In addition to its multiple uses in diagnostic imaging settings, ultrasound is playing an increasingly important role in the treatment of cancer.
In 2014 Bristol Haematology and Oncology Centre (BHOC) was the first in the UK to implement Clarity Autoscan (Elekta). Transperineal ultrasound (TPUS) provides therapeutic radiographers with a non-ionising means of localising, verifying and monitoring the prostate in three dimensions (3D) during radiotherapy. This means treatments can be more accurately targeted and intrafractional motion of the prostate can be corrected for, minimising the dose to healthy tissue.
Since starting Clarity intrafractional image guided radiotherapy, BHOC has treated close to 700 patients with the technique, saving more than 14,000 cone beam CT exposures.
During the initial radiotherapy planning CT scan, a transperineal ultrasound transducer is positioned by the therapeutic radiographers, the prostate is identified and a concurrent ultrasound image is taken at the isocentre. This ultrasound image, in conjunction with the CT scan, is used by the oncologist to inform the contouring of the prostate and a clinical target volume is created.
The physics team and radiographers mark and check a reference positioning volume (RPV) around the prostate and register this to the treatment plan for the patient. The therapeutic radiographers aim is to reproduce the position of the patient during their planning scan.
The Clarity transducer is placed against the perineum once the patient is in the treatment position. Once the patient begins treatment, a cone-beam CT scan (CBCT) is taken for the first three treatments, in addition to the ultrasound, to ensure the registration of the ultrasound is accurate and the match is reliable. Once it is confirmed that the setup is consistent and the ultrasound image is of good quality, the CBCT is no longer necessary.
For the daily treatment, the patient is aligned to treatment reference tattoos with the Clarity ultrasound probe in place. The probe height will have been recorded at planning CT, while the angle and pressure are seen with a visual cue, the 'fan' outlined in yellow on the Clarity computer.
Any moves to the isocentre from the tattoos are completed before acquiring the ultrasound image. The image can be manipulated to improve quality. When a CBCT is not required, therapeutic radiographers will perform the image match in the room with the patient. This matching is done by comparing the daily ultrasound image to the reference from the planning appointment. When matching the images, overall anatomy such as the penile bulb and urethra are used, as well as calcifications within the prostate (known locally as 'naturally occurring markers' or NOMs). A grid is placed on the computer and radiographers match the planning RPV to the treatment RPV based on these markers. Once the 3D match is complete, the scans will be synchronised with each other through each plane - sagittal, coronal, and transverse.
The couch shifts to isocentre are given by the Clarity system and compared to daily moves captured in MOSAIQ to see if the shifts are consistent. Any inconsistencies cause a confirmatory check through the ultrasound images. The shifts are sent to the treatment couch and displacements are corrected to within 1mm. When the couch is in position, the system is set to monitor the prostate position.
Once the system picks up the monitoring, it gives a displacement of the RPV throughout the treatment. The system will alert when the movement is greater than 3mm anterior and 5mm posterior, left-right and superior-inferior. The bed will be adjusted accordingly throughout the beam delivery to ensure accuracy of treatment is maintained.
A study from our department showed that at 3mm, threshold displacement occurred for 52% of fractions and that specific patients exhibited much higher displacement values than others. Posterior displacement was the most common with 10% of patients exhibiting more than a 10mm displacement in this direction. This study highlights the need for this intrafractional ultrasound monitoring to identify and help correct for prostate motion, improving treatment accuracy.