At this year’s SNMMI conference, researchers discussed the use of PET and SPECT to determine the precise radiotherapy dose that would deliver the “most powerful kill” to a patient's neuroendocrine tumors (NET) while protecting the kidney.

Researchers from the University of Iowa presented a study demonstrating the use of yttrium-90 DOTA0-Tyr3-octreotide, or Y-90 DOTATOC, on 12 patients who had malignant neuroendocrine tumors. Their goal was to limit radiation dose to kidneys since they receive most of the drug that does not bind to the tumors.

“DOTATOC is a peptide that binds with somatostatin receptors that are often highly expressed in neuroendocrine cancers,” said Mark T. Madsen, Ph.D., from the University of Iowa, in a statement.
“With molecular imaging, we are able to see whether the DOTATOC imaging agent is taken up by the tumor. If it is, we know that the Y-90 DOTATOC radiotherapy will also reach the tumor and be able to kill tumor cells,” he said.

For the first treatment, the adult patients in the study underwent 4.44 GBq (gigabecquerel, a unit of radioactivity) of Y-90 DOTATOC while the pediatric patients received 1.85 GBq per m2. To protect kidney toxicity, all the patients were given an infusion of amino acids.

After the first and second treatment, the researchers evaluated blood and renal dosimetry, the calculation of absorbed dose in tissue resulting from exposure to ionizing radiation, by using PET/CT five hours after the injection of Y-90 DOTATOC, and then SPECT/CT at 24, 48, and 72 hour intervals.

In the image below, the white arrows indicate kidneys and the yellow arrows are the tumors. The researchers acknowledge that although the position associated with Y-90 is rarely emitted, there is still a sufficient signal to acquire a quantitative PET/CT image after the injection of Y-90.


The team completed 20 dosimetry evaluations and found that the accumulation of Y-90 ranged from 1.4 to 3.6 percent. The kidney dose ranged from 0.6 to 2.7 mGy (milligray, a unit of absorbed radiation dose) per MBq, megabequerel, and the blood dose ranged from 0.04 to 0.24 mGy per MBq.

From the findings, the dose of radiotherapy was not increased for two of the children’s injections and one patient’s treatment ended after only one treatment. For the rest of the patients, the dose of radiotherapy was increased in eight courses and decreased in three courses. Overall, the prescribed dose of radiotherapy changed by more than 15 percent.

“Our approach combines the advantages of quantitative Y-90 PET and SPECT imaging to gather all the information required to accurately estimate kidney dose,” said Madsen. He added that they expect better outcomes in radionuclide therapy with less complications because they can adjust patient dose either up or down as needed.

While personalized dosimetry was used to determine the maximum dose that is safe for kidneys, it can also be used to determine the most effective dose to fight against NETs, according to the announcement, but further studies need to be done to elaborate on how to best apply PET and SPECT to improve neuroendocrine cancer care.

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