A common treatment option for low grade prostate cancers is low dose rate (LDR) seed brachytherapy. It is usually employed as a monotherapy. Early developments of this treatment technique would implant the seeds into the prostate via retropubic surgery. This was later improved so that the seeds were implanted via the perineum without the need for surgery . This procedure is done by placing the seeds into large gauge needles which are then inserted to the required depth before the seeds are deployed as the needle is withdrawn. To guide the placement of these seeds some form of image guidance is typically employed. The most commonly used is a combination of ultrasound (US) and fluoroscopy, where the ultrasound imaging is achieved via a trans-rectal ultrasound (TRUS) probe.
LDR seed brachytherapy has been performed at the Royal Adelaide Hospital (RAH) since September 2004. Initially the two step treatment technique was employed where the patient had a TRUS volume study on a separate day to the implant. The TRUS scan enabled the assessment of prostate volume (prostate volumes ranging from 15 cc to 50 cc were deemed eligible) and provided images for contouring and treatment planning. The time gap between the TRUS scan and the seed implant was usually 4 weeks. On the morning, prior to the implant the needles were manually loaded with the planned seed configurations. These needles were then implanted after aligning the planning US images of the prostate with the live US images of the prostate as seen on the implant day.
However, the time gap between the TRUS scan and the implant procedure was shown to lead to suboptimal consequences due to difficulties in reproducing the patient alignment. Often it was quite difficult to align the planning US images with the treatment images. This challenge was possibly caused by several factors, such as: (1) changes in patient’s anatomy between the two scans; (2) changes in patient positioning on the operating couch. Thus the delivered seed dosimetry was likely to be significantly different from the initially planned one . To eliminate this problem and, at the same time, to optimise treatment flow, the quality of the implant and hence treatment outcome, it was decided to move to a one step live planning technique. In a one step live planning technique the TRUS scan and implant are performed on the same day, significantly reducing the chance for movement and the changes in prostate position between pre-implant TRUS and live-implant.
The treatment procedure on the day is as follows: First the patient in anaesthetised, catheterised and placed into stirrups. Next, an ultrasound scan (BK Medical Flex Focus 1202–400 US mated with Nucletron SPOT PRO™ Version 3.1 TPS) is taken of the prostate upon which the prostate, PTV and OAR are contoured by both the urologist and radiation oncologist in consultation with each other. In most cases the urethra is both identified and contoured through the use of a contrast medium (aerated jelly) which is injected into the catheter. The visualised catheter (5 mm diameter) is then taken as being representative of the urethral volume. It should be noted that patients who are eligible to be treated via LDR seeds are generally not given androgen deprivation therapy (ADT) prior to the treatment day. A small number (~5%) are given ADT in order to bring the prostate down to a more manageable size. Literature suggests that shrinkage of around 30% is achieved through the use of ADT and the maximum shrinkage occurs after a period of three months .
Once this process is complete, the planner (medical physicist or radiation therapist) will create a plan for the distribution of seeds within the prostate using the Nucletron SPOT Pro system and a modified uniform loading of seeds [4, 5]. After treatment plan optimisation and approval, pre-loaded needles (Oncura Rapid Strand RX) are inserted by the urologist. The seeds are then deployed to their respective positions as indicated by the plan under fluoroscopic and ultrasound guidance. Anchor needles (or fixation needles) are not used at our centre. However all needles of the same retraction distance (relative to the prostate base plane) are inserted and then aligned (with US and fluoroscopy guidance) prior to seed deployment. At each retraction the prostate base plane is checked and amendments made if necessary again prior to seed deployment. After the completion of the planned implant the radiation oncologist will inspect the plan on the TPS (using the dosimetry from the live updates of the actual implanted seed positions), plus check the ultrasound and fluoroscopic images, to determine if the implant dose coverage is satisfactory. If there were any gaps in the dose coverage of the target the radiation oncologist may request more seeds to be implanted at a given location within the prostate by the urologist.
The final quality of the implant is assessed via post implant dosimetry. In our centre, post implant dosimetry is completed based on US-CT image co-registration, with the pelvic CT scan taken on day 0 or 1 after the implant. Details on post implant dosimetry technique and results have been recently published . According to the latest ABS guidelines  the post implant dosimetry (PID) is considered to be a mandatory component of the implant’s quality assurance and should be undertaken within 60 days of the implant. While CT-MR fusion-based PID is recommended, CT-based PID is also considered acceptable.
It should be noted that prior to the treatment day the medical physicist will have performed an independent assay of the seeds used to ensure the correct activity is used by the TPS. Initially for this technique the needles were manually pre-loaded on site prior to implant, but later we moved to purchasing manufacturer pre-loaded needles. In each case a set of generic seed-needle configurations were determined from analysing the seed configurations used for previous implants. The large number of seeds ordered per patient (110 seeds) allow ample seed configuration combinations to be pre-loaded.
The SPOT Pro planning software used at the RAH allows the user to update the location of the implanted needles in real-time on the treatment planning system and hence automatically recalculate the plan dosimetry, to accurately reflect what was actually implanted. This is important, as the theoretically planned grid positions might not always be practically attainable. There are a few factors which can possibly impede the perfect reproduction of the treatment plan:
Operator-related: needles can miss the planned grid position by a few millimetres, so they do not exactly end up where the implanting physician intended them to go. Though the error is usually small, overall it contributes to the difference between the planned and implanted dosimetry.
Patient anatomy-related: there are situations when the physician may need to implant at another grid position than the planned one due to interference such as pubic arch. Calcifications inside the prostate gland could also lead to similar interferences necessitating slight adjustments in needle positions.
Prostate gland-related: small changes in the prostate glad volume during implant due to swelling (from oedema) might influence the decision of needle placement.
These offsets (both small and large) could potentially add up to give a significantly different dosimetry to that which was initially planned and approved.
The current work will identify any significant differences for the main dosimetric parameters used in the reporting of LDR prostate treatments when comparing the planned dosimetry and the final updated real-time implant dosimetry.