Aspects of PET-CT in prostate cancer. Protocol optimization, diagnostic accuracy, and dosimetry

Abstract: The PET-CT imaging modality is based on positron emission tomography combined with computed tomography. Before a PET-CT examination, a radioactive tracer (also called a radiopharmaceutical) is intravenously injected into the patient. PET technology images the distribution of this tracer element by detecting the decay. In recent years, PET technology has been developed and improved in hardware and software. In addition, new radioactive tracers have been introduced.Prostate cancer is one of the most common cancers affecting men. PET-CT is used to evaluate the suspected primary tumour, for staging, to evaluate the treatment, and to diagnose suspected recurrences. In 2017, the PET-CT machine park at Skåne University Hospital was upgraded from conventional PET-CT systems to newly developed and improved PET-CT systems.Previously, the radioactive tracer [18F]fluorocholine ([18F]FCH) was used in PETCT examinations of prostate cancer at Skåne University Hospital. However, [18F]FCH is not specific or sensitive enough to examine prostate cancer, and therefore, in 2019, it was replaced by [18F]prostate-specific membrane antigen-1007 ([18F]PSMA-1007).In a optimization study, the protocol for the [18F]FCH examination method was optimized for the new PET-CT system in terms of frame duration time and value of the noise reduction factor, β. For injection of 4 MBq/kg [18F]FCH, a frame duration of 1.5 min/bed position with a β-value of 400–550 was required to obtain good image quality.The accuracy of [18F]FCH PET-CT for staging primary lymph node metastases has been validated, with low sensitivity (43%) and moderate specificity (70%) reported. To examine these results more in-depth, a second study was conducted. In this study,the conventional and novel PET-CT systems were validated based on their ability to accurately detect regional lymph node metastases. This study found large differences in sensitivity and specificity between the PET-CT systems but with almost similar total diagnostic ability. Both studies used histopathology from extended pelvic lymph node dissection as a reference method.Through a biokinetic and dosimetric study of [18F]PSMA-1007 in patients with prostate cancer, the last study aimed to calculate the absorbed doses and effective dose. The effective dose was calculated to 25 µSv/MBq, corresponding to approximately 8 mSv for a ”standard patient”.In conclusion, this thesis shows that the reconstruction algorithm BSREM is advantageous over OSEM using [18F]FCH PET-CT, but that the diagnostic accuracy of [18F]FCH PET-CT in detecting pelvic lymph node metastases is not good enough. Furthermore, the thesis shows that the effective dose generated by [18F]PSMA-1007 supporting continued use with the radiopharmaceutical in diagnostic imaging.