The Italian multicentre dosimetric study for lesion dosimetry in Ra 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 Ra therapy and evaluate the effects of differences in gamma camera calibration procedures into activity quantification.

Methods: Calibrations with Ra were performed with four gamma cameras (3/8-inch crystal) acquiring planar static images with double-peak (82 and 154keV, 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. Ra 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 Ra images as true positive/true negative group, and using as score value the lesion/soft tissue contrast ratio (CR) derived from Tc-MDP WB scan.

Results: Sensitivity was nearly constant varying activity and distance (maximum s.d.=2%). Partial volume effects were negligible for object area ⩾960mm. 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 Tc-MDP CR higher than 10, not overlapping the GI tract, are generally visible on Ra images acquired at 24h after the administration, and possibly eligible for dosimetric studies.