Before we introduce Ga-PSMA PET/CT, we need to understand PET/CT first.
What is PET/CT?
- CT is known as Computed Tomography (CT), which uses X-rays to examine the human body at the body level.
- PET, known as positron emission tomography (PET), is an imaging device that reflects the genetic, molecular, metabolic, and functional status of a lesion.
- PET/CT is a fusion of PET (functional metabolic imaging) and CT (anatomical structural imaging), combining the advantages of both PET’s functional imaging and spiral CT’s fine structural imaging into one, making it the most perfect medical imaging technology today and the most important invention in the field of tumor diagnosis in the last 20 years.
The various functional imaging of PET relies mainly on the radioactive contrast agent to perform, and basically the common PET/CT used in major hospitals, its positron radionuclide is F- FDG PET/CT, which is the most commonly used radioactive contrast agent for tumor diagnosis in clinical practice.
Malignant tumors have a significantly higher glucose metabolism than normal cells, and based on this property, in general, if FDG PET shows abnormal uptake of a local lesion, it suggests the presence of a malignant tumor, and, the higher this uptake value, the more malignant it is.
What are the advantages of PET/CT for prostate cancer?
High accuracy
Prostate specific antigen (PSA) is routinely selected for prostate cancer screening and is a very important biological indicator for prostate cancer screening, early diagnosis, and prognosis, but while its screening positive rate is about 80%, PET/CT screening has a positive rate of 93% and a specificity of 96%. Compared to PSA, PET/CT is more accurate.
In addition, PET/CT can clarify whether the prostate cancer is confined to the peri-peritoneum or breaks through it, and whether there is seminal vesicle invasion or bladder-rectum invasion.
The presence or absence of metastasis of prostate cancer can be determined
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The early symptoms of prostate cancer are not obvious, and when patients develop symptoms, they may have local infiltration or distant metastases. Bone and lymph node metastases are common in prostate cancer, with 70% to 80% of patients eventually developing bone metastases and 63% to 88% developing lymphatic metastases.
Common PET/CT uses radioactive F-FDG as a tracer, a type of glucose that is chemically similar to normal glucose and produces labeled metabolites in the body. Malignant tumor tissues are exceptionally metabolically active, characterized by rapid cell growth, high metabolism, and proliferation, and will require more glucose than normal tissues, and therefore, it has higher glucose concentrations than other tissues. This allows PET/CT to quickly identify prostate cancer whenever it appears as a bone or lymph node metastasis.
But F-FDG is not a tumor-specific tracer; inflammatory cells and granulomatous tissue can all take up F-FDG, so in some cases, relying solely on F-FDG PET/CT to identify benign and malignant can have a false-positive presentation.
Currently, efforts are underway worldwide to study and explore new effective and specific tracers through rigorous, scientific evidence-based medical strategies, with the aim of achieving higher clinical application of PET-CT.
What is Ga-PSMA PET/CT?
Ga-PSMA PET/CT is a new diagnostic imaging tool for prostate cancer by binding the metal radionuclide labeled drug Ga to the prostate-specific membrane antigen (PSMA), which has high sensitivity and specificity for prostate cancer and can be used throughout the body The results of this study are summarized below.
In which, PSMA is a type II cell membrane surface glycoprotein (also known as folate hydrolase I or glutamate carboxypeptidase II) with a structure that can be divided into extracellular, transmembrane, and intracellular segments, and its extracellular protein hydrolase contains a binuclear zinc finger structure and an arginine surface that binds to the substrate, is overexpressed in almost all prostate cancer cells, with higher levels of expression in intermediate to advanced prostate cancer or metastatic CRPC, and also will have normal physiological distribution in the following organs: salivary glands, liver, spleen, kidney, small intestine, and bladder.
That is, if high concentrations of radionuclides are found in the above organs, it does not necessarily mean that the tumor has spread. PSMA-mediated nuclide-targeted therapy shows great promise in the treatment of metastatic CRPC.
In addition, Ga-PSMA PET/CT compares favorably with conventional prostate cancer screening tools, including CT, MRI, bone scan, PET/MRI, cholinergic PET/CT, and Fluorodeoxyglucose, in terms of sensitivity, specificity, positive predictive value, and negative predictive value.
Of course, any test is subject to false-positive rates, and Ga-PSMA PET-CT is no exception: when combined with benign disease such as thyroid adenoma, Paget’s disease, nerve sheath tumor, tuberculosis, adrenal adenoma, splenic granuloma, and abdominal ganglion tumor, and malignant disease such as liver, lung, and kidney cancer, and thyroid cancer, there are False-positive results.
What are the unique advantages of Ga-PSMA PET/CT in the diagnosis of prostate cancer?
A 2016 article published in the prestigious international medical journal European Urology evaluated that Ga-PSMA PET/CT is far more accurate for prostate cancer diagnosis than traditional imaging, such as MRI, CT, and prostate ultrasound.
A domestic study also concluded that Ga-PSMA-11 PET/CT has a high diagnostic value for primary and metastatic foci, which can guide clinical decision making and treatment.
The Ga-PSMA PET/CT patient screening currently conducted by the Department of Urology at Beijing Hospital is mainly used for preoperative systemic assessment of prostate cancer, systemic assessment in case of biochemical recurrence after radical prostatectomy (radical resection or radical radiotherapy), and castration resistant prostate cancer (CRPC) This opens a window of hope for early detection, early diagnosis, and early treatment of the disease for patients whose recurrent and metastatic lesions could not be found with previous conventional imaging.
FAQs and Answers for Ga-PSMA PET/CT
1. Do I need to fast before doing Ga-PSMA PET/CT?
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No.
Because this scan is not testing the glucose metabolism function of the tumor, the ingested diet will not affect the functional imaging of the tumor, so fasting is not required, whereas fasting is required for FDG PET/CT scans.
2. Is it safe to do Ga-PSMA PET/CT?
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It is relatively safe.
This scan exposes the body to less radiation than X-rays and CT. the half-life of Ga is even shorter than the half-life of In-111 used in octreotide scans, and it is recommended to drink plenty of water after the test.
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3. Can I get a santoprene (lanreotide) before a Ga-PSMA PET/CT?
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No.
A Ga-PSMA PET/CT scan cannot be done until the artificial growth inhibitors have finished working. For example, a Ga-PSMA PET/CT scan cannot be done within 28 days of a santoprene injection. If both are planned for the same day, the scan can be done first, then the injection, and the order cannot be reversed.