clinical Trial with Intrabeam IORT for a Realistic Spherical Target Volume

H Zhang*, Northwestern Memorial Hospital, Chicago, IL


(Sunday, 7/12/2020)   [Eastern Time (GMT-4)]

Room: AAPM ePoster Library

Purpose: breast TARGIT A clinical trial, A 20 Gy dose of single fraction IORT treatment was compared with 50 Gy/25 fractions external beam radiation therapy (EBRT), because nominally the both regimens have the same biological equivalent dose (BED) of 60 Gy at the a/ß=10 Gy. However, the equivalent uniform dose (EUD) of Intrabeam (IB) IORT is significantly lower than 20 Gy if cancer cell infiltration depth is considered. This study postulated the required IB-IORT doses for different cancer cell infiltration depths within the tumor bed.
Methods: on commissioning data, three dimensional (3D) radiation dose distribution of IB-IORT using a 4-cm diameter spherical applicator at the energy of 50 keV was calculated. The modified linear quadratic model (M-LQ) was used to estimate the radiation killing of the cancer cells. Cancer cell distributions in the postsurgical tissue field were assumed to be the half-Gaussian with the standard deviations of 0.5, 1 and 2 mm respectively, which also correspond to the cancer cell infiltration depth of 1.5, 3 and 6 mm respectively. EUDs were calculated for breast cancer cell (a/ß=10 Gy). Doses of 20, 25 and 30 Gy were separately prescribed at the applicator surface.
Results: 1.5, 3 and 6 mm of the cancer cell infiltration depths, EUDs respectively were 18.03, 16.49 and 13.56 Gy at 20 Gy prescription dose, 22.24, 20.25 and 16.46 Gy at 25 Gy, 26.34, 23.90 and 19.04 Gy at 30 Gy.
Conclusion: EUD decreases with the cancer cell infiltration depth. Our study indicated that the prescription dose needs to be increased to 25 Gy for a 3-mm, or to 30 Gy for a 6-mm residual cancer cell infiltration. If the cancer cell depth is unknown, 30 Gy dose was recommended for taking care of residual cancer cells populated at as far as 6 mm depth.


IORT, Breast, Linear Quadratic Model


TH- External Beam- Photons: intraoperative

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