Estimation of radiation dosimetry for 68Ga-HBED-CC (PSMA-11) in patients with suspected recurrence of prostate cancer

Preclinical Evaluation and Pilot Clinical Study of 18F-Labeled Inhibitor Peptide for Noninvasive Positron Emission Tomography Mapping of Angiotensin Converting Enzyme 2

Angiotensin-converting enzyme 2 (ACE2) is the main molecular target for coronavirus SARS-CoV-2 to enter cells. Molecularly specific tracers that bind to ACE2 with high affinity can be used to determine the tissue distribution of this important receptor, noninvasively. A novel targeting PET imaging probe, [18F]AlF-DX600-BCH, was developed to detect the in vivo expression of ACE2 and monitor response to therapy. Preclinical experiments, including biodistribution, PET imaging, and tissue section analysis, were conducted after tests of in vitro and in vivo stability and pharmacokinetics. The agent was advanced to clinical evaluation in 10 volunteers who received [18F]AlF-DX600-BCH PET/CT at 1 and 2 h after injection (NCT04542863). Preclinical results of both biodistribution and PET demonstrated [18F]AlF-DX600-BCH accumulation in rat kidney (standardized uptake value; SUVkidney/normal > 50), along with specific uptake in testes (SUVtestis/normal > 10) tissues. Kidney, gastrointestinal, and bronchial cell labeling were correlated to ACE2 positive by immunohistochemistry (IHC) staining. In clinical imaging, significant tracer accumulation was predominantly observed in the urinary and reproductive system (SUVrenal cortex = 32.00, SUVtestis = 4.56), and the conjunctiva and nasal mucosa saw elevated uptake in several cases. This work is the first report of a radioisotope probe, [18F]AlF-DX600-BCH, targeting ACE2 with promising preliminary preclinical and translational outlook, thereby demonstrating the potential of noninvasive mapping of ACE2.

Biodistribution and radiation dosimetry of [99mTc]Tc-N4-BTG in patients with biochemical recurrence of prostate cancer

BACKGROUND

In patients with prostate cancer (PCa), imaging with gastrin-releasing peptide receptor (GRPR) ligands is an alternative to PSMA-targeted tracers, particularly if PSMA expression is low or absent. [99mTc]Tc-N4-BTG is a newly developed GRPR-directed probe for conventional scintigraphy and single photon emission computed tomography (SPECT) imaging. The current study aims to investigate the safety, biodistribution and dosimetry of [99mTc]Tc-N4-BTG in patients with biochemical recurrence (BCR) of PCa.

RESULTS

No adverse pharmacologic effects were observed. Injection of [99mTc]Tc-N4-BTG resulted in an effective dose of 0.0027 ± 0.0002 mSv/MBq. The urinary bladder was the critical organ with the highest mean absorbed dose of 0.028 ± 0.001 mGy/MBq, followed by the pancreas with 0.0043 ± 0.0015 mGy/MBq, osteogenic cells with 0.0039 ± 0.0005 mGy/MBq, the kidneys with 0.0034 ± 0.0003 mGy/MBq, and the liver with 0.0019 ± 0.0004 mGy/MBq, respectively. No focal tracer uptake suggestive of PCa recurrence could be revealed for any of the patients.

CONCLUSION

[99mTc]Tc-N4-BTG appears to be a safe diagnostic agent. Compared to GRPR-targeted PET tracers, this 99mTc-labelled SPECT agent could contribute to a broader application and better availability of this novel approach. Further research to assess its clinical value is warranted.

Comparison of tracer kinetic models for 68Ga-PSMA-11 PET in intermediate-risk primary prostate cancer patients

BACKGROUND

68Ga-PSMA-11 positron emission tomography enables the detection of primary, recurrent, and metastatic prostate cancer. Regional radiopharmaceutical uptake is generally evaluated in static images and quantified as standard uptake values (SUVs) for clinical decision-making. However, analysis of dynamic images characterizing both tracer uptake and pharmacokinetics may offer added insights into the underlying tissue pathophysiology. This study was undertaken to evaluate the suitability of various kinetic models for 68Ga-PSMA-11 PET analysis. Twenty-three lesions in 18 patients were included in a retrospective kinetic evaluation of 55-min dynamic 68Ga-PSMA-11 pre-prostatectomy PET scans from patients with biopsy-demonstrated intermediate- to high-risk prostate cancer. Three kinetic models-a reversible one-tissue compartment model, an irreversible two-tissue compartment model, and a reversible two-tissue compartment model, were evaluated for their goodness of fit to lesion and normal reference prostate time-activity curves. Kinetic parameters obtained through graphical analysis and tracer kinetic modeling techniques were compared for reference prostate tissue and lesion regions of interest.

RESULTS

Supported by goodness of fit and information loss criteria, the irreversible two-tissue compartment model optimally fit the time-activity curves. Lesions exhibited significant differences in kinetic rate constants (K1, k2, k3, Ki) and semiquantitative measures (SUV and %ID/kg) when compared with reference prostatic tissue. The two-tissue irreversible tracer kinetic model was consistently appropriate across prostatic zones.

CONCLUSIONS

An irreversible tracer kinetic model is appropriate for dynamic analysis of 68Ga-PSMA-11 PET images. Kinetic parameters estimated by Patlak graphical analysis or full compartmental analysis can distinguish tumor from normal prostate tissue.

Comparison of Tracer Kinetic Models for 68Ga-PSMA-11 PET in Intermediate Risk Primary Prostate Cancer Patients

BACKGROUND

68Ga-PSMA-11 positron emission tomography enables the detection of primary, recurrent, and metastatic prostate cancer. Regional radiopharmaceutical uptake is generally evaluated in static images and quantified as standard uptake values (SUV) for clinical decision-making. However, analysis of dynamic images characterizing both tracer uptake and pharmacokinetics may offer added insights into the underlying tissue pathophysiology. This study was undertaken to evaluate the suitability of various kinetic models for 68Ga-PSMA-11 PET analysis. Twenty-three lesions in 18 patients were included in a retrospective kinetic evaluation of 55-minute dynamic 68Ga-PSMA-11 pre-prostatectomy PET scans from patients with biopsy-demonstrated intermediate to high-risk prostate cancer. A reversible one-tissue compartment model, irreversible two-tissue compartment model, and a reversible two-tissue compartment model were evaluated for their goodness-of-fit to lesion and normal reference prostate time-activity curves. Kinetic parameters obtained through graphical analysis and tracer kinetic modeling techniques were compared for reference prostate tissue and lesion regions of interest.

RESULTS

Supported by goodness-of-fit and information loss criteria, the irreversible two-tissue compartment model was selected as optimally fitting the time-activity curves. Lesions exhibited significant differences in kinetic rate constants (K1, k2, k3, Ki) and semiquantitative measures (SUV) when compared with reference prostatic tissue. The two-tissue irreversible tracer kinetic model was consistently appropriate across prostatic zones.

CONCLUSIONS

An irreversible tracer kinetic model is appropriate for dynamic analysis of 68Ga-PSMA-11 PET images. Kinetic parameters estimated by Patlak graphical analysis or full compartmental analysis can distinguish tumor from normal prostate tissue.

Biodistribution and Internal Dosimetry of 68 Ga-DOTA-IBA PET Imaging for Patients With Bone Metastases

PURPOSE

We have developed a new pharmaceutical, ibandronic acid (IBA), and preliminarily demonstrated that it is an efficient bisphosphonate for the diagnosis and treatment of bone metastases. This study aims to examine the biodistribution and internal dosimetry of the diagnostic 68 Ga-DOTA-IBA in patients.

PATIENTS AND METHODS

68 Ga-DOTA-IBA was intravenously injected based on 1.81-2.57 MBq/Kg into 8 patients with bone metastases. Each patient underwent 4 sequential static whole-body PET scans at 0.1, 0.45, 0.8, and 1.8 hours after injection. The acquisition time for each scan was 20 minutes with 10 bed positions. Image registrations and volume of interest delineation were first performed on Hermes, whereas percentage injected activity (%IA), absorbed dose, and effective dose were measured for source organs, using OLINDA/EXM v2.0. Dosimetrics for the bladder was based on a bladder voiding model.

RESULTS

No adverse effects were observed on all patients. After the injection, 68 Ga-DOTA-IBA rapidly accumulated in bone metastases and cleared from nonbone tissues, as indicated by visual analysis and %IA measured on the sequential scans. High activity uptake was presented in the expected target organs, that is, bone, red marrow, and the drug-excretion organs such as kidneys and bladder. The mean total body effective dose is 0.022 ± 0.002 mSv/MBq.

CONCLUSIONS

68 Ga-DOTA-IBA has high bone affinity and is promising in the diagnosis of bone metastases. Dosimetric results show that the absorbed doses for critical organs and total body are within the safety limit and with high bone retention. It also has the potential to be used in 177 Lu-therapy as a theranostic pair.

Predicting the effect of different folate doses on [68Ga]Ga-PSMA-11 organ and tumor uptake using physiologically based pharmacokinetic modeling

BACKGROUND

Folate intake might reduce [⁶⁸Ga]Ga-PSMA-11 uptake in tissues due to a competitive binding to the PSMA receptor. For diagnostic imaging, this could impact decision making, while during radioligand therapy this could affect treatment efficacy. The relationship between folate dose, timing of dosing and tumor and organ uptake is not well established. The aim of this study was to develop a physiologically based pharmacokinetic (PBPK) model to predict the effect of folates on [⁶⁸Ga]Ga-PSMA-11 PET/CT uptake in salivary glands, kidneys and tumors.

METHODS

A PBPK model was developed for [⁶⁸Ga]Ga-PSMA-11 and folates (folic acid and its metabolite 5-MTHF), with compartments added that represent salivary glands and tumor. Reactions describing receptor binding, internalization and intracellular degradation were included. Model evaluation for [⁶⁸Ga]Ga-PSMA-11 was performed by using patient scan data from two different studies (static and dynamic), while for folates data from the literature were used for evaluation. Simulations were performed to assess the effect of different folate doses (150 µg, 400 µg, 5 mg and 10 mg) on accumulation in salivary glands, kidney and tumor, also for patients with different tumor volumes (10, 100, 500 and 1000 mL).

RESULTS

Final model evaluation showed that predictions adequately described data for both [⁶⁸Ga]Ga-PSMA-11 and folates. Predictions of a 5-MTFH dose of 150 µg and folic acid dose of 400 µg (in case of administration at the same time as [⁶⁸Ga]Ga-PSMA-11 (t = 0)) showed no clinically relevant effect on salivary glands and kidney uptake. However, the effect of a decrease in salivary glands and kidney uptake was determined to be clinically relevant for doses of 5 mg (34% decrease for salivary glands and 32% decrease for kidney) and 10 mg (36% decrease for salivary glands and 34% decrease for kidney). Predictions showed that tumor uptake was not relevantly affected by the co-administration of folate for all different folate doses (range 150 µg-10 mg). Lastly, different tumor volumes did not impact the folate effect on [⁶⁸Ga]Ga-PSMA-11 biodistribution.

CONCLUSION

Using a PBPK model approach, high doses of folate (5 and 10 mg) were predicted to show a decrease of [⁶⁸Ga]Ga-PSMA-11 salivary glands and kidney uptake, while intake by means of folate containing food or vitamin supplements showed no relevant effects. In addition, tumor uptake was not affected by folate administration in the simulated dose ranges (150 µg-10 mg). Differences in tumor volume are not expected to impact folate effects on [⁶⁸Ga]Ga-PSMA-11 organ uptake.

Assessing extra-prostatic extension for surgical guidance in prostate cancer: Comparing two PSMA-PET tracers with the standard-of-care

BACKGROUND

Incontinence and impotence occur following radical prostatectomy due to injury to nerves and sphincter muscle. Preserving nerves and muscle adjacent to prostate cancer risks positive surgical margins. Advanced imaging with MRI has improved cancer localization but limitations exist.

OBJECTIVE

To measure the accuracy for assessing extra-prostatic extension at nerve bundles for 2 PSMA-PET tracers and to compare the PET accuracy to standard-of-care predictors including MRI and biopsy results.

MATERIALS AND METHODS

We studied men with PSMA-targeted PET imaging, performed prior to prostatectomy in men largely with intermediate to high-risk prostate cancer, and retrospectively evaluated for assessment of extra-prostatic extension with whole-mount analysis as reference standard. Two different PSMA-PET tracers were included: 68Ga-PSMA-11 and 68Ga-P16-093. Blinded reviews of the PET and MRI scans were performed to assess extra-prostatic extension (EPE). Sensitivity and specificity for extra-prostatic extension were compared using McNemar's Chi2.

RESULTS

Pre-operative PSMA-PET imaging was available for 71 patients with either 68Ga-P16-093 (n = 25) or 68Ga-PSMA-11 (n = 46). There were 24 (34%) with pT3a (EPE) and 16 (23%) with pT3b (SVI). EPE Sensitivity (87% vs. 92%), Specificity (77% vs. 76%), and ROC area (0.82 vs. 0.84) were similar between P16-093 and PSMA-11, respectively (P = 0.87). MRI (available in only 45) found high specificity (83%) but low sensitivity (60%) for EPE when using a published grading system. MRI sensitivity was significantly lower than the PSMA-PET (60% vs. 90%, P = 0.02), but similar to PET when using a >5 mm capsular contact (76% vs. 90%, P = 0.38). A treatment change to "nerve sparing" was recommended in 21 of 71 (30%) patients based on PSMA-PET imaging.

CONCLUSIONS

Presurgical PSMA-PET appeared useful as a tool for surgical planning, changing treatment plans in men with ≥4+3 or multi-core 3+4 prostate cancer resulting in preservation of nerve-bundles.

68Ga-DOTA-DiPSMA PET/CT Imaging: Biodistribution, Dosimetry, and Preliminary Application in Prostate Cancer

Purpose: This prospective trial aimed to evaluate the safety, dosimetry, and biodistribution of a novel theranostic probe ⁶⁸Ga-DOTA-DiPSMA. Also, we have performed the first preliminary application with ⁶⁸Ga-DOTA-DiPSMA in prostate cancer (PCa) patients.

Methods: Five healthy volunteers and ten PCa patients were injected with an intravenous bolus of ⁶⁸Ga-DOTA-DiPSMA. They received serial whole-body PET scans from the time of injection up to 60 min post-injection, with a second PET/CT scanning at 120 min post-injection. In PCa patients, low-dose CT scan and whole-body PET were performed with 2 min per bed position in 40 min post-injection. Absorbed organ doses and effective doses were calculated using OLINDA/EXM. Normal organ uptake and tumor lesion uptake were measured. A lesion-by-lesion analysis was performed.

Results: ⁶⁸Ga-DOTA-DiPSMA administration was safe and well-tolerated. The kidneys received the highest absorbed dose (114.46 ± 29.28 μSv/MBq), followed by the urinary bladder wall (100.82 ± 46.22 μSv/MBq) in accordance with the expected Prostate-Specific Membrane Antigen (PSMA) renal excretion of the tracer. The mean effective dose was 19.46 ± 1.73 μSv/MBq. The SUV_max of ⁶⁸Ga-DOTA-DiPSMA PET/CT for PCa lesions, bone metastases, and lymph node metastases was 4.41 ± 2.72, 2.95 ± 1.11, and 3.26 ± 1.20, respectively.

Conclusion: Injection of ⁶⁸Ga-DOTA-DiPSMA is safe and associated with low absorbed and effective doses. ⁶⁸Ga-DOTA-DiPSMA shows favorable kinetics and imaging characteristics in patients who warrant further head-to-head comparison to validate ⁶⁸Ga-DOTA-DiPSMA as an alternative for gallium-68-labeled PSMA clinical PET. Low nonspecific uptake in normal organs of ⁶⁸Ga-DOTA-DiPSMA indicates potential radioligand therapy (RLT) application when labeled with ¹⁷⁷Lu, ⁹⁰Y, or ²²⁵Ac.

[68Ga]Ga-P16-093 as a PSMA-Targeted PET Radiopharmaceutical for Detection of Cancer: Initial Evaluation and Comparison with [68Ga]Ga-PSMA-11 in Prostate Cancer Patients Presenting with Biochemical Recurrence

PURPOSE

This study was undertaken to evaluate radiation dosimetry for the prostate-specific membrane antigen targeted [⁶⁸Ga]Ga-P16-093 radiopharmaceutical, and to initially assess agent performance in positron emission tomography (PET) detection of the site of disease in prostate cancer patients presenting with biochemical recurrence.

PROCEDURES

Under IND 133,222 and an IRB-approved research protocol, we evaluated the biodistribution and pharmacokinetics of [⁶⁸Ga]Ga-P16-093 with serial PET imaging following intravenous administration to ten prostate cancer patients with biochemical recurrence. The recruited subjects were all patients in whom a recent [⁶⁸Ga]Ga-PSMA-11 PET/X-ray computed tomography (CT) exam had been independently performed under IND 131,806 to assist in decision-making with regard to their clinical care. Voided urine was collected from each subject at ~ 60 min and ~ 140 min post-[⁶⁸Ga]Ga-P16-093 injection and assayed for Ga-68 content. Following image segmentation to extract tissue time-activity curves and corresponding cumulated activity values, radiation dosimetry estimates were calculated using IDAC Dose 2.1. The prior [⁶⁸Ga]Ga-PSMA-11 PET/CT exam (whole-body PET imaging at 60 min post-injection, performed with contrast-enhanced diagnostic CT) served as a reference scan for comparison to the [⁶⁸Ga]Ga-P16-093 findings.

RESULTS

[⁶⁸Ga]Ga-P16-093 PET images at 60 min post-injection provided diagnostic information that appeared equivalent to the subject's prior [⁶⁸Ga]Ga-PSMA-11 scan. With both radiopharmaceuticals, sites of tumor recurrence were found in eight of the ten patients, identifying 16 lesions. The site of recurrence was not detected with either agent for the other two subjects. Bladder activity was consistently lower with [⁶⁸Ga]Ga-P16-093 than [⁶⁸Ga]Ga-PSMA-11. The kidneys, spleen, salivary glands, and liver receive the highest radiation exposure from [⁶⁸Ga]Ga-P16-093, with estimated doses of 1.7 × 10^-1, 6.7 × 10^-2, 6.5 × 10^-2, and 5.6 × 10^-2 mGy/MBq, respectively. The corresponding effective dose from [⁶⁸Ga]Ga-P16-093 is 2.3 × 10^-2 mSv/MBq.

CONCLUSIONS

[⁶⁸Ga]Ga-P16-093 provided diagnostic information that appeared equivalent to [⁶⁸Ga]Ga-PSMA-11 in this limited series of ten prostate cancer patients presenting with biochemical recurrence, with the kidneys found to be the critical organ. Diminished tracer appearance in the urine represents a potential advantage of [⁶⁸Ga]Ga-P16-093 over [⁶⁸Ga]Ga-PSMA-11 for detection of lesions in the pelvis.

Development of an Albumin-Based PSMA Probe With Prolonged Half-Life

Prostate-specific membrane antigen (PSMA) is an attractive target for the diagnosis and therapy of prostate cancer as it is specifically overexpressed in prostate cancer cells. Improving the circulation of radioligands in the blood is considered as an effective strategy that can improve tumor burden, which benefits detection of small lesions and improves the effect of PSMA radioligand therapy (PRLT). In this study, we introduced maleimidopropionic acid (MPA) to a PSMA-targeted tracer and developed Al¹⁸F-PSMA-CM, which targets human serum albumin (HSA) binding and PSMA. Al¹⁸F-PSMA-CM is evaluated in vitro and in vivo for stability, PSMA specificity, and biodistribution in 22Rv1 tumor-bearing mice. Al¹⁸F-PSMA-CM was prepared with a radiochemical purity of >99% and specific activity of 11.22–18.70 MBq/nmol. Al¹⁸F-PSMA-CM was stable in vitro and in vivo and prolonged circulation in blood with a binding ratio of 47 ± 3.2% and Kd value of 3.08 ± 0.45 nM to HSA. The uptake of Al¹⁸F-PSMA-CM in PSMA(+) 22Rv1 cells was increased in 2 h, and the uptake was blocked by a PSMA inhibitor, ZJ-43. The Kd value of Al¹⁸F-PSMA-CM to PSMA was 8.46 ± 0.24 nM. Al¹⁸F-PSMA-CM was accumulated in kidneys and 22Rv1 tumors [74.76 ± 15.42 and 6.16 ± 0.74 ID%/g at 2 h post injection (p.i.)], which were decreased by −80.0 and −84.3% when co-injected with ZJ-43. Al¹⁸F-PSMA-CM showed high PSMA specificity and accumulated in 22Rv1 tumors with increasing uptake in 4 h. MPA moiety showed the ability to prolong the half-life of tracers, and the MPA-conjugated tracer showed the potential to improve tumor uptake. MPA may be a choice to develop radiopharmaceuticals for PRLT of prostate cancer.

Biodistribution and Radiation Dosimetric Analysis of [68Ga]Ga-RM2: A Potent GRPR Antagonist in Prostate Carcinoma Patients

[68Ga]Ga-RM2 is a promising innovative positron emission tomography (PET) tracer for patients with primary or metastatic prostate carcinoma. This study aims to analyze the biodistribution and radiation dosimetry of [68Ga]Ga-RM2 in five prostate cancer patients. The percentages of injected activity in the source organs and blood samples were determined. Bone marrow residence time was calculated using an indirect blood-based method. OLINDA/EXM version 2.0 (Hermes Medical Solutions, Stockholm, Sweden) was used to determine residence times, organ absorbed and effective doses. Physiological uptake was seen in kidneys, urinary bladder, pancreas, stomach, spleen and liver. Blood clearance was fast and followed by rapid clearance of activity from kidneys resulting in high activity concentrations in the urinary bladder. The urinary bladder wall was the most irradiated organ with highest mean organ absorbed dose (0.470 mSv/MBq) followed by pancreas (0.124 mSv/MBq), stomach wall (0.063 mSv/MBq), kidneys (0.049 mSv/MBq) and red marrow (0.010 mSv/MBq). The effective dose was found to be 0.038 mSv/MBq. Organ absorbed doses were found to be comparable to other gallium-68 labelled GRPR antagonists and lower than [68Ga]Ga-PSMA with the exception of the urinary bladder, pancreas and stomach wall. Remarkable interindividual differences were observed for the organ absorbed doses. Therefore, [68Ga]Ga-RM2 is a safe diagnostic agent with a significantly lower kidney dose but higher pancreas and urinary bladder doses as compared to [68Ga]Ga-PSMA.

68Ga/64Cu PSMA Bio-Distribution in Prostate Cancer Patients: Potential Pitfalls for Different Tracers

BACKGROUND

68Ga-PSMA is a widely useful PET/CT tracer for prostate cancer imaging. Being a transmembrane protein acting as a glutamate carboxypeptidase enzyme, PSMA is highly expressed in prostate cancer cells. PSMA can also be labeled with 64Cu, offering a longer half-life and different resolution imaging. Several studies documented bio-distribution and pitfalls of 68Ga-PSMA as well as of 64Cu- PSMA. No data are reported on differences between these two variants of PSMA. Our aim was to evaluate physiological distribution of these two tracers and to analyze false positive cases.

METHODS

We examined tracer bio-distribution in prostate cancer patients with negative 68Ga-PSMA PET/CT (n=20) and negative 64Ga-PSMA PET/CT (n=10). A diagnostic pitfall for each tracer was documented.

RESULT

Bio-distribution of both tracers was similar, with some differences due to renal excretion of 68Ga- PSMA and biliary excretion of 64Cu-PSMA. 68Ga-PSMA uptake was observed in sarcoidosis while 64Cu- PSMA uptake was recorded in pneumonitis.

DISCUSSION

Both tracers may present similar bio-distribution in the human body, with similar uptake in exocrine glands and high intestinal uptake. Similarly to other tracers, false positive cases cannot be excluded in clinical practice.

CONCLUSION

The knowledge of difference in bio-distribution between two tracers may help in interpretation of PET data. Diagnostic pitfalls can be documented, due to the possibility of PSMA uptake in inflammation. Our results are preliminary to future studies comparing diagnostic accuracies of 68Ga-PSMA and 64Cu-PSMA.

Radiation dosimetry of [68Ga]PSMA-11 in low-risk prostate cancer patients

BACKGROUND

68Ga-labeled Glu-NH-CO-NH-Lys(Ahx)-HBED-CC ([68Ga]PSMA-11) has been increasingly used to image prostate cancer using positron emission tomography (PET)/computed tomography (CT) both during diagnosis and treatment planning. It has been shown to be of clinical value for patients both in the primary and secondary stages of prostate cancer. The aim of this study was to determine the effective dose and organ doses from injection of [68Ga]PSMA-11 in a cohort of low-risk prostate cancer patients.

METHODS

Six low-risk prostate cancer patients were injected with 133-178 MBq [68Ga]PSMA-11 and examined with four PET/CT acquisitions from injection to 255 min post-injection. Urine was collected up to 4 h post-injection, and venous blood samples were drawn at 45 min, 85 min, 175 min, and 245 min post-injection. Kidneys, liver, lungs, spleen, salivary and lacrimal glands, and total body where delineated, and cumulated activities and absorbed organ doses calculated. The software IDAC-Dose 2.1 was used to calculate absorbed organ doses according to the International Commission on Radiological Protection (ICRP) publication 107 using specific absorbed fractions published in ICRP 133 and effective dose according to ICRP Publication 103. We also estimated the absorbed dose to the eye lenses using Monte Carlo methods.

RESULTS

[68Ga]PSMA-11 was rapidly cleared from the blood and accumulated preferentially in the kidneys and the liver. The substance has a biological half-life in blood of 6.5 min (91%) and 4.4 h (9%). The effective dose was calculated to 0.022 mSv/MBq. The kidneys received approximately 40 mGy after an injection with 160 MBq [68Ga]PSMA-11 while the lacrimal glands obtained an absorbed dose of 0.12 mGy per administered MBq. Regarding the eye lenses, the absorbed dose was low (0.0051 mGy/MBq).

CONCLUSION

The effective dose for [68Ga]PSMA-11 is 0.022 mSv/MBq, where the kidneys and lacrimal glands receiving the highest organ dose.

ESTIMATION OF THE ORGAN ABSORBED DOSES AND EFFECTIVE DOSE FROM 68Ga-PSMA-11 PET SCAN

68Ga-PSMA-11 PET/CT has been proven to have high clinical value for imaging of prostate cancer and rapidly gained popularity. In this study, we aimed to investigate absorbed doses of 68Ga-PSMA-11. Seven patients (mean age = 66.9 ± 6.6 years, range: 57-79 years) were enrolled in the study. Whole body PET images were acquired with multiple time points. MIRD method, NUKFIT and OLINDA/EXM software were used for dosimetry calculations. Kidneys, bladder wall, salivary and lacrimal glands received the highest absorbed dose. Estimated absorbed doses to these organs after injection of 150 MBq 68Ga-PSMA-11 were 37.0, 12.6, 14.4 and 6.3 mSv, respectively. Effective dose from PET scanning with 150 MBq injected 68Ga-PSMA-11 was 2.5 mSv. In conclusion, 68Ga-PSMA-11 has a favorable dosimetry profile similar to the 68Ga labeled octreotide analogs, which are used safely in routine clinical practices for many years. No adverse effects were reported. The kidneys were the dose-limiting organs.

Prostate-specific membrane antigen positron emission tomography in the management of recurrent prostate cancer

INTRODUCTION

There is an unmet clinical need for early, accurate imaging of recurrent prostate cancer to improve patient outcomes. Staging, by conventional bone scintigraphy and CT have become outdated. 68Ga-PSMA PET/CT imaging in this setting has developed rapidly, with widespread International adoption in line with evidence-based guidelines in this group of patients.

SOURCES OF DATA

A PubMed search of English language articles was performed using following keywords: PSMA, PET/CT, biochemical recurrence, prostate cancer. The search revealed 85 articles, of which 75 were original; 70 of these involved use of the most widely available type of PSMA tracer (HBED). The review also relied on the clinical experience of reporting over 1000 PSMA PET/CT studies at a major tertiary referral centre for uro-oncology, with the majority of cases having been performed in the biochemical recurrence setting from 2015 to 2018.

AREAS OF AGREEMENT

68Ga-PSMA PET is a game changer and superior to choline PET and other established tracers which have been used in prostate cancer evaluation. Detection of recurrence at the prostate bed remains challenging due to bladder and urethral tracer accumulation. The main strength of PSMA PET/CT is its ability to identify small (<8 mm) pathological lymph nodes, upstaging nodal status in up to two-thirds of cases. Additionally, PSMA PET/CT, detects bone and bone marrow metastases missed by conventional bone and CT imaging. Thus, PSMA PET/CT has major impact on patient management, with studies reporting overall changes in 39-76% of cases.

AREAS OF CONTROVERSY

Controversy exists regarding patient access and NHS affordability of PSMA PET/CT imaging. Currently, no reimbursement is available under the NHS tariff system. The cost outlay for tertiary hospital linked PET centres ranges from £150-170 K. Large referral volumes, and technical advances in manufacturing process will make this tracer cost neutral and similar to the current funded, but less sensitive, choline PET. Current NICE guidelines for prostate cancer management do not include a recommendation on when PSMA PET/CT should be used and this is likely to remain the case in the next revision, due in 2019.

GROWING POINTS

Although PSMA PET/CT imaging results in significant management change, there is a need for high quality economic evaluation and cost analysis for this modality. Lack of this data will result in poor adoption of this technique and thus limit patient access. Furthermore, it is hoped that future tracers will become even more sensitive and identify disease at earlier thresholds.

AREAS TIMELY FOR DEVELOPING RESEARCH

Well-designed clinical trials with consideration of the health economic benefit of using PSMA PET/CT will be essential to provide a basis for entry into guidelines such as NICE and to provide a rationale for reimbursement.