The Italian multicentre dosimetric study for lesion dosimetry in 223Ra therapy of bone metastases: Calibration protocol of gamma cameras and patient eligibility criteria

Purpose The aims of this work were to explore patient eligibility criteria for dosimetric studies in 223Ra therapy and evaluate the effects of differences in gamma camera calibration procedures into activity quantification. Methods Calibrations with 223Ra were performed with four gamma cameras (3/8-inch crystal) acquiring planar static images with double-peak (82 and 154 keV, 20% wide) and MEGP collimator. The sensitivity was measured in air by varying activity, source-detector distance, and source diameter. Transmission curves were measured for attenuation/scatter correction with the pseudo-extrapolation number method, varying the experimental setup. 223Ra images of twenty-five patients (69 lesions) were acquired to study the lesions visibility. Univariate ROC analysis was performed considering visible/non visible lesions on 223Ra images as true positive/true negative group, and using as score value the lesion/soft tissue contrast ratio (CR) derived from 99mTc-MDP WB scan. Results Sensitivity was nearly constant varying activity and distance (maximum s.d. = 2%). Partial volume effects were negligible for object area ⩾960 mm2. Transmission curve measurements are affected by experimental setup and source size, leading to activity quantification errors up to 20%. The ROC analysis yielded an AUC of 0.972 and an optimal threshold of CR of 10, corresponding to an accuracy of 92%. Conclusion The minimum calibration protocol requires sensitivity and transmission curve measurements varying the object size, performing a careful procedure standardisation. Lesions with 99mTc-MDP CR higher than 10, not overlapping the GI tract, are generally visible on 223Ra images acquired at 24 h after the administration, and possibly eligible for dosimetric studies.