An analytic model to calculate voxel s-values for ¹⁷⁷Lu

Abstract

Objective: 177Lu is one of the most employed isotopes in targeted radionuclide therapies and theranostics, and 3D internal dosimetry for such procedures has great importance. Voxel S-Values (VSVs) approach is widely used for this purpose, but VSVs are available for a limited number of voxel dimensions. The aim of this work is to develop an analytic model for the calculation of 177Lu-VSVs in any cubic voxelized geometry of practical interest. Approach: Monte Carlo (MC) simulations were implemented with the toolkit GAMOS to evaluate VSVs in voxelized geometries of soft tissue from a source of 177Lu homogeneously distributed in the central voxel. Nine geometric setups, containing 15×15×15 cubic voxels of sides l ranging from 2 mm to 6 mm, in steps of 0.5 mm, were considered. For each l, the VSVs computed as a function of the “normalized radius”, Rn = R/l (with R = distance from the center of the source voxel), were fitted with a parametric function. The dependencies of the parameters as a function of l were then fitted with appropriate functions, in order to implement the model for deducing 177Lu-VSVs for any l within the aforementioned range. Main results: The MC-derived VSVs were satisfactorily compared with literature data for validation, and the VSVs computed with the analytic model agree with the MC ones within 2% for Rn ≤ 2 and within 6% for Rn > 2. Significance: The proposed model enables the easy and fast calculation, with a simple spreadsheet, of 177Lu-VSVs in any cubic voxelized geometry of practical interest, avoiding the necessity of implementing ad-hoc MC simulations to estimate VSVs for specific voxel dimensions not available in literature data.

Relevance of artefacts in 99mTc-MAA SPECT scans on pre-therapy patient-specific 90Y TARE internal dosimetry: a GATE Monte Carlo study

“…Such approach was similarly employed in masking A and B of the present work, in order to set to zero the probability of decays in air. In Pistone et al (2020), relative percent differences of about 50% between lung absorbed doses calculated using native-PET and masked-PET were found. These quantitative results cannot be directly compared with the ones of the present work, considered the difference in radionuclide distribution between a selective internal radiotherapy and a systemic radiopharmaceutical administration.…”

“…The mentioned artefacts may lead to an inaccurate estimation of the absorbed dose, especially in low-density air-rich regions, e.g. the lungs, as highlighted in a recent work dealing with the dosimetry of 18 F-choline PET (Pistone et al 2020).…”

Technical note: Impact of dose voxel kernel (DVK) values on dosimetry estimates in 177Lu and 90Y radiopharmaceutical therapy (RPT) applications

“…For 177 Lu, nine DVKs with voxel sizes of 3 × 3 × 3mm 3 and matrix sizes ranging from 11 × 11 × 11 to 275 × 275 × 275 were compared. Among them, five were generated by direct MC simulation (Lanconelli, 6 Pistone, 7 Salvat, Tranel and UMICH DPM), two were derived from DPKs (Graves 8 and Vergara 9 ), and two were computed based on analytical methods (Amato 10 and Pistone_AM 7 ). For 90 Y, nine DVKs with voxel sizes of 3 × 3 × 3 mm 3 and matrix sizes from 11 × 11 × 11 to 17 × 17 × 17 were selected.…”

Development of a GPU-accelerated Monte Carlo dose calculation module for nuclear medicine, ARCHER-NM: demonstration for a PET/CT imaging procedure

“…(3) The phase space module generates the decay particles. The PET images indicate the intensity distribution of radionuclides expressed in Bq/ml (Pistone et al 2020). The probability density distribution of the decay particles in MC simulations is constructed through a linear conversion of the PET value.…”

“…Neira et al (2020) calculated individualized dose distributions in patients undergoing 18 F-FDG PET/CT based on MC software GATE and PET/CT images. Piston et al (Pistone et al 2020) compared the dose-rate results between GATE and GOMOS MC simulation based on the same 18 F-Choline PET/CT patient data. However, MC simulations are notoriously slow due to the massive amount of calculations required to reach an acceptable accuracy, which prevents their clinical use.…”