PET/CT
Technical information
The integration of positron emission tomographie (PET) with computer tomographie (CT) has a long tradition in our department. One of the first scanner combining the two imaging modalities for clinical studies was installed in March 2001 in our clinic. Since then PET/CT is an established imaging method world wide. In Switzerland every university hospital and most of the large clinics have a PET/CT (a total of 30-35 scanners in Switzerland).
The main principle of PET imaging is the detection of radioactive decay within the body. Injected radiotracers will decay and emits two photons, with multiple detector rings surrounding the patient this decay can be localized. The great advantage of PET over conventional radiological imaging modalities such as CT or MRI is the high sensitivity for the injected substances. This allows the visualization of biochemical processes and therefore enables the detection of lesions before anatomical changes can be detected. Thanks to a relatively short half-life of the used isotopes (18F = 110 min, 68Ga = 68 min), the exams have a low radiation burden to the patients. However it forces us to stick to a fixed time schedule, since any delay leads to a decrease in tracer activity.
Depending on the tumor subtype, different tracers are used for PET/CT imaging (18F-FDG, 18F-Cholin, or 68Ga-DOTATATE).
18F-FDG PET/CT:
18F-FDG is the most commonly used tracer in PET imaging today. It consists of a modified and radioactive labeled sugar. After the injection into the body,
18F-FDG accumulates in all cells with a glucose uptake such as the brain, the heart, tumors and metastasis.
Figure 1
Preparation and scanning:
It is mandatory that patients are fasting for at least 4 hours prior to a
18F-FDG scan. No food, no sweets such as candies or chewing gums shall be consumed, even if they are sugar free. Only water or teas without sugar can be taken, together with the daily medication. Any stimulation of insulin production could interfere with the image quality. Medication that has be taken together with food, should be taken either 4h before the scan or after the exam.
After the injection patients are asked to rest for 50-60 minutes, so that the tracer can distribute in the body and does not accumulate in the muscles. After this the images are acquired (20-25 min).
18F-FDG PET/CT and diabetes?
High blood sugar values are reducing the image quality substantially. Therefore patients suffering from diabetes mellitus have to be careful regarding their blood sugar values before the scan. For the examination the blood glucose level must be under 12 mmol/l, ideally under 7 mmol/l. Ideally glucose values are checked the day before the FDG PET/CT and if there is any difficulty the PET team should be informed (Tel.: 044 255 35 55). If not instructed otherwise you can consider the following roles:
Typ I diabetes:
- With insulin (base / bolus): Take the regular base insulin in the evening, have a little breakfast and the regular bolus insulin early in the morning (before 7) and come to the scan around noon (after 11 am).
- If you have a continuous insulin pump, leave it on “night setting” the morning before the scan. Do not eat any breakfast or inject bolus insulin. The scan should be done in the early morning.
Typ II diabetes:
- You can take oral antidiabetics together with food, if it is at least 4 hours before the scan.
- Blood sugar will be checked before every scan.
18F-FDG PET/CT and renal insufficiency?
18F-FDG PET/CT can be performed even if the kidney function is impaired without risk of side effects. However, if the kidney function is reduced no contrast enhanced CT scan will be performed.
18F-FDG PET/CT during pregnancy or breast feeding?
Due to the radiation burden to the child a
18F-FDG PET/CT will not be performed during pregnancy.
If a mother is breast feeding she should not directly breast feed the child fro 6h after the scan. The radioactive tracer does not accumulate in the milk, but in the breast tissue – therefore the milk does not have to be discharged but can be give to the baby with the buddle by someone else.
18F-FDG PET/CT risks and side effects?
For PET examinations only minimal amounts of tracer are necessary thanks to the high sensitivity of the detectors. Therefore any interaction with other medication or allergic reactions are not expected. Thanks to the short half-life of
18F-FDG the radiation burden to the patient is only slightly higher compared to a conventional CT (8-10 mSv).
18F-FDG PET/CT indications:
The following indications for
18F-FDG PET/CT have been established and are covered by the Suisse public insurance:
- Staging and re-staging or various cancers with increase glucose metabolism (lung, ENT-tumors, pancreatic or hepatic cancer, ovarian or cervical cancer, seminoma, esophageal or gastrointestinal carcinomas,...).
- Lymphoma
- High risk melanoma
- Dementia
18F-Cholin PET/CT:
18F-Cholin PET/CT is a sensitive and very specific exam for patients with prostate cancer. In the case of a newly diagnosed high risk prostate cancer it will be used to rule out lymph node or distant metastasis. In patients previously treated for prostate cancer with a rising PSA it can be used for early detection of the recurrence and guide focal treatment.
Prostate cancer cells have an increase in Choline uptake over time, therefore we perform two scans for Choline PET to distinguish benign from malignant lesions.
Figure 2
Preparation and scanning:
Since the same transporters take up Choline and amino acids into the cell, we recommend that patients should not be eating for at least 4 hours.
The tracer injection in done on the scanner. First the whole body CT is acquired, than the early 18F-Cholin phase (2-17 min), directly followed by the late phase (20-35 min). The two time points allow a better distinction between inflammatory lesions (decrease in activity) and malign lesions (increase in activity).
18F-Cholin PET/CT and diabetes?
The
18F-Cholin PET/CT exam is not affected by a high blood glucose or increased insulin levels. Therefore no special preparation in necessary.
18F-Cholin PET/CT and renal insufficiency?
18F-Cholin PET/CT can be performed also with impaired renal function.
18F-Cholin PET/CT risk and side effects?
For PET examinations only minimal amounts of tracer are necessary thanks to the high sensitivity of the detectors. Therefore any interaction with other medication or allergic reactions are not expected. Thanks to the short half-life of
18F-Cholin the radiation burden to the patient is only slightly higher compared to a conventional CT (8-10 mSv).
18F-Cholin PET/CT indications:
Following indications are established and reimbursed by the swiss public insurances since 01.07.2014:
- Re-staging patients with biochemical relapse after primary therapy. However good detection rates for tumor recurrence can be reached when either the PSA Level is > 2 ng/ml or the PSA doubling time is < 7.3 months.
PSMA-PET/CT
or PET/MR:
In Switzerland 68Ga-PSMA-PET is approved
since April 2016 and 18F-PSMA-PET since October 2019 for patients
with prostate cancer. Both radiotracers are reimbursed by Swiss healthcare
insurances for the restaging of patients with biochemical recurrence of
prostate cancer. Currently (November 2019), there is no reimbursement for other
indications, such as the initial staging of prostate cancer patients with
high-risk tumors. Here, patients undergoing this exam might have to cover costs
themselves. PSMA-PET has higher sensitivity for lymph node metastases and bone
metastases than the previously used 18F-Choline-PET.
Abbildung 3:
Tiny PSMA-positive pararectal
lymph node metastasis in a patient with biochemical recurrence of prostate
cancer and a PSA serum level of 0.22 ng/ml. Despite its small size of only 3mm,
the node is quite active on PSMA-PET.
Preparation
and examination:
No specific dietary rules need to be followed. However, we recommend staying
n.p.o. for 6 h prior to the examination in order to reduce bowel motility. The
whole examination will take 2 – 2.5 hours. You will be injected with a
radioactive substance into a forearm vein. No adverse effects are expected.
After that, you are requested to rest for 40 – 90 min, depending on the type of
radiotracer and scanner used.
PSMA-PET with diabetes?
No specific preparations necessary.
PSMA-PET
with renal insufficiency (kidney failure)?
You may safely undergo a PSMA-PET even with renal insufficiency.
PSMA-PET
risks and side effects?
A PET examination uses only very small amounts of radioactive substances. No
interaction with medication and no allergic reactions are expected. Owing to
the short half-life of the substance, the radiation exposure is only slightly
higher than that of a normal CT examination (PET/CT, 6-7 mSv) or is in the
range of a normal CT examination (PET/MR, ca. 4 mSv)
PSMA-PET indications:
The main indication is the early biochemical recurence of prostate cancer (PSA
increase of 0.2 ng/ml or more after radical prostatectomy). Large studies have
shown that the detection rate in subject with PSA levels 0.2-0.5 ng/ML is slightly
higher than 50%. The initial staging of patients with high-risk prostate cancer
is another promising indication, and provides higher sensitivity for the
detection of lymph node metastases and distant metastases compared to 18F-Choline-PET.
PSMA-PET/CT or PET/MR?
One advantage of PET/MR over PET/CT is the reduced radiation exposure. Another
advantage is the increase soft-tissue contrast in the pelvis, which is
important particularly for the detection of local recurrence and the local
staging of primary tumors. For the detection of pathologic lymph nodes, both
modalities are equal. PET/CT takes less time than PET/MR (20 min vs. 35 min,
respectively). If you have contraindications against MR (cardiac devices,
neurostimulators, metallic foreign bodies in the eye), please call our front
office, if you have received a notification for PET/MR. In case you suffer from
an allergy to Gadolinium-based MR contrast agents or suffer from
claustrophobia, please let our on-site staff know on the day of the examination,
and do not travel by car (as a driver), because you might require additional
medication before the scan.
68Ga-DOTATATE PET/CT:
68Ga-DOTATATE PET/CT is the most sensitive exam to detect well-differentiated neuroendocrine tumors. Since these tumors have a low glucose consumption but high expression of somatostatine receptors (SSTR) on their surface,
68Ga-DOTATATE is superior to
18F-FDG for tumor detection. This new method has replaced the Octreotide scintigraphiy in our clinic, due to a higher sensitivity and an improved localization of the SSTR positive lesion, thanks to the CT component.
Figure 3
Preparation and scanning:
Patients should be fasting for at least 4 hours. Oral medication can be taken regularly. Sandostatine injections should always be performed after the
68Ga-DOTATATE PET/CT and not before.
To increase the detection rate of small lesions in the abdomen, an additional contrast enhanced CT is often required.
68Ga-DOTATATE PET/CT and diabetes?
The
68Ga-DOTATATE PET/CT exam is not affected by a high blood glucose or increased insulin levels. Therefore no special preparation in necessary.
68Ga-DOTATATE PET/CT and renal insufficiency?
68Ga-DOTATATE PET/CT can be performed also with impaired renal function.
68Ga-DOTATATE PET/CT während der Schwangerschaft & Stillzeit?
Due to the radiation burden to the child a
68Ga-DOTATATE PET/CT will not be performed during pregnancy.
If a mother is breast feeding she should not breast feed the child for 12h after the scan.
68Ga-DOTATATE PET/CT risks and side effects?
For PET examinations only minimal amounts of tracer are necessary thanks to the high sensitivity of the detectors. Therefore any interaction with other medication or allergic reactions are not expected. Thanks to the short half-life of
68Ga-DOTATATE the radiation burden to the patient is only slightly higher compared to a conventional CT (7-8 mSv).
68Ga-DOTATATE PET/CT indications:
Established and covered by most insurance companies are the following indication for
68Ga-DOTATATE PET/CT scans:
- Staging und re-staging of well differentiated neuroendocrine tumors.
- Therapy control
- Evaluation for potential 177Lu-DOTATATE therapy
