206 patients underwent MRI of the prostate in the study period.
6 patients were excluded due to non-diagnostic MR images.
Fig. 2
127 patients accounting for 62 % had additional finding on DWI that were not obvious on standard MRI sequences.
These are categorised and discussed as below:
Fig. 3
Tumour detection:
Anterior prostate tumors are difficult to image and biopsy on TRUS .
These contribute largely to the false negative rate of TRUS guided biopsy which has been reported to be between 11 and 25%.
Benign prostatic hypertrophy changes in the transitional zone make assessment difficult on conventional T2W MRI.
Diffusion weighted imaging has been shown to be useful in detecting prostate cancer and distinguishing it from other benign changes. It helps in targeting the tumour foci on TRUS biopsy with a high yield.
Fig. 5
In our series 66 MRI scan were performed in patient with previously negative biopsies.
MRI identified tumour in 21% of cases that were confirmed on subsequent biopsies.
More importantly no clinically significant tumour was identified in 59% of cases on MRI with good correlation with subsequent biopsies.
Fig. 4
Tumour volume estimation:
Accurate measurement of tumor is important in determining prognosis and aid the clinicians in choosing the best treatment strategy.
TRUS biopsies often underestimate the volume of disease especially in the central zone tumors.
TW2 MRI alone has been shown to often over estimate volume of P ca. Addition of DWI to T2W MRI significantly improves the accuracy of prostate PZ tumor volume measurement.
Fig. 6
Tumour staging:
Post biopsy haemorrhage within the prostate causes significant low signal changes on T2 weighted images.
Haemorrhage often exerts focal mass effect resulting in bulging of the capsule and signal distortion within the seminal vesicles.
These changes cause considerable interpretational difficulties with frequent over staging as extra capsular spread.
Diffusion weighed imaging has been shown to be helpful in detecting tumour within areas of haemorrhage . Fig. 7 ,
Fig. 8 and Fig. 9
Tumour Grade:
P ca is graded according to the pathological appearance using gleason score.
This is a sum of the primary and secondary pattern and is one of the most important prognostic factors. High Gleason score increases the potential of local spread,
early lymphovascular involvement and distant metastases.
Needle core biopsies have been known to underestimate the true aggressiveness of tumour.
A number of studies have demonstrated a good correlation between ADC values on DWI and the Gleason score.
Fig. 10
Tumour distribution:
IMRT Intensity Modulated Radiation Therapy (IMRT) is a technique of external conformal radiation planning and delivery.
Boost doses using fluence maps (i.e.,
intensity maps) can be used to provide higher dose to the tumour load ,with better sparing of surrounding normal tissue .
Fusion T2 and high B value images are able to delineate the high risk Clinical Tumour Volume ( CTV) which is vital for treatment planning .
Fig. 11,
Fig. 12 and Fig. 13
Apical tumours are under sampled on normal TRUS biopsies.
Also presence of disease at the apex of the prostate ,carries a significant risk of tumour spread along the apical neurovascular bundles.
DWI helps delineate the location and extent of tumour which is useful in surgical planning.
Fig. 15
In our case series,
50% of patients with apical tumour ,who went on to have radical surgery had positive margins on histology.
Extraprostatic findings :
Life expectancy in patients with prostate cancer is very good due to natural history of prostate cancer,
early detection and advances in treatment .
However this cohort still carries the same risk of developing other cancers which often have more serious implications.
Diffusion principles are similar in most neoplastic processes.
6 incidental tumours in the rectum ,
sigmoid and bladder were identified on high B value images which were not apparent of standard sequences.
Fig. 18, Fig. 19 and Fig. 20