A limitation of MRI post-biopsy is commonly from haemorrhage and oedema within the prostate gland occurred during biopsy. Haemorrhage artefacts and prostatitis make tumour detection very difficult in MRI (Sutton 2003). Higgins and Hricak (1997) recommend a delay of atleast 3 weeks between time of biopsy and MRI study, to allow the haemorrhage to clear.
Yu and Hricak (2000) have found that the use of endorectal coil not only allows for high-resolution images but also the potential of acquiring metabolic information using magnetic resonance spectroscopy. This has also been suggested by Cheong and Krebs (2000) who add that spectroscopy could improve the sensitivity and specificity of MRI, expanding its diagnostic and staging usefulness. Alterations in levels of certain metabolites may provide prognostic information useful for treatment planning. Dynamic Contrast-enhanced MRI can also be used to demonstrate hypervascularity in prostate cancer (Ito et al. 2003).
The endorectal coil is often used in conjunction with a surface pelvic coil; this results in poor image quality of information received from the pelvic coil. This is because, as well as imaging anatomical structures within the pelvis, the endorectal coil is also imaged in situ, resulting in streaking artefacts on the image. When the endorectal coil is used on its own, there is less coverage of the pelvic area with the field of view limited to the prostate gland, seminal vesicles, bladder and close structures. However, the image resolution is very good. A study by Tewari et al. (2000) included 73 patients with biopsy- proven Prostate cancer who were examined by endorectal coil MRI. It concluded that the overall accuracy in defining local tumour stage was 82%, which suggests that endorectal MRI is an accurate method for local staging. Table 1 summarises the sensitivity and specificity found by the study.
Table 1: The sensitivity and Specificity of endorectal MRI (Tewari et al. 2000)
MRI has very good spatial resolution and tissue contrast. It has multiplanar capabilities and it does not use ionising radiation. MRI is an expensive modality and is of limited availability, with long waiting lists in NHS hospitals. The limitation of MRI equipment is that it can be claustrophobic for some patients. It is contraindicated for patients with metallic implants, pacemaker, neurostimulators, surgical clips, etc. (Grainger and Allison 2001). The use of an endorectal coil is invasive to the patient and the imaging time can exceed 35 minutes. Therefore, a good explanation of procedure must be given to patient to ensure their cooperation, and it is important for patients to keep very still during scanning.
Yu and Hricak (2000) have determined that “Computed Tomography (CT) is not recommended for local staging of prostate cancer due to its low diagnosing accuracy; 24% for extracapsular extension and 69% for seminal vesicle invasion”. Enlargement of the prostate gland is not indicative of cancer; it could be benign or malignant. If the carcinoma is within the capsule, causing enlargement, it cannot be differentiated from BPH (Yu and Hricak 2000).
Grainger and Allison (2001) argue that CT is highly accurate in advanced stages of the cancer, particularly in detection of metastases to lymph nodes, liver, lung and bone. It also demonstrates extraprostatic spread into adjacent organs. However, when looking for lymph node metastases, CT does not demonstrate the internal structure of the nodes. Therefore, microinvasions in normal sized nodes (<10mm) cannot be detected. This results in false negative results. Grainger and Allison (2001) recommend performing a percutaneous needle biopsy under CT-guidance in enlarged or suspicious nodes. This is also suggested by Yu and Hricak (2000); “Combined CT and CT-guided fine needle aspiration proves high specificity up to 100% for diagnosing lymph node metastases”.
CT is a relatively expensive modality. However, it is much cheaper than MRI and is widely available in most NHS hospitals. The scanning time is very short (<30seconds) and the procedure is much more acceptable to the patient. Its disadvantage includes the high skin dose from ionising radiation.
CT is excellent in distinguishing osteoblastic, osteolytic and mixed bony metastases. However, Grainger and Allison (2001) argue that CT should not be used to screen for bony metastases. Radionuclide Scintigraphy (RNS) is preferred as it examines the entire skeleton and is more sensitive at detecting small and early metastatic lesions.
Bruggmoser et al. (2003) state that bone Scintigraphy is not recommended for all prostate cancer patients, it should be routinely used for those cases with PSA >10ng/ml or with skeletal symptoms in the spine and pelvis. Tewari et al. (2000) reported that no patients in their study had a positive bone scintigram with PSA level <15ng/ml. Therefore, it can be suggested that RNI is performed with elevated PSA levels >15ng/ml.
Yu and Hawkins (2000) claim that RNS has a high sensitivity for bone metastases, but it has a low specificity. Radionuclide bone imaging is quick, relatively inexpensive and widely available (Love et al. 2003). The radiological appearance is of multiple sclerotic deposits. False positive results occur due to Paget’s disease and Osteoarthritis, which are prevalent in this age group (Merrick 1998). Love et al. (2003) suggest that Single Photon Emission Computed Tomography (SPECT) is useful in differentiating between metastases and Osteoarthritis in the spine. Sections of vertebrae can be examined on transaxial, coronal, sagittal and three-dimensional structures, which facilitates both localisation and characterisation of an abnormality. Yu and Hawkins (2000) suggest that plain films are useful, for selective bones, to evaluate inconclusive findings on the bone scan. False negative results can occur due to residual radioactivity in the bladder and underclothes, which can obscure parts of the pelvis (Merrick 1998).
In conclusion, TRUS-guided biopsy is the first line investigation to diagnose a suspected prostate cancer. MRI is used to stage biopsy proven carcinoma and evaluate ECE and SVI. CT can be used where MRI is contraindicated, or to evaluate spread in advanced stages >T3. RNS is used in advanced stages to evaluate bone metastases in patients with skeletal symptoms or PSA levels >15ng/ml. All these investigations have their limitations, but their benefits result in successful evaluation of prostate cancer, leading to the best method of treating the patient.
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Appendix 1
a. Zonal Anatomy
b. TNM Staging Classification System