PET scan in Urology. Goh Eng Hong. Wednesday, March 13, 13

Transcription

1 PET scan in Urology Goh Eng Hong

2 PET (Positron Emission Tomography) Image of a molecular process is obtained. Use of radiopharmaceutical which participates in a metabolic or (patho-)physiologic process, and accumulate at the sites the process is most active. Main difference of PET vs conventional nuclear medicine is the type of (radio) isotope used. Examples are 18F, 15O, 13N, 11C, 68Ga, 82Rb, 89Zr, and 124I. PET images are a volumetric set of data that can be displayed as tomographic images in

3 PET: isotopes are characterized by a surplus of positive charge in the nucleus, creating an unstable nucleus. Stability is regained by either capturing an electron (=negative charge) into the nucleus, or by emitting the surplus positive charge in the form of a positron. The positron is a particle with the mass of an electron, but with a positive charge. It should be considered the anti-matter of an electron. The positron is expulsed from the nucleus with a certain kinetic energy and migrates through the tissue for a few millimeters, losing its kinetic energy when traveling. The distance traveled is depending on the isotope used. When the positron has come to a complete standstill it will form a positronium. Together with an electron in its vicinity the positronium will be annihilated instantly, a process in which the available mass of the particles is converted into energy following Einstein s formula E = mc2. It has been calculated that the energy present in this system is 1022 kilo-electron Volts (kev). However, in order to fulfill the law on conservation of momentum, this energy is released as 2 gamma-photons with energy of 511 kev which are expulsed diametrically. The radiation resulting

4 Summary Unstable nucleus Emission of Positron, an equivalent of electron but with positive charge Formation of positronium at the end of travelling The annihilation process of positronium releases energy, detected by PET scan

5 Disadvantages Lack of an anatomical reference frame. The combined PET/CT device offers optimal fusion of images, which allows the localization of functional findings detected by PET in morphological structures as shown by CT during one imaging procedure. FDG is also taken up in inflammatory tissue, macrophages in particular Excretion into urine, with subsequent accumulation of FDG in the bladder causes high radiation dose to the bladder wall, and hampering interpretation of the pelvic areas. Bladder irrigation or forced diuresis in combination with an indwelling catheter only

6 Radiopharmaceuticals 18F-FDG (fluorodeoxyglucose) Most commonly used radiopharmaceutical An analogue of glucose, it is taken up by the cell by glucose transporters (mainly glut-1) and phosphorylized to FDG-6-phosphate by the enzyme glucose-6- phosphokinase. The resultant FDG-6-phosphate is not a substrate for other enzymes anymore. As a result, the FDG is trapped in the cell. Most cancer cells have a high expression of glut-1 transporters in the cell membrane. 11C-choline and analogues Choline is one of the components of phosphatidylcholine, an essential element of phospholipids in the cell membrane. 11C-acetate Increased 11C-acetate uptake in malignant tissue is caused by accelerated lipid synthesis. Radioimmunology and PET Immuno-PET involves radio labeled monoclonal antibodies

7 Renal cell carcinoma Overall sensitivity and specificity of FDG-PET for re-staging were 64 87% and %. Overall sensitivity of FDG-PET varied, ranging from 32% to 100%, for initial primary staging. As compared with primary tumors, it seems that recurrent or metastatic foci of RCC tended to be FDG-avid, resulting in higher sensitivity of FDG-PET In papillary RCC, the sensitivity was 100% in papillary RCC, 75% in clear cell RCC, p not significant Prostate carcinoma Low glucose metabolic rate in primary prostate carcinoma Expression of GLUT1 was higher in poorly differentiated cell lines (Gleason 7 & high PSA). Less sensitive than bone scan in detecting osteoblastic metastasis Provision of prognostic information: an increase of 33% in the average standardized uptake value (SUVmax) from up to 5 lesions, or the appearance of new lesions, could be

8 Bladder carcinoma Sensitivity 82% specificity 89% Testicular Cancer No evidence for using FDG-PET scan in staging. But recommended in patients with seminoma who have any residual mass at least 6 wk after chemotherapy, to help decide between watchful waiting and active treatment. Not recommended in the restaging of patients with NSGCTs after chemotherapy. False negative FDG-PET scans can occur if the residual lesion contains only mature teratoma tissue, a negative predictive value of 51% and a positive predictive value of 59% for predicting viable tumor. Penile Cancer The sensitivity and specificity in the detection of nodal metastases were 80% and 100% respectively. PET/CT showed a sensitivity of 89% in the detection of metastases in the superficial inguinal lymph node basins and a sensitivity of 100% in the deep inguinal and obturator lymph node basins. Positive predictive value (PPV) was 93.8%, while negative predictive

9 PET probe-guided surgery Technique to improve intra-operative staging and localize sites of occult disease in cancers of the colon, stomach, lung, breast and thyroid. Non-oncological utilization of PET The only area currently studied is the central control of bladder function, detrusor overactivity and the female pelvic floor. Cerebral blood flow, measured by H215O is

10 Precaution & contraindication Short duration, 24hrs No restricted activites Pregnant women & children Diet restriction prior to scan: carbohydrate

11 Conclusion No additional value for PET compared to conventional imaging has been shown in tumor detection and initial staging of prostate, bladder and renal cancer. In metastatic disease the studies are ongoing. At present there are two accepted clinical indications for FDG PET, i.e. restaging of renal cell cancer and treatment evaluation after chemotherapy in seminomatous