C.E. Wooten

Implementation and early clinical results utilizing Cs-131 permanent interstitial implants for gynecologic malignancies.

OBJECTIVE Permanent interstitial brachytherapy is an ideal yet underutilized treatment modality for accessible, small volume gynecological malignancies. We present early clinical results utilizing a new permanent isotope, Cs-131. METHODS A retrospective review was performed evaluating patients treated with Cs-131 permanent interstitial radiation at our institution from July 2011 through June 2013. Doses were most commonly prescribed and calculated to a depth of 5mm using Paterson-Parker planar implant rules for Au-198. This activity was converted to air-kerma strength (U). A conversion factor of 1.1 was applied based on RBE calculations, clinical observation and experience. RESULTS 14 patients were identified among whom 17 Cs-131 implants were performed. Seven patients were implanted as sole therapy, and a median dose of 50 Gy was delivered. Ten implants were performed as boost within a more extensive radiation treatment plan. In these patients, a median implant dose of 27.5 Gy was used and the median total dose delivered in combination was 78.25 Gy. After a median follow up of 12 months, the actuarial local control rate was 84.4%. A very low level of grade 1-3 reactions was observed with no fistula formations or other severe side effects. CONCLUSIONS Permanent interstitial brachytherapy with Cs-131 was well tolerated with favorable early results compared to other series. Cs-131 has multiple favorable properties, including minimal radiation exposure to treating staff, and should be considered as a therapeutic option in appropriately selected patients. A methodology for dose prescription, calculation of radioactivity required and distribution of the isotope is also presented.

SU-E-T-114: Dose Modification for Cs-131 Permanent Implants Using Resensitization-Corrected Normal Tissue BED

PURPOSE To apply resensitization (redistribution and reoxygenation) correction to normal tissue BED calculation and have it verified with clinical outcomes. METHOD AND MATERIALS The BED formalism without resensensitization for permanent implants was BED = D*RE - BF, where D is the prescribed dose, RE = 1 + (β/α)R0/(μr+λ), BF = K*Teff, K = ln(2)/(αTp), and Teff = Taveln(αDTp/T1/2). α and β are LQ parameters, R0 the initial dose rate, μr the repair constant, λ the source decay constant, and Tp the repopulation time. Resensitization can be included in the extended LQ equation (LQR) S = exp[-αD - βG(Tr)D2 + 1/2σ2 G(Ts)D2 + Teff/Tp], where G(Tr) and G(Ts) describe repair and resensitization, and 1/2σ2 represents cell-to-cell diversity. Combining Dales formalism with LQR led to RE = 1+(β/α)R0/(μr+λ)-(1/2σ2 /α)R0/(μs+λ), where μs is the resensitization constant. We used this formula to calculate the BED for normal tissue based on the prescribed dose for Au-198 GYN permanent implants from which we have gained extensive clinical experience. Then, we calculated the dose with Cs-131 which has the equal BED as Au-198. RESULTS The prescribed doses for Au-198 ranged from 10 to 120 Gy. The converted doses for Cs-131 implants ranged from 9 to 161 Gy (without resensitization correction) and 8.9 to 156 Gy (with resensitization correction), which resulted in the average value of dose conversion factor, Fn (no resensitization correction) = 1.14, and 1.10 for Fr (with resensitization correction) which agreed with the results from the calculation for tumor. The doses derived with 1.10 reduced the complications such as brisk moist desquamation in actual clinical cases. CONCLUSION Resensitization correction in BED for normal tissues led to significant reduction in prescription dose and thus in toxicity. The results further show that resensitization correction is needed for permanent implant dose calculation.