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Hyperthyroidism is one of the most frequent MN treatments; however, quantification of 131I activity in thyroid using Single Photon Computer Tomography (SPECT) and combined with Computer Tomography (SPECT/CT techniques has almost no bibliographic history. This work studied the impact on 131I activity accuracy quantification of scatter corrections using CT information. Three sources of known activity were used for the calibration process (1.06 MBq 2.61 MBq and 4.66 MBq) with a typical uncertainty of 2.4 % and the classic thyroid phantom. The calibration factors and their uncertainty were calculated for each case. To analyze the accuracy of the system, a set of 11 sources (with activities between 1.37 MBq-3.92 MBq) were calculated and the uncertainty was estimated in each case. The relative discrepancies of the calculated activity versus reference were compared (p<0.05). The lowest average discrepancy resulted was 3 % (from 5 to 7 %) corresponding with any correction using CT. The uncertainties were found in the range of 9 to 11 %, being consistent with the discrepancies already found. Conclusions: The corrections of attenuation and dispersion by MC using CT information did not improve the 131I activity quantification accuracy in the geometry of classical thyroid phantom; although it is recommended to expand the range of masses of the thyroid to better represent the wide specific patient clinical situation and confirm these results. [ABSTRACT FROM AUTHOR]

In 2013, the European Association of Nuclear Medicine Dosimetry Committee recommends a "Standard Operational Procedures for Hyperthyroidism Pre-Therapeutic Dosimetry" based on the assessment of the individual 131I uptake and kinetics. To estimate the 3D dose delivery deviations from prescribed dose during patient specific application of this SOP, a computer Matlab application was developed and verified. It was design to execute: radiopharmaceutical curve fitting, cumulated activity calculations, functional thyroid mass estimation, obtain the therapeutic planning activity to warranty the prescribed dose and produce the 3D planning dose map and related dosimetry parameters. 6 patients with 150-400Gy prescribed dose data planning (average 241,67Gy) were analysed using the developed application. The developed system was verify successfully using a test image phantom and 6 known pharmacokinetics data. The tridimensional thyroid volume cumulated activity and dose distributions were heterogeneous. 3D dose distribution showed standard deviations between 18.01- 27.08 % of prescribed dose. The differences between maximum and minimum dose value per voxel/ MBq were 74-129%. According to the result, between 50,2 % and 71,4 % of patient's thyroid will be treat with a dose of DP±20 % of planned dose, the rest will be overdose or sub dose. Conclusions: the 3D treatment planning dose distribution were completely no-homogenous, the significant difference observed should be study in the future more deeply in order to optimized the hyperthyroidism iodine treatment. [ABSTRACT FROM AUTHOR]

Scattering is quite important for image activity quantification. In order to study the scattering factors and the efficacy of 3 multiple window energy scatter correction methods during 131I thyroid studies with a pinhole collimator (5 mm hole) a Monte Carlo simulation (MC) was developed. The GAMOS MC code was used to model the gamma camera and the thyroid source geometry. First, to validate the MC gamma camera pinhole-source model, sensibility in air and water of the simulated and measured thyroid phantom geometries were compared. Next, simulations to investigate scattering and the result of triple energy (TEW), Double energy (DW) and Reduced double (RDW) energy windows correction methods were performed for different thyroid sizes and depth thicknesses. The relative discrepancies to MC real event were evaluated. The accuracy of the GAMOS MC model was verified and validated. The image's scattering contribution was significant, between 27-40 %. The discrepancies between 3 multiple window energy correction method results were significant (between 9-86 %). The Reduce Double Window methods (15 %) provide discrepancies of 9-16 %. Conclusions: For the simulated thyroid geometry with pinhole, the RDW (15 %) was the most effective. [ABSTRACT FROM AUTHOR]