68Ga-PSMA PET/CT in prostate cancer patients - patterns of disease, benign findings and pitfalls

A Case Series Depicting PSMA Expression in Nonmalignant Lesions

Prostate-specific membrane antigen (PSMA) is a widely accepted and used tracer in staging and biochemical recurrences of prostate cancer. PSMA is extensively expressed in normal prostatic epithelial cells and prostate cancer cells, with some amount of expression also in nonprostatic cells. False-positive PSMA uptake in nonmalignant lesions creates ambiguity in disease detection. In such cases, histopathological correlation and radiological follow-up assist in clinical decision-making. In this case series, we illustrate a few cases where PSMA uptake was incidentally found in some of the commonly occurring benign conditions.

Intense prostate-specific membrane antigen receptor expression in coronary artery pypass graft scar tissue: A potential molecular imaging pitfall

68Gallium-PSMA positron emission tomography/computer tomography has been utilized recently for the diagnosis and staging of prostate cancer. PSMA is a transmembrane protein that is expressed not only in the prostate gland but also in other tissues. While some pitfalls have been addressed, there are still uncertainties. Herein, we report a 79-year-old male with prostate cancer who underwent a PSMA scan after coronary artery bypass graft surgery, revealing disease progression and PSMA-avid foci at the surgical stitch sites. This report discusses the immunohistochemical and molecular imaging mechanisms underlying PSMA expression in surgical scar tissues, providing critical insights for optimizing radiologic reporting in such situations.

How to Report PSMA PET

Prostate cancer (PCa) is the most common cancer diagnosed in men in most developed countries and a leading cause of cancer-related morbidity and mortality. Prostate-specific membrane antigen positron emission tomography (PSMA-PET) has become a valuable tool in the staging and assessment of disease recurrence in PCa, and more recently for assessment for treatment eligibility to PSMA radioligand therapy (RLT). Harmonization of PSMA-PET interpretation and synoptic reports are needed to communicate concisely and reproducibly PSMA-PET/CT to referring physicians and to support clinician therapeutic management decisions in various stages of the disease. Uniform image interpretation is also important to provide comparable data between clinical trials and to translate such data from research to daily practice. This review provides an overview of the value of PSMA-PET across the different clinical stages of PCa, discusses published reporting criteria for PSMA-PET, identifies pitfalls in reporting PSMA, and provides recommendations for synoptic reports.

The diagnostic accuracy of 68Ga-PSMA PET/CT versus 99mTc-MDP bone scintigraphy for identifying bone metastases in persons with prostate cancer: A systematic review

INTRODUCTION

Prostate cancer (PCa) is the second most common cause of cancer related death in men. Accurate diagnosis of bone metastases is essential to treatment decision-making and follow-up. Recent primary studies have compared the accuracy of 68Ga-PSMA PET/CT versus 99mTc-MDP bone scintigraphy in the detection of PCa bone metastases. These studies suggest 68Ga-PSMA PET/CT to be superior. Comprehensive syntheses of these studies are now warranted.

PURPOSE

To synthesize studies comparing the accuracy of 68Ga-PSMA PET/CT versus 99mTc-MDP bone scintigraphy, the most used modality in the identification of bone metastases in PCa patients.

METHODS

A systematic review was conducted evaluating diagnostic accuracy studies which compared 68Ga-PSMA PET/CT and 99mTc-MDP bone scintigraphy. Bias and quality were assessed using the QUADAS-2 tool. Searches in three databases using search terms: Positron-Emission Tomography, prostatic neoplasm, 68Ga, and bone were conducted. Image acquisitions between modalities had to be performed within 3 months of each other.

RESULTS

Five single-centered studies were included in this review. Across all measures of accuracy, 68Ga PSMA PET/CT was superior to 99mTc-MDP bone scintigraphy in the detection of skeletal metastases. Patient-based sensitivities and specificities across included studies ranged from (91%-100% vs. 50%-91%) and (88%-100% vs 19%-96%) for 68Ga-PSMA PET/CT and 99mTc-MDP bone scintigraphy respectively. The overall risk of bias was moderate primarily due to the retrospective nature of most included studies.

CONCLUSION

68Ga-PSMA PET/CT was more accurate than 99mTc-MDP bone scintigraphy in the detection of PCa bone metastases. Future studies should seek to define the clinical relevance of these findings.

Traditional and novel imaging modalities for advanced prostate cancer: A critical review

Accurate detection of metastatic prostate cancer in the setting of preoperative staging as well as posttreatment recurrence is crucial to provide patients with appropriate and timely treatment of their disease. This has traditionally been accomplished with a combination of computed tomography, magnetic resonance imaging, and bone scan. Recently, more novel imaging techniques have been developed to help improve the detection of advanced and metastatic prostate cancer. This review discusses the efficacy of the traditional imaging modalities as well as the novel imaging techniques in detecting metastatic prostate cancer. Articles discussed were gathered through a formal PubMed search.

Predicting Outcomes of Indeterminate Bone Lesions on 18F-DCFPyL PSMA PET/CT Scans in the Setting of High-Risk Primary or Recurrent Prostate Cancer

Indeterminate bone lesions (IBLs) on prostate-specific membrane antigen (PSMA) PET/CT are common. This study aimed to define variables that predict whether such lesions are likely malignant or benign using features on PSMA PET/CT. Methods: 18F-DCFPyL PET/CT imaging was performed on 243 consecutive patients with high-risk primary or biochemically recurrent prostate cancer. IBLs identified on PSMA PET/CT could not definitively be interpreted as benign or malignant. Medical records of patients with IBLs were reviewed to determine the ultimate status of each lesion. IBLs were deemed malignant or benign on the basis of evidence of progression or stability at follow-up, respectively, or by biopsy results; IBLs were deemed equivocal when insufficient or unclear evidence existed. Post hoc patient, lesion, and scan variables accounting for clustered data were evaluated using Wilcoxon rank-sum and χ2 tests to determine features that favored benign or malignant interpretation. Results: Overall, 98 IBLs within 267 bone lesions (36.7%) were identified in 48 of 243 patients (19.8%). Thirty-seven of 98 IBLs were deemed benign, and 42 were deemed malignant, of which 8 had histologic verification; 19 remained equivocal. Location and SUVmax categorical variables were predictive of IBL interpretation (P = 0.0201 and P = 0.0230, respectively). For IBLs with new interpretations, 34 of 37 (91.9%) considered benign showed an SUVmax of less than 5 or exhibited focal uptake without coexisting bone metastases; 37 of 42 (88.1%) deemed malignant demonstrated an SUVmax of at least 5 or were present with coexisting bone metastases. Logistic regression predicted IBLs with a high SUVmax (univariable: odds ratio [OR], 9.29 [P = 0.0016]; multivariable: OR, 13.87 [P = 0.0089]) or present with other bone metastases (univariable: OR, 9.87 [P = 0.0112]; multivariable: OR, 11.35 [P = 0.003]) to be malignant. Conclusion: IBLs on PSMA PET/CT are concerning; however, characterizing their location, SUV, and additional scan findings can aid interpretation. IBLs displaying an SUVmax of at least 5 or present with other bone metastases favor malignancy. IBLs without accompanying bone metastases that exhibit an SUVmax of less than 5 and are observed only in atypical locations favor benign processes. These guidelines may assist in the interpretation of IBLs on PSMA PET/CT.

The Utility of 68Ga-PSMA PET/CT in Decisions Regarding Administering Salvage Radiotherapy to Men with Prostate Cancer

BACKGROUND

Numerous papers have described 68Ga-prostate-specific membrane antigen (PSMA) positron emission tomography/computed tomography (PET/CT)'s sensitivity in identifying prostate cancer (PCa) recurrence. This study aimed to characterize the role of 68Ga-PSMA PET/CT in deciding to re-irradiate pelvic structures.

METHODS

68Ga-PSMA PET/CT scans performed at Sheba Medical Center over seven years in 113 men were reviewed. All had undergone radiation to the prostate (70, 61.9%) or post-radical prostatectomy radiation to the prostate fossa (PF) (43, 48.1%), and had local or oligometastatic PCa recurrence and received salvage radiotherapy (SRT) based on PET/CT findings.

RESULTS

Mean age was 70.7 years. The mean grade group was 2.9; the mean prostate-specific antigen was 9.0. The 68Ga-PSMA PET/CT positive findings included: 37 (32.7%) in the prostate, 23 (20.4%) in seminal vesicles, 7 (6.2%) in the PF, and 3 (2.7%) in the seminal vesicle fossa. The mean standardized uptake value was 10.6 ± 10.2 (range: 1.4-61.6); the mean lesion size was 1.8 ± 3.5 mm (range: 0.5-5.1). SRT was directed toward the prostate and seminal vesicles in 48 (42.5%), PF in 18 (15.9%), and intrapelvic lymph node and bone in 47 (41.6%). Toxicities were mostly mild to moderate.

CONCLUSION

68Ga-PSMA PET/CT-identified relapse with targeted SRT was well-tolerated and may result in less onerous treatments.

Late-Term Findings of Pancreatitis on 68 Ga-PSMA PET/CT in a Patient With Prostate Cancer

ABSTRACT

The 68 Ga-PSMA PET/CT imaging modality is used to evaluate biochemical recurrence, response to treatment, and staging in prostate cancer. Prostate-specific membrane antigen (PSMA) receptor activation can be seen in benign and malignant diseases as well as in many physiological tissues. Many pitfalls and artifacts have been reported when reporting 68 Ga-PSMA PET/CT. In this case, diffuse moderate PSMA receptor activation in pancreatic tissue due to the previous pancreatitis is presented in 68 Ga-PSMA PET/CT imaging modality that was performed for restaging of prostate cancer.

Prostate specific membrane antigen positron emission tomography in primary prostate cancer diagnosis: First-line imaging is afoot

Prostate specific membrane antigen positron emission tomography (PSMA PET) is an excellent molecular imaging technique for prostate cancer. Currently, PSMA PET for patients with primary prostate cancer is supplementary to conventional imaging techniques, according to guidelines. This supplementary function of PSMA PET is due to a lack of systematic review of its strengths, limitations, and potential development direction. Thus, we review PSMA ligands, detection, T, N, and M staging, treatment management, and false results of PSMA PET in clinical studies. We also discuss the strengths and challenges of PSMA PET. PSMA PET can greatly increase the detection rate of prostate cancer and accuracy of T/N/M staging, which facilitates more appropriate treatment for primary prostate cancer. Lastly, we propose that PSMA PET could become the first-line imaging modality for primary prostate cancer, and we describe its potential expanded application.

Bone Uptake in Prostate Cancer Patients: Diagnostic Performances of PSMA-RADS v1.0, Clinical, Biological, and 68 Ga-PSMA-11 PET Features to Predict Metastasis After Biochemical Recurrence

PURPOSE

68 Gallium-labeled prostate-specific membrane antigen-11 (PSMA) PET/CT is the new reference to identify relapse during biochemical recurrence of prostate cancer (PCa). However, this method lacks specificity for bone foci. This study aimed to report the prevalence of PCa bone metastases and to assess the diagnostic performances of PSMA reporting and data systems (RADS), clinical, biological, and imaging features for identification.

PATIENTS AND METHODS

A multicentric retrospective cohort of consecutive patients with biochemical recurrence after local treatment was analyzed. Clinical and biological features at initial staging and during recurrence were retrieved from medical reports. The metastatic status of each bone uptake on PSMA PET/CT was determined according to histopathology, comparisons with concomitant and previous conventional imaging, prostate-specific antigen kinetic, and follow-up. Two nuclear medicine physicians assessed PSMA-RADS, anatomic location, radiological patterns, SUV max , and the presence of other molecular lesions. Univariate and multivariate analyses were conducted to identify independent predictors of PCa metastases.

RESULTS

In the eligible population, 98/298 patients (32.9%) showed bone uptake on PSMA PET/CT. In patients with a final diagnosis, 28/81 lesions (34.6%) were metastases. PSMA-RADS-4 or 5 showed sensitivity of 79%, specificity of 94%, and accuracy of 89%. PSMA-RADS had a significantly higher area under the receiver operating characteristic curve than the initial reading in clinical practice (0.91 vs 0.83, P = 0.0074). Initial Gleason score ≥8, age ≤71 years at recurrence, and SUV max >6.21 were independent predictors of PCa metastases in multivariate logistic regression ( P = 0.0314, 0.0179, and 0.0003, respectively).

CONCLUSIONS

Most bone uptakes at PSMA PET/CT were benign lesions. PSMA-RADS, patients and tumor characteristics, and SUV max could help identify PCa bone metastases.

Gallium-68 Prostate-Specific Membrane Antigen Positron Emission Tomography: A Practical Guide for Radiologists and Clinicians

Prostate cancer (PCa) is the third most common form of cancer and the most common cancer diagnosis in men in the United States. It is known that prostate-specific membrane antigen (PSMA) is overexpressed in PCa. PSMA is a type II transmembrane glycoprotein expressed in several benign and malignant tissues. In December 2020, FDA approved Gallium-68 (68Ga) PSMA, which is a PSMA-targeted positron emission tomography (PET) imaging agent. Molecular imaging targeting PSMA has shown substantial advancement in PCa imaging. In this article, we discuss the radiopharmaceutical, indications to do PSMA PET, technical aspects of PSMA PET imaging, normal biodistribution of PSMA, other benign and malignant conditions that can take up PSMA, staging of prostate cancer, and how to report PSMA PET.

68Ga-PSMA PET/CT Scan Leading to Diagnosis of PSMA-Positive Rectal Adenocarcinoma in a Patient With Prostate Cancer

ABSTRACT

A 59-year-old prostate cancer patient, status post radical prostatectomy, was referred to our department for restaging with 68Ga-prostate-specific membrane antigen (PSMA) PET/CT scan. Aside multiple metastatic lesions involving pelvic lymph nodes as well as the right femoral trochanter, a PSMA-positive rectal wall thickening was detected. Colonoscopy correlation and tissue diagnosis, recommended to rule out accompanying primary malignancy, confirmed the presence of rectal adenocarcinoma. This case signifies the importance of requesting pathological correlation for unexplained PSMA-positive lesions, emphasizing the potential role PSMA ligands may play in detecting occult second primary malignancies, especially synchronous/metachronous colorectal cancers.

Prostate-specific membrane antigen (PSMA) fusion imaging in prostate cancer: PET-CT vs PET-MRI

OBJECTIVES

To investigate whether PET-CT or PET-MRI is more appropriate for imaging prostate cancer, in terms of primary tumor detection, local staging and recurrence, as well as lymph nodes and distant metastases.

METHODS

A systematic literature search was conducted on Embase, PubMed/MEDLINE, and the Cochrane Library database. Studies evaluating the diagnostic performance of PET-CT vs PET-MRI in prostate cancer patients were emphasized.

RESULTS

We reviewed 57 original research articles during the period 2016-2021: 14 articles regarding the radiotracer PSMA; 18 articles regarding the primary tumor detection, local tumor staging, managing local recurrence; 17 articles for managing lymph node metastases; and eight articles for managing bone and other distant metastases. PSMA PET could be complementary to mpMRI for primary prostate cancer localization and is particularly valuable for PI-RADS three lesions. PET-MRI is better than PET-CT in local tumor staging due to its specific benefit in predicting extracapsular extension in MRI-occult prostate cancer patients. PET-MRI is likely superior as compared with PET-CT in detecting local recurrence, and has slightly higher detection rates than PET-CT in lymph node recurrence. PET-CT and PET-MRI seem to have equivalent performance in detecting distant bony or visceral metastases.

CONCLUSION

In conclusion, PET-MRI is suitable for local and regional disease, either primary staging or restaging, whereas PET-CT is valuable for managing distant bony or visceral metastasis.

ADVANCES IN KNOWLEDGE

We reviewed the emerging applications of PET-MRI and PET-CT in clinical aspects. Readers will gain an objective overview on the strength and shortfalls of PET-MRI or PET-CT in the management of prostate cancer.

Multimodal Characterization of a PSMA-Positive Thyroid Nodule Using 68Ga-PSMA and 124Iodine PET/US Fusion Imaging

A 54-year-old male diagnosed with prostate cancer was referred for 68Gallium-PSMA-11 PET/CT. The scan revealed a solitary PSMA-positive thyroid lesion. On PET/ultrasound fusion imaging, a nodule with moderate risk of malignancy (TIRADS 4B) could be unambiguously correlated. Additional 124Iodine PET/ultrasound fusion imaging revealed normal iodine uptake within the PSMA-positive thyroid nodule. Fine-needle aspiration cytology was performed using an ultrasound needle-guidance system. The cytopathological investigation confirmed a benign thyroid nodule and excluded a thyroid carcinoma as well as a prostate cancer metastasis. Immunohistochemistry was positive for thyroglobulin staining.

The modern therapeutic & imaging landscape of metastatic prostate cancer: a primer for radiologists

Prostate cancer represents one of the leading causes of cancer-related mortality in the United States and the most common cancer among men. Treatment paradigms for the management of advanced stages of prostate cancer have continued to evolve in recent years. These advancements in the therapeutic landscape of metastatic prostate cancer and diagnostic imaging modalities have fundamentally changed the treatment of patients with prostate cancer. In this review article we provide a primer for radiologists highlighting the most recent developments in treatment options and imaging techniques utilized in the modern oncologic management of metastatic prostate cancer. We will examine current therapy options and associated toxicities with an emphasis on relevant imaging findings commonly encountered by radiologists. We also summarize the role of modalities including CT, MRI, PET, bone scintigraphy, and PET in the diagnosis and follow-up of patients with metastatic prostate cancer.

The differential diagnostic value of dual-phase 18F-DCFPyL PET/CT in prostate carcinoma

OBJECTIVE

Binding of ¹⁸F-DCFPyL at prostate cancer (PC) cells increases over time. The dual-phase protocol may be helpful in separating benign lesions from malignant ones associated with prostate cancer. The purpose of this study was to retrospectively analyze the incremental diagnostic value of ¹⁸F-DCFPyL dual-time imaging in patients with prostate cancer.

METHOD

114 prostate-related malignant lesions and 43 benign lesions in 38 patients with prostate cancer were retrospectively analyzed. Maximum standardized uptake value (SUVmax) for benign and prostate-related malignant lesions were calculated at min 60 and min 120 of PET/CT imaging. In order to calculate SUV ratio, the SUVmax of left gluteus maximus was measured at the same time. The difference of SUVmax metrics and SUV ratio between malignant and benign lesions was statistically analyzed, the cut-off value of ROC curve was calculated, and the diagnostic efficacy of SUVmax index and SUV ratio at two time points was compared.

RESULTS

SUVmax metrics and SUV ratio of early and delayed imaging of PC-related malignant lesions were significantly higher than those of benign lesions (p < 0.05). In terms of individual indicators, the highest accuracy and sensitivity was in the delayed SUV ratio (89.2% and 94.7%), the best specificity was in the early SUVmax (93.0%). When the individual and combined indicators were compared together, the SUV ratio in the delay period still showed the best diagnostic sensitivity and accuracy, and the best specificity were SUVmax early and ▵SUVmax, SUVmax early and RI.

CONCLUSIONS

Uptake of ¹⁸F-DCFPyL increased over time in prostate-associated malignant lesions compared with benign tissue. For single-phase imaging, 2-hour (delayed) imaging has better diagnostic performance. However, the dual-phase imaging (1 and 2 h) are helpful in the differential diagnosis of prostate-associated malignant lesions and benign lesions.

Clinical insignificance of [18F]PSMA-1007 avid non-specific bone lesions: a retrospective evaluation

PURPOSE

[18F]PSMA-1007 offers advantages of low urinary tracer excretion and theoretical improved spatial resolution for imaging prostate cancer. However, non-specific bone lesions (NSBLs), defined as mild to moderate focal bone uptake without a typical morphological correlate on CT, are a common finding on [18F]PSMA-1007 PET/CT. The purpose of this study was to investigate the clinical outcomes of patients with [18F]PSMA-1007 avid NSBLs, to determine whether patients with NSBLs represent a higher risk clinical cohort, and to determine whether SUVmax can be used as a classifier of bone metastasis.

METHODS

A retrospective audit of 214 men with prostate cancer was performed to investigate the clinical outcomes of [18F]PSMA-1007 avid NSBLs according to defined criteria. We also compared the serum PSA, Gleason score, and uptake time of patients with [18F]PSMA-1007 avid NSBLs to patients without [18F]PSMA-1007 avid bone lesions. Finally, we analysed an SUVmax threshold to identify bone metastases using ROC curve analysis.

RESULTS

Ninety-four of 214 patients (43.9%) demonstrated at least one NSBL. No [18F]PSMA-1007 avid NSBLs met criteria for a likely malignant or definitely malignant lesion after a median 15.8-month follow-up interval (11.9% definitely benign, 50.3% likely benign, and 37.7% equivocal). There were no statistically significant differences in serum PSA, Gleason score, and uptake time between patients with [18F]PSMA-1007 avid NSBLs and those without [18F]PSMA-1007 avid bone lesions. All NSBLs with adequate follow-up had SUVmax ≤ 11.1. The value of the highest SUVmax distinguished between NSBLs and definite prostate cancer bone metastases, whereby an SUVmax threshold of ≥ 7.2 maximized the Youden's index.

CONCLUSION

[18F]PSMA-1007 avid NSBLs rarely represent prostate cancer bone metastases. When identified in the absence of definite metastatic disease elsewhere, it is appropriate to classify those with SUVmax < 7.2 as likely benign. NSBLs with SUVmax 7.2-11.1 may be classified as equivocal or metastatic, with patient clinical risk factors, scan appearance, and potential management implications used to guide interpretation.

Head-to-head comparison between 68Ga-PSMA and 18F-FDG-PET/CT in lymphomas: a preliminary analysis

PURPOSE

Isolated case reports mention the uptake of radiolabeled PSMA in lymphoma. However, it is not clear if the intensity of 68Ga-PSMA expression varies among different histological subtypes or if it correlates with 18F-FDG uptake. This study compared both tracers in patients with diverse lymphoma subtypes.

METHODS

Ten patients with biopsy-proven-lymphoma underwent 18F-FDG and 68Ga-PSMA-PET/CT (maximum time interval: 6 days). Lymphoma subtypes included Hodgkin's lymphoma (HL, three patients) and aggressive and indolent non-Hodgkin's lymphoma (NHL, seven patients). The intensity of PSMA uptake was classified visually as low, intermediate, or high, using blood pool, liver and parotid gland uptake as references. Maximum standardized-uptake value (SUVmax) of each affected site was measured in both sets of images.

RESULTS

FDG detected 59/59 involved sites in 10 patients and PSMA 47/59 sites in nine patients. PSMA uptake was generally low, regardless of the intensity of FDG uptake, but it was classified as intermediate in two patients. The median SUVmax varied from 2.0 (2.0-8.2) to 30.9 for FDG and from 1.7 (1.7-1.7) to 4.4 for PSMA, P < 0.0001. The primary lesion of one patient had a marked intralesional mismatch uptake pattern of the tracers, with areas of higher PSMA expression than FDG uptake, and vice-versa. A brain lesion was more easily identified with PSMA than with FDG images.

CONCLUSION

HL and several NHL subtypes may present PSMA uptake. The intensity of PSMA expression is generally lower than that of FDG uptake and seems to present less variation among the different histological subtypes of lymphomas.

Patterns of Metastatic Disease in Patients with Cancer Derived from Natural Language Processing of Structured CT Radiology Reports over a 10-year Period

Background Patterns of metastasis in cancer are increasingly relevant to prognostication and treatment planning but have historically been documented by means of autopsy series. Purpose To show the feasibility of using natural language processing (NLP) to gather accurate data from radiology reports for assessing spatial and temporal patterns of metastatic spread in a large patient cohort. Materials and Methods In this retrospective longitudinal study, consecutive patients who underwent CT from July 2009 to April 2019 and whose CT reports followed a departmental structured template were included. Three radiologists manually curated a sample of 2219 reports for the presence or absence of metastases across 13 organs; these manually curated reports were used to develop three NLP models with an 80%-20% split for training and test sets. A separate random sample of 448 manually curated reports was used for validation. Model performance was measured by accuracy, precision, and recall for each organ. The best-performing NLP model was used to generate a final database of metastatic disease across all patients. For each cancer type, statistical descriptive reports were provided by analyzing the frequencies of metastatic disease at the report and patient levels. Results In 91 665 patients (mean age ± standard deviation, 61 years ± 15; 46 939 women), 387 359 reports were labeled. The best-performing NLP model achieved accuracies from 90% to 99% across all organs. Metastases were most frequently reported in abdominopelvic (23.6% of all reports) and thoracic (17.6%) nodes, followed by lungs (14.7%), liver (13.7%), and bones (9.9%). Metastatic disease tropism is distinct among common cancers, with the most common first site being bones in prostate and breast cancers and liver among pancreatic and colorectal cancers. Conclusion Natural language processing may be applied to cancer patients' CT reports to generate a large database of metastatic phenotypes. Such a database could be combined with genomic studies and used to explore prognostic imaging phenotypes with relevance to treatment planning. © RSNA, 2021 Online supplemental material is available for this article.

Matched-Pair Comparison of 68Ga-PSMA-11 and 18F-rhPSMA-7 PET/CT in Patients with Primary and Biochemical Recurrence of Prostate Cancer: Frequency of Non-Tumor-Related Uptake and Tumor Positivity

Radiohybrid prostate-specific membrane antigen (rhPSMA) ligands are a new class of prostate cancer theranostic agents. 18F-rhPSMA-7 offers the advantages of 18F labeling and low urinary excretion compared with 68Ga-PSMA-11. Here, we compare the frequency of non-tumor-related uptake and tumor positivity with 68Ga-PSMA-11 and 18F-rhPSMA-7 in patients with primary or recurrent prostate cancer. Methods: This retrospective matched-pair comparison matched 160 18F-rhPSMA-7 with 160 68Ga-PSMA-11 PET/CT studies for primary staging (n = 33) and biochemical recurrence (n = 127) according to clinical characteristics. Two nuclear medicine physicians reviewed all scans, first identifying all PET-positive lesions and then differentiating lesions suggestive of prostate cancer from those that were benign, on the basis of known pitfalls and ancillary information from CT. For each region, the SUVmax of the lesion with the highest PSMA ligand uptake was noted. Tumor positivity rates were determined, and SUVmax was compared separately for each tracer. Results:18F-rhPSMA-7 and 68Ga-PSMA-11 PET revealed 566 and 289 PSMA ligand-positive lesions, respectively. Of these, 379 and 100 lesions, equaling 67.0% and 34.6%, respectively, of all PSMA-positive lesions, were considered benign. The distribution of their etiology was similar (42%, 24%, and 25% with 18F-rhPSMA-7 vs. 32%, 24%, and 38% with 68Ga-PSMA-11 for ganglia, bone, and unspecific lymph nodes, respectively). All primary tumors were positive with both agents (n = 33 each), whereas slightly more metastatic lesions were observed with 68Ga-PSMA-11 in both disease stages (113 for 18F-rhPSMA-7 and 124 for 68Ga-PSMA-11). The SUVmax of 18F-rhPSMA-7 and 68Ga-PSMA-11 did not differ (P > 0.05) in local recurrence or primary prostate cancer; however, the tumor-to-bladder ratio was significantly higher with 18F-rhPSMA-7 (4.9 ± 5.3 vs. 2.2 ± 3.7, P = 0.02, for local recurrence; 9.8 ± 9.7 vs. 2.3 ± 2.6, P < 0.001, for primary prostate cancer). Conclusion: The tumor positivity rate was consistently high for 68Ga-PSMA-11 and 18F-rhPSMA-7. Both tracers revealed a considerable number of areas of uptake that were reliably identified as benign by trained physicians making use of corresponding morphologic imaging and known PSMA pitfalls. These were more frequent with 18F-rhPSMA-7. However, the matched-pair comparison could have introduced a source of bias. Adequate reader training can allow physicians to differentiate benign uptake from disease and be able to benefit from the logistical and clinical advantages of 18F-rhPSMA-7.

Incidental 68Ga-Prostate-Specific Membrane Antigen Uptake in Compression Fracture of a Vertebral Body

ABSTRACT

Prostate-specific membrane antigen (PSMA) is a membrane glycoprotein, which is overexpressed in prostate cancer cells. With its wide use, there is a growing number of case reports describing non-prostate cancer-related benign and malignant lesions showing increased 68Ga-PSMA uptake. We herein present the case of an 89-year-old man with prostate cancer who was referred for 68Ga-PSMA PET/CT for restaging, which revealed incidental 68Ga-PSMA uptake in compression fracture of a vertebral body. This case demonstrates that PSMA expression may occur in acute compression fractures, and it can be a potential pitfall when reporting 68Ga-PSMA PET/CT images.

PET/CT Variants and Pitfalls in Liver, Biliary Tract, Gallbladder and Pancreas

A wide variety of pathological anomalies may occur in the liver, biliary system, and pancreas. It is a necessity to use many different imaging techniques in order to distinguish such varied pathologies, especially those from malignant processes. Positron Emission Tomography/Computed Tomography (PET/CT) is an imaging method that has proven its diagnostic value in oncology and can be used for different clinical purposes. Fluoro-18 fluoro-2-deoxy-D-glucose has a wide range of uses as a dominant radiopharmaceutical in routine molecular imaging, however, molecular imaging has started to play a more important role in personalized cancer treatment in recent years with new Fluoro-18 and Gallium-68 labeled tracers. Although molecular imaging has a strong diagnostic effect, the surprises and pitfalls of molecular imaging can lead us to unexpected and misleading results. Prior to PET/CT analysis and reporting, information about possible technical and physiological pitfalls, normal histological features of tissues, inflammatory pathologies, specific clinical features of the case, treatment-related complications and past treatments should be evaluated in advance to avoid misinterpretation. In this review, the physiological and pathophysiological variants as well as pitfalls encountered in PET/CT imaging of the liver, biliary tract, gallbladder, and pancreas will be examined. Other benign and malignant pathologies that have been reported to date and that have led to incorrect evaluation will be listed. It is expected that the devices, software, and artificial intelligence applications that will be developed in the near future will enable much more effective and faster imaging that will reduce the potential causes of error. However, as a result of the dynamic and evolving structure of the information obtained by molecular imaging, the inclusion of the newly developed radiopharmaceuticals in routine practice will continue to carry new potentials as well as new troubles. Although molecular imaging will be the flagship of diagnostic oncology in the 21st century, the correct analysis and interpretation by the physician will continue to form the basis of achieving optimal performance.

Ring Sclerosis in Prostate Cancer: Circle of Malignancy or Benignity?

ABSTRACT

Prostate cancer osteoblastic metastases may have different morphologies, and some of these may overlap with certain benign bone lesions. In this series of 5 prostate cancer patients, we describe bone lesions with central lucency and surrounding peripheral sclerosis and their varying appearances on different imaging modalities. Although prostate cancer metastases are commonly associated with sclerotic lesions, they can also present as osteolytic or lucent lesions, and these lesions should be carefully evaluated. The findings emphasize the importance of correlation with prior imaging, comparing findings on different imaging techniques and follow-up to differentiate benign disease from metastatic disease in these situations.

Parameters predicting [18F]PSMA-1007 scan positivity and type and number of detected lesions in patients with biochemical recurrence of prostate cancer

BACKGROUND

Detection of the site of recurrence using PSMA-PET/CT is important to guide treatment in patients with biochemical recurrence of prostate cancer (PCa). The aim of this study was to evaluate the positivity rate of [¹⁸F]PSMA-1007-PET/CT in patients with biochemically recurrent PCa and identify parameters that predict scan positivity as well as the type and number of detected lesions. This monocentric retrospective study included 137 PCa patients with biochemical recurrence who underwent one or more [¹⁸F]PSMA-1007-PET/CT scans between August 2018 and June 2019. PET-positive malignant lesions were classified as local recurrence, lymph node (LN), bone or soft tissue lesions. The association between biochemical/paraclinical parameters, as PSA value, PSA doubling time, PSA velocity, Gleason score (GS) and androgen deprivation therapy (ADT), and scan positivity as well as type and number of detected lesions was evaluated using logistic regression analysis (binary outcomes) and Poisson models (count-type outcomes).

RESULTS

We included 175 [¹⁸F]PSMA-1007-PET/CT scans after radical prostatectomy (78%), external beam radiation therapy (8.8%), ADT (7.3%), brachytherapy (5.1%) and high intensity focused ultrasound (0.7%) as primary treatment (median PSA value 1.6 ng/ml). Positivity rate was 80%. PSA value and PSA velocity were significant predictors of scan positivity as well as of the presence of bone and soft tissue lesions and number of bone, LN and soft tissue lesions, both in uni- and/or multivariable analysis. Multivariable analysis also showed prior ADT as predictor of bone and soft tissue lesions, GS as predictor of the number of bone lesions and ongoing ADT as predictor of the number of LN lesions.

CONCLUSION

[¹⁸F]PSMA-1007-PET/CT showed a high positivity rate in patients with biochemically recurrent PCa. PSA value and PSA velocity were significant predictors of scan positivity as well as of the presence and number of bone and soft tissue lesions and the number of LN lesions. Our findings can guide clinicians in optimal patient selection for [¹⁸F]PSMA-1007-PET/CT and support further research leading to the development of a prediction nomogram.

Intra-Individual Comparison of 18F-PSMA-1007 and 18F-FDG PET/CT in the Evaluation of Patients With Prostate Cancer

Purpose

18F labelled PSMA-1007 presents promising results in detecting prostate cancer (PC), while some pitfalls exists meanwhile. An intra-individual comparison of 18F-FDG and 18F-PSMA-1007 in patients with prostate cancer were aimed to be performed in the present study. Then, the pitfalls of 18F-PSMA-1007 PET/CT in imaging of patients with prostate cancer were analyzed.

Methods and Material

21 prostate cancer patients underwent 18F-PSMA-1007 PET/CT as well as 18F-FDG PET/CT before treatment. All positive lesions were noticed in both 18F-PSMA-1007 PET/CT and 18F-FDG PET/CT, then differentiated PC metastasis from benign lesions. the SUVmax, SUVmean and TBR of lesions, up to 10 metastases and 10 benign lesions per patients were recorded (5 for bone, 5 for soft tissue metastasis ). The distribution of positive lesions were analyzed for two imaging. Detection rates, SUVmax, SUVmean and TBR in 18F-PSMA-1007 PET/CT and 18F-FDG PET/CT were compared, respectively. The optimal cut-off values of SUVmax, SUVmean for metastases vs. benign lesions was found through areas under ROC in 18F-PSMA-1007.

Results

The detection rates of primary lesions in 18F-PSMA-1007 PET/CT was higher than that of 18F-FDG PET/CT(100% (21/21) vs. 67%(14/21)). For extra- prostatic lesions, 18F-PSMA-1007 PET/CT revealed 124 positive lesions, 49(49/124, 40%) attributed to a benign origin; 18F-FDG PET/CT revealed 68 positive lesions, 14(14/68, 21%) attributed to a benign origin. The SUVmax, SUVmean, TBR of primary tumor in 18F-PSMA-1007 PET/CT was higher than that in 18F-FDG PET/CT (15.20 vs. 4.20 for SUVmax; 8.70 vs. 2.80 for SUVmean; 24.92 vs. 4.82 for TBR, respectively); The SUVmax, SUVmean, TBR of metastases in 18F-PSMA-1007 PET/CT was higher than that in 18F-FDG PET/CT (10.72 vs. 4.42 for SUVmax; 6.67 vs. 2.59 for SUVmean; The TBR of metastases was 13.3 vs. 7.91). For 18F-FDG PET/CT, the SUVmax, SUVmean in metastases was higher than that in benign lesions (4.42 vs. 3.04 for SUVmax, 2.59 vs. 1.75 for SUVmean, respectively). Similarly, for 18F-PSMA-1007 PET/CT, the SUVmax, SUVmean in metastases was significantly higher than that in benign lesions(10.72 vs. 3.14 for SUVmax, 6.67 vs. 1.91 for SUVmean, respectively), ROC suggested that SUVmax=7.71, SUVmean=5.35 might be the optimal cut-off values for metastases vs. benign lesions.

Conclusion

The pilot study suggested that 18F-PSMA-1007 showed superiority over 18F-FDG because its high detecting rate of PC lesions and excellent tumor uptake. While non-tumor uptake in 18F-PSMA-1007 may lead to misdiagnosis, recognizing these pitfalls and careful analysis can improve the accuracy of diagnosis.

68Ga-Prostate-Specific Membrane Antigen Focal Pulmonary Activity With No Corresponding CT Abnormality

ABSTRACT

We describe a case of 68Ga-prostate-specific membrane antigen PET/CT scan with focal tracer concentration in the lung without any abnormalities in the CT part. 18F-FDG focal uptake in lungs without corresponding CT lesions is described. Two possible mechanisms proposed to explain this finding in FDG PET/CT scans are iatrogenic pulmonary microembolism and 18F-FDG-avid infection or inflammation that precedes structural findings. Logically, these proposed mechanisms seem to be applicable to 68Ga-prostate-specific membrane antigen as well. In our case, there were no features of any lung infection/inflammation, and the most plausible explanation would be iatrogenic microembolism.

False-Positive 18F-Prostate-Specific Membrane Antigen-1007 PET/CT Caused by Hepatic Multifocal Inflammatory Foci

ABSTRACT

A 68-year-old man with history of treated prostate cancer was referred to 68Ga-prostate-specific membrane antigen (PSMA)-11 PET/CT imaging. Moderate bone involvement was observed, with a normal liver tracer uptake. Bone biopsy confirmed metastases of prostate cancer. After therapy adjustment, a follow-up 18F-PSMA-1007 PET/CT revealed good response in bone lesions, although it showed 3 new rounded liver intense uptake foci. Concurrent PSA was 0.6 ng/mL. Liver function tests were normal. PET/CT-guided hepatic biopsy demonstrated no malignant cells, focal inflammation, and steatosis, being possibly the inflammation the cause of false-positive multifocal uptake of 18F-PSMA-1007. Following PET/CT, controls showed a normal liver.

PSMA whole-body tumor burden in primary staging and biochemical recurrence of prostate cancer

PURPOSE

The objective of this study was to evaluate whether 68Ga-PSMA PET/CT whole-body tumor burden (PSMAwbtb) is associated with clinical parameters and laboratory parameters in prostate cancer patients.

METHODS

We retrospectively evaluated prostate cancer patients submitted to PSMA PET/CT for primary staging purposes or due to biochemical recurrence (BR). PSMAwbtb metrics (total volume of PSMA-avid tumor (PSMA-TV)) and total uptake of PSMA-avid lesions (PSMA-TL) were calculated semi-automatically. Spearman's rank correlations between PSMAwbtb metrics and clinical, laboratory parameters (age, time-to-BR, years of diagnosis of prostate cancer, free and total serum PSA levels, and the Gleason score) and with the highest SUVmax of a lesion (hSUVmax) were analyzed.

RESULTS

Among the 257 PSMA PET/CT studies, there were 46 scans (17.9%) performed for primary staging and 211 (82.1%) for BR. PSMA-TV and PSMA-TL were calculated for the 157 positive scans (58.8%), which were 43 patients (93.5%) in the primary staging group and 114 patients (54.0%) in the BR group. In the primary staging group, we observed a significant correlation between PSMA-TL and hSUVmax (p = 0.0021). In the BR group, there was a significant direct correlation between PSMA-TL and the variables age (p = 0.0031), total serum PSA values (p = < 0.0001), free serum PSA values (p = < 0.0001), and the hSUVmax (p = < 0.0001). Similar results were obtained for PSMA-TV.

CONCLUSION

PSMAwbtb has a direct and positive correlation with serum PSA values and age in prostate cancer patients with BR.

Assessment of the role of Ga-68 PSMA I&T PET/CT in response evaluation to docetaxel therapy in castration resistant prostate cancer patients

OBJECTIVE

There have been only few studies investigating the role of PSMA ligands in the therapy response assessment of metastasized castration resistant prostate cancer (mCRPC) cases. In this study we aimed at evaluating the capability of ⁶⁸Ga- prostate-specific membrane antigen (PSMA) I&T positron emission tomography/computerized tomography (PET/CT) in the assessment of therapeutic response in patients under docetaxel therapy for prostate cancer (PCa).

MATERIAL AND METHODS

The clinical records of all mCRPC patients treated with docetaxel and referred to our department for ⁶⁸Ga-PSMA I&T PET/CT imaging were retrospectively analysed. Sixteen patients (mean age 69 years, range 52-82 years) with castration-resistant prostate cancer patients receiving palliative docetaxel therapy and had undergone ⁶⁸Ga-PSMA I&T PET/CT scan were included in the study. ⁶⁸Ga-PSMA I&T PET/CT imaging was done and prostate specific antigen (PSA) levels were measured at baseline before administration of docetaxel (PET1) and after at least 3 cycles (range 4-12) of chemotherapy (PET2). Patient-based as well as lesion-based comparison of PET2 findings with PET1 findings were done.

RESULTS

The change (decrease) observed in lymph node and prostate gland/prostatic bed SUVmax values after treatment compared to pretreatment was found to be statistically significant (P=.033). 3/16 patients (19%) were classified as progressive disease (PD), 4/16 (25%) as stable disease (SD), 9/16 (56%) as partial remission (PR) radiologically. An increasing PSA trend (IT) was observed in 4 patients (25%) and a decreasing PSA trend (DT) in 3 patients (18%). Nine patients showed a PSA response of ≥ 50% (56%). Of the 4 patients showing SD, 3 had IT, 3 had BR. Of the 9 patients who showed PR on PET studies, 8 patients showed BR and 1 patient showed DT.

CONCLUSION

Imaging with ⁶⁸Ga-PSMA PET/CT showed great concordance with biochemical response evaluation in terms of PSA levels, especially in patients showing good response to therapy. ⁶⁸Ga-PSMA PET/CT was also successful in identifying progressive disease in patients showing paradoxical decline in PSA levels.

Potential Causes of False-Negative Interpretations in 68Ga-PSMA PET/CT for the Detection of Local and Recurrent Prostate Cancer: An Underexposed Issue

Ga-PSMA PET/CT has emerged as a highly promising diagnostic method for the evaluation of prostate cancer. Although it is increasingly recognized that its specificity is imperfect, data on false-negatives in the prostate have been underreported. We present 3 cases with false-negatives for local (recurrent) prostate cancer on Ga-PSMA PET/CT, using multiparametric MRI and histopathology as reference standard. Metal artifact of prostate gold markers placed for external beam radiation therapy, low level of PSMA uptake, and bladder spillover are potential causes of misinterpretation. Awareness of these diagnostic pitfalls may improve Ga-PSMA PET/CT interpretation.

Contribution of 5th minute and 2nd hour images to standard imaging in (68Ga)PSMA 11 PET/CT

OBJECTIVE

To investigate the superiority or contribution of 5th minute pelvic and 2nd hour whole body Gallium68-prostate-specific membrane antigen-HBED-CC [(⁶⁸Ga)PSMA 11] Positron Emission Tomography/Computed Tomography (PET/CT) images to 1st hour imaging in patients with prostate cancer (PCa).

MATERIALS AND METHODS

A total of 63 patients diagnosed with PCa who underwent (⁶⁸Ga)PSMA 11 PET/CT between April 2019 and June 2019 and who had 5th minute and 1st and 2nd hour images were included in the study. Early (5th minute) pelvic region and 1st and 2nd hour full body images were obtained from all patients. The regions of interest (ROI) were drawn from the background tissues and the physiological uptake sites in a way to include the same lesions from primary and metastatic lesions in all three imagings, and SUVmax values, and tumor-background ratio (TBR) were calculated.

RESULTS

The mean age of the patients was 69.81 ± 8.78 (min/max: 51/91) years. In the 5th minute images, prostate gland and bed were easier to evaluate, because of low bladder activity. However, lymph node evaluation was more difficult due to high vascular activity. In the prostate gland, lymph nodes and bone metastases, both SUVmax values and TBR rates increased with time from the 5th minute (p < 0.001). At the 2nd hour, some lesions became more visible, while decreased activity was observed in some lesions. However, none of the patients required a change in the stage or treatment.

CONCLUSION

In conclusion, the 5th minute pelvic images in (⁶⁸Ga)PSMA 11 PET/CT were helpful in visual evaluation of the prostate gland and bed, while 2nd hour images showed high SUVmax and TBR rates in malignant lesions. As the SUVmax values of benign lesions were found to be lower in the 2nd hour, when compared to the 1st hour, it was thought that the 2nd hour imaging could be used in the additional imaging for suspicious lesions without the need for very long waiting times.

Matched-Pair Comparison of 68Ga-PSMA-11 PET/CT and 18F-PSMA-1007 PET/CT: Frequency of Pitfalls and Detection Efficacy in Biochemical Recurrence After Radical Prostatectomy

¹⁸F-labeled prostate-specific membrane antigen (PSMA)-ligand PET has several principal advantages over ⁶⁸Ga-PSMA-11. The purpose of this retrospective study was to evaluate the frequency of non-tumor-related uptake and the detection efficacy comparing ⁶⁸Ga-PSMA-11 PET/CT and ¹⁸F-PSMA-1007 PET/CT in recurrent prostate cancer (PC) patients. Methods: The study included 102 patients with biochemically recurrent PC after radical prostatectomy undergoing ¹⁸F-PSMA-1007 PET/CT imaging. On the basis of various clinical variables, patients with corresponding ⁶⁸Ga-PSMA-11 PET/CT scans were matched. All PET/CT scans (n = 204) were reviewed by 1 nuclear medicine physician. First, all PET-positive lesions were noted. Then, lesions suspected of being recurrent PC were differentiated from lesions attributed to a benign origin on the basis of known pitfalls and information from CT. For each region, the SUV_max of the lesion with the highest PSMA-ligand uptake was noted. Detection rates were determined, and SUV_max was compared separately for ⁶⁸Ga-PSMA-11 and ¹⁸F-PSMA-1007. Results: In total, ¹⁸F-PSMA-1007 PET and ⁶⁸Ga-PSMA-11 PET revealed 369 and 178 PSMA-ligand-positive lesions, respectively. ¹⁸F-PSMA-1007 PET revealed approximately 5 times more lesions attributed to a benign origin than did ⁶⁸Ga-PSMA-11 PET (245 vs. 52 lesions, respectively). The benign lesions most frequently observed were ganglia, unspecific lymph node, and bone lesions, at a rate of 43%, 31%, and 24% for ¹⁸F-PSMA-1007 PET and 29%, 42%, and 27% for ⁶⁸Ga-PSMA-11 PET, respectively. The SUV_max of lesions attributed to a benign origin was significantly higher (P < 0.0001) for ¹⁸F-PSMA-1007 PET. Further, a similar number of lesions was attributed to recurrent PC (124/369 for ¹⁸F-PSMA-1007 PET and 126/178 for ⁶⁸Ga-PSMA-11 PET). Conclusion: The number of lesions with increased PSMA-ligand uptake attributed to a benign origin is considerably higher for ¹⁸F-PSMA-1007 PET than for ⁶⁸Ga-PSMA-11 PET. This finding indicates the need for sophisticated reader training emphasizing known pitfalls and reporting within the clinical context.

68Ga-PSMA PET thyroid incidentalomas

INTRODUCTION

Thyroid incidentaloma is defined as a thyroid lesion incidentally and newly detected by imaging techniques performed for an unrelated purpose and especially for a non-thyroid disease. The aim of this review is to evaluate the prevalence and clinical significance of focal incidental radiolabelled prostate-specific membrane antigen (PSMA) uptake in the thyroid gland [PSMA thyroid incidentaloma (PTI)] revealed by PET/CT or PET/MRI.

METHODS

A comprehensive literature search of the PubMed/MEDLINE, Scopus, and Embase databases was conducted to find relevant published articles about the prevalence and clinical significance of PTIs detected by PET/CT or PET/MRI in patients studied for other oncologic purposes.

RESULTS

Twelve articles were included in the systematic review. Among 23 PTIs, 6 were malignant (5 primary thyroid tumors and one metastasis from renal cell carcinoma), one was a follicular lesion of undetermined significance, and the rest were benign.

CONCLUSION

Despite being very rare, though probably underestimated, PTIs frequently signal the presence of unexpected lesions in the thyroid which differ from the indicated reason for which the patient was initially scanned and concerning which the risk of malignancy is not negligible.

68Ga-PSMA PET/CT for monitoring response to 177Lu-PSMA-617 radioligand therapy in patients with metastatic castration-resistant prostate cancer

PURPOSE

To evaluate the use of ⁶⁸Ga-PSMA PET/CT for monitoring response to ¹⁷⁷Lu-617 PSMA radioligand therapy in patients with metastatic castrate-resistant prostate cancer (mCRPC).

METHODS

Patients from the University Hospital Bonn and the University Hospital Aachen were retrospectively reviewed for this study. We included 48 patients with mCRPC who were treated with ¹⁷⁷Lu-PSMA-617 and whose records included ⁶⁸Ga-PSMA PET/CT imaging before the first and after the third or fourth treatment cycle. A treatment response based on ⁶⁸Ga-PSMA PET/CT was defined according to a modified version of the PERCIST criteria. A decline in PSA level of ≥50% was considered the reference standard. The sensitivity, specificity, positive and negative predictive values, and ROC curves were calculated, and patient survival times in relation to the PET results were also analysed.

RESULTS

⁶⁸Ga-PSMA PET/CT had a sensitivity of about 85% and a specificity of between 55% and 65%. The negative and positive predictive values ranged between 70% and 78%. The fitted ROC area was 0.70. The survival time was about 19.6 months in patients with a treatment response, while nonresponders had a survival time of about 15.9 months. However, this difference between the groups was not statistically significant.

CONCLUSION

Our results indicate that ⁶⁸Ga-PSMA PET/CT could be a useful tool for the evaluation of response to ¹⁷⁷Lu-PSMA-617 radioligand therapy within a theranostic framework.