A Whole Body Dosimetry Protocol for Peptide-Receptor Radionuclide Therapy (PRRT): 2D Planar Image and Hybrid 2D+3D SPECT/CT Image Methods

The method can be used in peptide-receptor radionuclide therapy and enable us to improve the accuracy of absorbed dose estimation in the presence of overlapping organs. The uptake distribution provided by the 3D SPECT/CT images is combined with the effective half-life calculated for planar images, while the non-overlapping structural data is derived for planar images alone. This method requires high-performance teamwork, including the assistance of physicians, physicists, medical radiology technicians, nurses, for the success of the dosimetry evaluation.

This method enables whole-body information to be obtained, while also providing 3D information of the abdominal region in which the organs overlapping may impair dosimetric readings. The imaging settings, including the position of the cameras and the patient, are of paramount importance to obtain suitable images for dosimetry evaluation. This methodology can be used with other beta and gamma radiopharmaceuticals.

Demonstrating the procedure with Paola Caroli and me will be Elisabeth Canali, a technician from our laboratory. To evaluate the patient’s water equivalent thickness for the attenuation correction of counts derived from 2D planar images acquired after radioligand injection, position the patient on the couch in a feet-first, supine position with arms at rest along the side of the body. It’s important to find a setup that is comfortable for the patient, as they must remain still for the entire acquisition time and for all subsequent imaging.

After placing supports to help maintain the position as necessary, set the SPECT dual heads at the opposite positions and at the maximum distance from the field-of-view center. Raise the couch so that the patient is positioned at the field-of-view center and with head at the detector center, and position the Cobalt-57 flood support on the posterior camera and the Cobalt-57 flood on the support. Then, begin the image acquisition.

When the image has been acquired, repeat the acquisition without the patient on the couch. For an effective half-life and mean absorbed dose evaluation of different structures, position the patient on the couch as demonstrated. Use the teach pendant to manually adjust the position of the posterior camera to reach the minimum distance from the inferior couch profile.

Then, manually adjust the position of the anterior and posterior cameras to reach the minimum distance from the patient’s profile, and start the image acquisition. Collect a two-milliliter blood sample at each imaging session, placing the closed collection tube in a shielded box labeled with the time of collection. For 24-hour post-injection 3D SPECT/CT image acquisition, position the detector at the maximum distance from the center to avoid collision, and position the patient with the arms lifted over the head.

Then, position the patient table inside the camera until the region of interest is centered on the detector, and begin the image acquisition. For all post-infusion image analyses, select emission, low-and high-scatter images, and click on the right panel of the dedicated workflow to create a scatter-corrected 2D image. Rotate the posterior images, and draw regions of interest around the kidneys, liver, spleen as visible, parotid glands, submandibular glands, and the whole body.

If possible, contour any visible lesions and regions of interest on the most useful image between the anterior and posterior views, and contour a small region of interest adjacent to each contoured structure for background. For each image, take note of the average counts and pixel dimension inside each region of interest and background region of interest for both the anterior and posterior views. Then, evaluate the water equivalent thickness for each structure to estimate the self-attenuation.

To perform the blood sample measurements, first position the blood sample tube inside the high-purity germanium detector within the proper holder. After closing the detector, select the proper high-purity germanium calibration file corresponding to the two-milliliter collection tube geometry holder, and initiate the sample measurements to obtain a minimum of 12-hour measurements. For each image and structure, calculate the counts on the anterior and posterior views as indicated.

For each region of interest, calculate the uptake at each image time point as indicated by the formula. Then, calculate the relative uptake. For hybrid 2-and 3D SPECT/CT image analysis, open the SPECT/CT image of interest, and contour the volumes of interest based on both the uptake information and CT morphology.

To avoid the problem of high-uptake intestine overlap on the kidney structure, scale the kidney 2D time-activity curve using the acquired scaling factor. To perform scaling for blood values to calculate the red marrow dose, calculate the relative blood uptake and rescale the data to the red marrow mass. To perform fits of time-activity curves, for each organ, insert the relative uptake at each image time point and click Refresh.

Perform a curve fitting using an appropriate mono-bi-or tri-exponential curve fitting model, flag the required parameters, and enter starting values. Then, click Fit until the fit is performed. In the Main Input Form, click Doses and Modify Input Data.

In the Multiply all masses by box, enter the ratio between the patient’s weight and the adult male phantom weight, and click on the Multiply all masses by button. All the organ masses will then be rescaled accordingly. Enter the single organ masses as calculated from the CT delineation for the analyzed organs.

For paired organs, such as the kidneys, insert the sum of left and right kidney masses. The report will display the mean absorbed dose normalized to injected activity, expressed in milligrays per megabecquerels. Note the total absorbed dose for the contoured organs.

For this representative patient, a full 3D SPECT/CT evaluation was performed by acquiring both planar images and 3D SPECT/CT data for all of the days dedicated to the dosimetry. The hybrid method intestine overlap correction was valid for all of the time points for the right kidney, whereas the correction underestimated the relative uptake on day one for the left kidney. Nevertheless, a discrepancy of only 1.6%was observed between the hybrid and 2D methods in terms of the mean absorbed dose of the hybrid method.

The implementation of a dosimetry methodology requires several technical solutions. Sharing existing experiences can help other teams through this process and could promote standardization of the methodology. Using the 3D SPECT/CT data, dose distribution and dose-volume histograms can be calculated, allowing the combination of absorbed doses of PRRT and radiotherapy treatments.

This method can be further optimized, owing the implementation of attenuation correction based on the CT image or the use of a reference source of known activity. Dosimetric results from beta-emitter treatments exhibit lower absorbed dose than organ if there is constraint. This is useful information for alpha-emitter therapy, too, where severe side effect to salivary gland may impair treatment outcomes.