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

Abstract

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 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. 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 ⩾960mm². 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 24h after the administration, and possibly eligible for dosimetric studies.

Keywords: (223)Ra-dichloride; Bone metastases; Dosimetry; Radionuclide therapy.