Whole Body and SPECT/CT Imaging for Dosimetry of 177Lu-labeled Radiopharmaceutical Therapies: A Technologist’s Perspective

Natalie Jung; Natasha Aga; Minh Lu; Carlos Uribe; Nadine Colpo

Abstract

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Introduction: Radiopharmaceutical therapies (RPT) have shown promise in the treatment of disease like prostate cancer and neuroendocrine tumors. ¹⁷⁷Lu is an ideal radioisotope because it emits both beta particles useful to treat and gamma rays which make imaging with a gamma camera possible. Here, we aim at sharing our perspective from our clinical protocol used for quantitative imaging of ¹⁷⁷Lu-labeled radiopharmaceuticals (RP) using whole body (WB) and SPECT/CT that enable accurate dosimetry calculations.

Methods: Calibration of our Atomlab 100 Plus and 500 Plus dose calibrators (DC) and Siemens Symbia T gamma camera were performed in conjunction with the medical physicist. DC calibration factors were determined for a 30 ml glass vial (common geometry containing the RP) and a 20 ml plastic syringe (used to perform the patient injections). An imaging protocol for both WB and SPECT has been configured that includes 4 energy windows: 1) photopeak [187.2 keV – 228.8 keV], 2) low scatter [166.4 keV – 187.2 keV], 3) upper scatter [228.8 keV – 249.6 keV], and 4) general scatter [55 keV – 165 keV]. Our medium energy general purpose collimator is used for all the acquisitions with ¹⁷⁷Lu. Planar scans of a point source prepared in an Eppendorf tube were performed to enable the calculation of the camera CF by our medical physicist. SPECT images are generated with 96 projections (48 per head), 15 seconds per projection, with 3 stitched bed positions to cover from the top of the head to middle of femurs. Image reconstruction is performed with triple energy window scatter correction, CT-based attenuation correction, and resolution recovery using a matrix size of 128x128. This generates quantitative images that allow for retrospective dosimetry calculations.

For ¹⁷⁷Lu RPTs at our institution, each patient is intravenously injected with approximately 7.4 ± 10% GBq. Our protocol that involves dosimetry calculations requires 2 SPECT/CT acquisitions for the first cycle (between 20-28 h and at 42-100 h post injection) and 1 SPECT/CT (42-100 h post injection) for subsequent cycles. Planar WB and SPECT/CT are acquired with arms positioned down by the side. Images are visually examined and interpreted by a nuclear medicine physician, and the medical physicists perform dosimetry calculations when required.

Results: The calibration numbers for our Atomlab 100 Plus and Atomlab 500 DCs were 114 and 125 for the 30 ml glass vial, respectively, and 111 and 123 for the 20 ml syringe, respectively. The protocol setup on our gamma camera allows sequential acquisition of WB planar images and SPECT/CT with minimal interaction by the technologists during a patient exam. As total acquisition time is in the order of 90 minutes, special considerations are needed to ensure minimal patient movement. We include additional pillows and straps to help patient arm positioning for optimal patient comfort. Also, given that our planar scan is not performed with a standard source in the field of view, the only way for physicists to correctly quantify it is by correlating it with the SPECT images. However, the level of radioactivity should be the same in both types of images. We ask patients to void prior to the WB acquisition to avoid the need of a washroom break before the whole procedure is completed. This has also ensured minimal changes to patient positioning between planar and SPECT/CT. Consistent patient positioning facilitates the assessment for extent of metastatic disease and progression between therapy cycles.

Conclusions: Working closely with the medical physicist has highlighted the importance of equipment calibration and protocols for quantitative imaging. This has allowed the technologist to detect and correct issues at early stages in the dosimetry workflow. We have successfully implemented planar WB and SPECT/CT imaging of ¹⁷⁷Lu radiopharmaceuticals to support its use as a practical therapeutic option for patients at our centre, and to enable dosimetry calculations.