90Y, 177Lu, and 131I therapy optimisation: Monte Carlo calculation of dose distribution and radiobiological evaluations

F BOTTA, M CREMONESI, A DI DIA, M FERRARI, M VALENTE, C DE CICCO, L. BODEI, M BARTOLOMEI, G PEDROLI , AND G PAGANELLI.

Toronto, Ontario, Canada

Congreso; Society of Nuclear Medicine: 2009 SNM Annual Meeting; 2009

Society of Nuclear Medicine

Aim: To evaluate energy deposition inside and outside lesions treated with radionuclide therapy and absorbed dose (D) distribution in tumors and infiltrated disease or radiosensitive bordering tissues. Secondly, to estimate tumor control probability (TCP, linear-quadratic model) and to assess possible non-negligible biological efficacy of apparently negligible doses. Methods: 5 tissue spheres (radii (R) 6 to 36 mm) uniformly filled with 90Y, 177Lu, 131I were simulated with PENELOPE code (107 histories/run). Dose distribution was tallied in 1 mm3 voxels inside spheres and up to 20mm outside. TCP was evaluated for cases derived from clinical experience (30% washout). Its dependence on lesion dimension, radioisotope and activity was investigated for typical radiobiological parameters. Results: D% profiles of 131I and 177Lu show uniform irradiation inside and a rapid fall with 20% of Dmax at 0.3-0.5 mm from the edge. 90Y offers less homogeneity in the sphere, especially for smaller radii, and a longer tail (20% of Dmax at 0.8-1.2 mm outside). As regards TCP, assuming D=100 Gy to be therapeutic, 177Lu can cope with control within the sphere completely sparing tissue 0.5 mm beyond R. 131I and 90Y show equal TCP profiles if D is prescribed at 1.3, 0.9, 0.7 mm from the sphere edge (for R =6, 15, 36 mm). For prescription at farther distances, 131I offers wider local control (g contribution) than 90Y at the expense of higher activities, thus a tailored balance between TCP and activity may turn to 90Y as optimum choice. Conclusions: Treatments should be optimized by tailoring the choice of radionuclide – if possible – and activity. Dose distribution and TCP profiles as obtained in this work are a quantitative support for case by case physicians’ considerations.