Abstract
Purpose The purpose of this study was to calibrate different gamma cameras in the framework of a multicentre research for lesion dosimetry in 223Ra therapy of bone metastases. Equipment of several manufacturers and different models were available. Moreover, differences in gamma camera calibration procedures into activity quantification were explored. Methods Eleven gamma cameras of different crystal thickness (3/8- and 5/8-inches) were used, acquiring planar static images with double-peak windows (at 82 and 154 keV, 20% wide) and a MEGP collimator in all cases. Sensitivity was measured in air, varying the activity, the source-detector distance and the diameter of the source. Transmission curves were measured, calculating the parameters used for attenuation/scatter correction with the pseudo-extrapolation number method, and assessing their variations with the source size. Results The values of the calibration factors (considering the geometric mean of both detector sensitivities) ranged from 41 to 114 cps/MBq. For the smallest source (diameter of 3.5 cm), the calibration factor decrease ranged from 30% to 4%, highlighting the importance of partial volume effect according to the equipment involved. Partial volume effects were negligible for an object area of 960 mm2. The sensitivity was nearly constant varying activity and distance, and its variation with the source-detector distance, with respect to the 15 cm-value, reached 10% (in absolute value) in the range of 5–30 cm. Fixing the distance between the two heads, the calibration factor variation with the distance from the midline was within 3.6%. Examining the results obtained with six gamma cameras, appreciable variation of the transmission curves with the source size and experimental setup were observed, leading to activity quantification errors up to 20%. Conclusion The calibration protocol that should be regularly implemented for Ra22 acquisitions requires sensitivity and transmission curve measurements varying the source size, and performing a careful procedure of standardisation. The results of this study represent a compendium useful for dosimetric purposes.
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