Discordance between Y-PET/CT(MR)-estimated activity and dose calibrator measured administered activity: an international study in SIRT patients treated with resin and glass microspheres.

PURPOSE

Therapeutic administration of Y-loaded microspheres is routinely used for primary and secondary liver tumours. For activity-based therapeutic prescription the activity must be within 10% of the intended activity. Previous studies reported significant discrepancies between manufacturer-declared vial activities and both experimental and Monte-Carlo assessments, greater than 10%, for resin/glass Y-microspheres. The objective of this work was to investigate whether these discrepancies were also seen in patients.

METHODS

We analysed patient Y-PET reconstructions (99 glass and 15 resin microspheres) from 4 different institutions and 4 different systems. We considered tail-fitting background scaling (TFBS) and absolute scaling (ABS), for scatter correction. Residuals after therapeutic injection were measured. Eighty-one patients were imaged with PET/CT and 33 with PET/MR. The PET measured activity (A) was assessed in the whole liver. The ratio A/A was calculated for each patient, where A was the injected activity measured by the dose calibrator corrected for residual and lung shunt.

RESULTS

Quantification ratio between calibrators and PET was significantly different from 1, regardless of the scatter correction used. In glass microspheres, the mean A/A was 0.84 ± 0.06 for TFBS and 0.90 ± 0.06 for ABS (0.66 ± 0.09 and 0.76 ± 0.07 for (A/A)). The mean A/A ratio for resin microspheres was 1.16 ± 0.09 for TFBS and 1.30 ± 0.12 for ABS.

CONCLUSIONS

We observed in patients similar activity discrepancies as reported for vials, with a relative difference of 44 ± 16% between glass and resin Y-loaded microspheres. In Y hepatic radioembolization, the 10% accuracy prerequisite on knowing the administered therapeutic activity is then unlikely to be met.