JNM: Planar/radiopharma studies provide diagnostic, dose information
A patient-specific, 3D dosimetry protocol using the radiopharmaceutical Tc-HYNIC-TOC is clinically feasible to demonstrate large dose variations in neuroendocrine tumors and normal organs, according to study findings published Sept. 1 in the Journal of Nuclear Medicine.

Joshua Grimes, PhD, of the department of physics and astronomy at the University of British Columbia in Vancouver, and colleagues assessed the biodistribution and radiation dosimetry of Tc-HYNIC-TOC in 28 patients that were imaged for suspected neuroendocrine tumors. Patients were scanned using a series of whole-body planar, dynamic planar and SPECT/CT studies, after injection with the radiopharmaceutical, according to the study.

Patient-specific dosimetry was performed using software with time-integrated coefficients estimated from a hybrid planar/SPECT technique, according to the study, and a phantom experiment was performed to establish adaptive thresholds to determine source region volumes and activities.

Tumors were revealed in 12 patients out of the 28. Researchers found that the spleen typically received the most significant uptake of the radiopharmaceutical, followed by the kidneys, the urinary bladder wall, liver and thyroid. 

“In general, the kidneys were found to reach maximum uptake after five to 10 minutes, followed by the washout phase," the authors wrote. "In the liver, maximum uptake occurred in less than five minutes, followed by a rapid decrease and then a more gradual washout phase. Uptake in the spleen was slower and usually extended beyond the 30-minute scan.” Tumor uptake did not reach a maximum until after the 30-minute scan as well.

“The image-based dose calculation method presented in this study was designed to provide accurate patient-specific dose estimates while working within the limitations of a busy clinical environment. To this end, a hybrid planar/SPECT technique was used to obtain source region cumulated activities, with image segmentation performed using adaptive thresholds,” wrote Grimes et al.

“This work outlines how hybrid planar/SPECT Tc-HYNIC-TOC studies can provide clinicians not only with diagnostic information, but also with patient-specific quantitative parameters such as time-activity curves, absolute activity and absorbed doses for tumors and normal organs,” the authors concluded.

“The large dose variations observed in this study demonstrate the significant impact that patient-specific considerations can have on treatment planning decisions,” Grimes and colleagues wrote. “In the future, similar comprehensive analyses may serve to supplement the standard clinical examination of lesions overexpressing somatostatin receptor and may aid in the selection of patients for therapy through the accurate quantification of tumor and normal-organ uptake of radiolabeled somatostatin analogs.”