Novel Imaging in Staging of Primary Prostate Cancer

Prostate cancer (PC) is the most common cancer among men and one quarter of diagnosed PC are metastatic at the time of diagnosis. Accurate staging is paramount as the stage is the most important factor when treatment decisions are made. The stage is also the single most important prognostic factor. Currently, traditional imaging methods for detection of PC metastasis, including bone scan (BS) and contrast enhanced whole-body computer tomography (CT), are rather inaccurate. Respectively, novel imaging techniques are evolving and novel imaging modalities are emerging in PC diagnostics and staging, but their clinical relevance is unclear and lacking prospective studies comparing traditional imaging with novel imaging. This prospective single-institutional study compares the diagnostic accuracy of novel imaging modalities to traditional imaging modalities aiming to find the most appropriate staging modality in high-risk PC at the time of initial staging.

Prostate cancer (PC) is the most common cancer among men. The incidence of PC has increased dramatically in Finland since 1980´s and lately approximately 4.500 new PC cases have been diagnosed annually in Finland. About one quarter of diagnosed PC are metastatic at the time of diagnosis. Accurate staging is extremely important, as the stage is single most important factor when treatment decisions are made and stage is the single most important prognostic factor. Localized PC is treated with active surveillance (low risk cases), or with treatment modalities with curative intent (radical prostatectomy or radiotherapy).Although recently radical treatments have been suggested to play a role in low volume metastatic disease, the standard treatment of metastatic disease is castration therapy. In PC staging the most important anatomic locations to be imaged are i) bone, ii) lymph nodes (especially pelvic lymph nodes), and iii) extranodal soft tissues. Detection of tumor bone metastases is commonly performed by BS. However, the results of recent studies have raised many doubts whether BS is as effective for confirming or excluding metastatic bone disease. Moreover, the sensitivity for 99mTc-methylene diphosphonate bone scintigraphy (99mTc-MDP BS) is only 50-70%. The detection of bone metastases in patients with high-risk PC is significantly improved by SPECT compared to planar BS. Other imaging modalities with potentially improved accuracy to detect bone metastases in PC include PET-scan and whole body MRI. The value of positron emission tomography (PET) imaging depends on the suitability of used isotope tracer to identify lesions of the imaged tumor type. When bone is imaged with PET, 18F-fluoride has been the most commonly used tracer. Other commonly used PET tracers in PC include 18F-FDG, and 18F/11C-choline, but both have been late more or less replaced by PSMA-PET. Prostate-specific membrane antigen (PSMA) is a trans-membrane protein with an increased expression on cell membranes of PC cells. 68Ga-PSMA HBED-CC (Glu-NH-CO-NH-Lys- (Ahx)-\[68Ga(HBED-CC)\]) was designed as an extracellular PSMA inhibitor for PET imaging and has been shown to demonstrate high specificity for PSMA-expressing tumor cell. PSMA-PET results have been reported in several studies, but only in three prospective, one including 20-30 patients. Of those studies only study by Fendler and coworkers investigated overall staging, the other two studies by van Leeuwen focused on intraprostatic tumor detection or nodal metastases. Nevertheless, PSMA-PET/CT is a promising imaging modality both for soft tissues and bone. Recently 68Ga-PSMA was reported to outperform 99mTc-DPD-SPECT in detection of bone metastases in PC. Recently, the novel PET tracer 18F-PSMA-1007 has been developed as a promising PSMA ligand to even outperform 68Ga-PSMA-PET in overall staging. 18F-PSMA-1007 have advantages in comparison to 68Ga-PSMA-PET including primary elimination of 18F-PSMA-1007 via the hepatobiliary excretion route leading to less isotope activity in urinary tract. Consequently, 18F-PSMA-1007 might lead to better local staging because of its favorable pharmacokinetics and tumor-specific uptake. 18F-PSMA-1007-PET combined with CT or even MRI could truly offer a 1-stop solution for both metastatic screening and local staging, but more prospective studies are needed to confirm this hypothesis. Whole-body T1-weighted MRI is an effective method for bone imaging and is superior when compared to 99mTc-MDP BS. If combined with soft tissue imaging, bone and nodal imaging may be performed in single imaging session. Diffusion-weighted imaging (DWI) as a part of routine MRI examination is a promising tool for detection of an early intramedullary malignant lesion before cortical destruction or reactive processes due to bone marrow metastasis. DWI performs high contrast resolution between tumor and normal tissue. Individual variability of the mean apparent diffusion coefficient (ADC) values, as the result of DWI, may decrease the diagnostic accuracy of DWI. Diagnostic accuracy of DWI for detection of malignant lesion is better than 18-fluoro-deoxy-glucose (FDG) and for detection of bone metastasis is comparable to 11C-choline. However, it is unclear if it is superior compared to the standard T1-weighted imaging or STIR fat suppression technique. Currently there is not sufficient data comparing MRI and PSMA-PET/CT accuracy on bone imaging in PC. In addition to bone, the possible tumor spread to soft tissues, especially pelvic lymph node is common in PC staging. Traditionally contrast enhanced abdomen and pelvic CT or MRI are used but the sensitivity of these imaging modalities is very limited. Diffusion-weighted MRI may improve the diagnostic accuracy when normal sized lymph nodes are evaluated. Still, different PET-tracers and recently especially 68Ga-PSMA and novel 18F-PSMA have both been considered as the most promising modalities for pelvic lymph node metastasis detection in PC and preliminary results suggest superior diagnostic accuracy of PSMA-PET compared to other modalities. The investigators have previously investigated different imaging modalities for detection of bone metastases in prospective setting (Skeleta-trial). According to that study, 18F-NaF PET-CT and whole-body MRI are superior when compared to 99mTc-MDP SPECT-CT or 99mTc-MDP planar bone scan. Nevertheless, that study needs validation and further investigations as it was limited by low number (n=27) of PC patients, and PSMA-PET was not included in the study. Clinicians face challenges when choosing optimal imaging modality/modalities for individual patient. Guidelines do not support any imaging in low risk cases. For some intermediate risk cases, and also for high-risk cases, if local treatment is planned, accurate staging of pelvic lymph node is important. In contrary, in very high-risk cases the knowledge of distant metastases is the single most important staging data. Optimally for clinicians most appropriate imaging technique would be chosen based on patient related risk factors or a single imaging modality would offer all aspects of needed staging information. The rationale for the present study is to find the most appropriate staging modality in high-risk PC at the time of initial staging.