Tumour control probability derived from dose distribution in homogeneous and heterogeneous models: Assuming similar pharmacokinetics, 125Sn 177Lu is superior to 90Y 177Lu in peptide receptor radiotherapy

Clinical trials on ¹⁷⁷Lu ⁹⁰Y therapy used empirical activity ratios. Radionuclides (RN) with larger beta maximal range could favourably replace ⁹⁰Y. Our aim is to provide RN dose-deposition kernels and to compare the tumour control probability (TCP) of RN combinations. Dose kernels were derived by integration of the mono-energetic beta-ray dose distributions (computed using Monte Carlo) weighted by their respective beta spectrum. Nine homogeneous spherical tumours (125mm in diameter) and four spherical tumours including a lattice of cold, but alive, spheres (1, 3, 5, 7mm in diameter) were modelled. The TCP for ⁹³Y, ⁹⁰Y and ¹²⁵Sn in combination with ¹⁷⁷Lu in variable proportions (that kept constant the renal cortex biological effective dose) were derived by 3D dose kernel convolution. For a mean tumour-absorbed dose of 180 Gy, 2mm homogeneous tumours and tumours including 3mm diameter cold alive spheres were both well controlled (TCP > 0.9) using a 7525% combination of ¹⁷⁷Lu and ⁹⁰Y activity. However, ¹²⁵Sn ¹⁷⁷Lu achieved a significantly better result by controlling 1mm-homogeneous tumour simultaneously with tumours including 5mm diameter cold alive spheres. Clinical trials using RN combinations should use RN proportions tuned to the patient dosimetry. ¹²⁵Sn production and its coupling to somatostatin analogue appear feasible. Assuming similar pharmacokinetics ¹²⁵Sn is the best RN for combination with ¹⁷⁷Lu in peptide receptor radiotherapy justifying pharmacokinetics studies in rodent of ¹²⁵Sn-labelled somatostatin analogues. © 2012 Institute of Physics and Engineering in Medicine.