SU-E-T-462: Calculation of Prescribed Dose for Permanent Implant with Cs-131 Using LQ Equation including Resensitization.

PURPOSE To develop a new biological equivalent dose (BED) formulism that includes repair, repopulation and resensitization (redistribution and reoxygenation), and can be used to calculate prescribed dose for permanent implant with Cs-131. METHODS Because of lack of sufficient clinical data, the prescribed dose with new isotopes is usually calculated based on BED compared with other isotopes that have already been used in clinical practice. The current widely used formulism of BED calculation for permanent implant which was given by Dale includes cell repair and repopulation but not resensitization. Resensitization can be included in the extended LQ equation (LQR) proposed by Brenner. Applying Dales formulism to Brenners LQR, we obtained a new BED calculation formula that accounts for resensitization. We used this new formulism to calculate the BED that corresponds to the prescribed dose for cervix (with Au-198) and prostate (with I-125) permanent implant. Then, we obtained the prescribed dose with Cs-131 which has the same BED as Au-198 or I-125. RESULTS The new formulism was used for prostate and cervix cancer permanent implant. The prescribed dose with Cs-131 for cervix was 40 Gy (42 Gy without resensitization) and 66 Gy (69 Gy without resensitization) which were equivalent to 35 Gy and 60 Gy with Au-198, respectively. For prostate implant, the prescribed dose with Cs-131 that corresponded to 144 Gy with I-125 was 135 Gy (124 Gy without resensitization). CONCLUSION Resensitization led to significant dose correction for permanent implant. The calculation of BED and dose was dependent on dose itself and thus should be patient-specific. Since this calculation relied on many biological parameters, and may not be accurate if the parameters are not accurate. Also, the dose prescription should be adjusted based on clinical outcomes.