Monte Carlo simulations of novel optical imaging methods for ¹³¹I radiometabolic therapy dosimetry

A new method for dosimetry and imaging (nodule size detection) in metabolic radiotherapy of thyroid gland with ¹³¹I is investigated using Monte Carlo simulations. Two approaches are compared, based on Cerenkov effect in human tissues and scintillation effect in a scintillator material placed on the detector. The feasibility is evaluated in terms of dosimetry and imaging performances, taking into account the patient variability and the geometrical configuration for the proposed detection device. The advantages and limitations are discussed with respect to the conventional approach. A good linear correlation between the number of photons and the absorbed dose was found for thyroid sick lobe (R²>0.95) and thyroid healthy lobe (R²>0.95) after gamma attenuation and optical absorption correction and for muscle (R²>0.94), while poorer results were found for skin (R²>0.85). No significant differences were found for Cerenkov and scintillation approach. The standard deviation of the nodule diameter was estimated 9.2 mm for Cerenkov approach and 7.4 nm for scintillation approach. The scintillator detector showed a definitely greater efficiency (a factor 10⁴), as expected. The proposed geometry was combined with a Silicon Photomultiplier detector in order to overcome the low light yield in Cerenkov effect and the estimated total number of photons detected at the average of patient parameters was estimated 1.8× 10⁸ for scintillation and 1.3 × 10³ for Cerenkov.