A practical dead time correction method in planar activity quantification for dosimetry during radionuclide therapy

Aim. Gamma camera saturation is the first quantification problem in dosimetric studies following therapeutic administrations of ¹³¹I labeled radiopharmaceuticals. A new approach for dead time correction (DTC) is here proposed. It employs planar whole-body (WB) images without the need of standard radionuclide sources or of preliminary phantom calibrations. Methods. Step and shoot WB acquisitions of the patient are required. A program was developed to compensate for the image discontinuities ("Continuity DTC method") between two adjacent static fields of view (FOVs) caused by different dead time count losses. For its validation, authors used two ^99mTc 6 GBq phantom scans after administration of six patients with ¹³¹I labeled agents with different statistics and ten clinical scans taken between 16 h and 48 h after administration of ¹³¹I labeled agents, whose activity ranged from 4 to 10 GBq. The deviation from true decay corrected counts on phantoms and the constancy of monitor point-source counts in different patients' FOVs (root mean square error and maximum deviation) served as figures of merit. The accuracy of absorbed dose calculation was also estimated by comparison with the standard source correction method, computing the area under the time activity curve (AUC) of six lesions. Results. With respect to the true phantom counts, corrected images gave excellent results, giving a 6% maximum deviation. For what concerns the other figures of merit, continuity DTC reduced the average root mean square error from 36% to 2% and the mean maximum deviation from 50% to 2%, on phantom, while from 51% to 32/28% (absence/presence of triple energy window scatter correction) and from 72% to 21/14% on patients. Mean compensation of AUC gave a correction of +56% with our method, while +78% with standard source method. Conclusion. The "Continuity DTC method" is a useful tool in dosimetry during nuclear medicine treatment, showing good accuracy. Moreover, since it does not require the use of any source, it provides with several advantages in terms of practicability and applicability, with respect to the standard source method and to methods based on the count rate characteristic curve.