iii) - High Dose Rate Brachytherapy

HDR brachytherapy for prostate cancer is becoming a mainstream method for dose escalation in intermediate to high risk cases.

Rationale

Multiple studies have shown that dose escalation improves outcomes for prostate cancer. Dose escalation with external beam techniques is limited in some cases by bladder and bowel toxicity. Using brachytherapy, which deposits dose more conformally within the prostate, should be able to significantly escalate dose without the expected toxicity from external beam treatment.

Evidence

There is no randomised evidence comparing the external beam RT + HDR boost with other methods of dose escalation or with radical prostatectomy. Observational studies show 10 year biochemical free survival rates of 80-90% which is consistent with findings from other treatment modalities.

Toxicities

In general, dose escalation with HDR brachytherapy reduces rectal toxicity at the expense of urinary toxicity.

Requirements

The gland volume should be less than 60 cm3 to allow for adequate dose distribution. Glands over 60 cm3 have reduced dose conformality and may be difficult to implant due to conflict with the pubic arch.
Patients with a high IPSS score (> 15) have significantly reduced urine related quality of life with HDR brachytherapy. Radical prostatectomy or external beam radiotherapy alone should be used in these patients. Patients with high post-void residuals (> 100 ml), or who use prazosin or tamsulosin prior to therapy also have worse outcomes.
Prior to being accepted for HDR brachytherapy, patients should have a volume study (evaluation of prostate gland size and determination of suitability for catheter insertion) and have their urinary symptoms assessed (IPSS, post void residual).

Technique

The HDR component is typically done prior to EBRT as the fiducial markers for image guidance can be inserted at the same time. Patients also have 6 months of androgen deprivation; this helps to shrink the prostate gland to make it less than 60 cm3.

Insertion

The patient is anaesthetised and positioned in stirrups. The perineum is exposed. A catheter containing contrast and air within the balloon is inserted and the penis and scrotum are retracted and fixed anteriorly to prevent obstruction of the perineum.
The rectal ultrasound is attached to a stepper and inserted. A grid is fixed to the stepper. The grid matches the co-ordinates given by the ultrasound so that interstitial needles can be placed accurately.
Three fiducial markers are inserted initially.
Seventeen to eighteen interstitial needles are inserted into the prostate with spacing of about 1 cm. The urethra is avoided centrally. In general about 4-5 needs are inserted centrally and the remainder throughout the periphery of the gland.
Position is confirmed by ultrasound.
The patient is woken from anaesthetic and simulation and planning occurs.

Planning

The patient is CT simulated. The gold fiducial markers are used as a a guide. The CTV is the prostate gland. The PTV is a 0.3 cm expansion except posteriorly.
The technique I am familiar with uses 3 fractions. The first fraction is the same day as the insertion. The remaining two fractions are delivered the following day and require replanning due to potential for shift in the prostate needle position and from prostate swelling.

Organs at Risk

The organs at risk are the bladder, urethra and bowel. There are no dose conventions for the bladder.

  • A volume of rectum no greater than 2 cc should receive the prescription dose or higher.
  • The dose to the urethra (D10%) should be less than 130% of the prescribed dose

These dose constraints create a 'cold spot' around the urethra which limits potential toxicity.


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