ii) - Permanent Interstitial LDR Brachytherapy

This section is heavily based on the review by Crook et al in 2011, see reference below. Most textbooks are out of date in relation to this topic (and HDR brachytherapy) and the review by Crook is an excellent overview of the current state of the treatment.

Low dose rate brachytherapy became a reality with the development of several technologies:

  • Trans-rectal ultrasound (TRUS) that allowed realtime imaging of the prostate gland
  • Availability of low dose rate seed sources that decay into inert daughter products and have a short range dose distribution

Modern LDR brachytherapy for the prostate also uses post-implant CT to identify dose distribution for quality assurance purposes. Seeds are placed via needles inserted through the perineum, behind the scrotum and anterior to the anal orifice.


Patient selection

The size of the prostate and its relation to the pubic arch (inferior pubic ramus) can limit the placement of seeds within the prostate gland. Large prostates, as well as providing limitations relating to the pubic arch, also confer a higher risk of urinary retention after a procedure. Other factors include the IPSS score and peak urine flow rate (see the prostate cancer topic).
In general, the low dose rate of the seeds and the difficulty in covering extra-prostatic disease, permanent LDR seed implants are used primarily for low risk prostate cancer.


Simulation

Patients are simulated prior to having the seeds inserted. Simulation during seed insertion is usually not possible given the constraints of TRUS in detecting implanted seeds. Patients are placed in the lithotomy position with legs in stirrups and the buttocks and perineum placed at the end of a treatment couch. The position of the legs is recorded. A large spirit level is used to ensure the legs are evenly positioned. A catheter is inserted into the urethra and filled with a mix of ultrasound gel and air to allow for easy visualisation on the TRUS.
When in this position, a TRUS is performed and axial slices of the prostate are obtained from 5 mm above to 5 mm below its extent. This allows staff to evaluate the position of the prostate with respect to the pubic arch. The positions of the seeds are then determined while the patient is absent.


Planning

Planning occurs in two phases.

  • Pre-implant planning is performed between the simulation and the actual implantation. This step determines where the seeds should be placed to achieve the desired dose distribution. Modern planning for LDR brachytherapy is computer aided.
    • The most important dosimetric quantity is the D90, or the isodose that covers 90% of the prostate volume. This value should be no less than 90% of the prescribed dose and ideally should be at least 140 Gy (usual prescribed dose is 145 Gy). There is no evidence of increased toxicities with higher doses (up to 200 Gy). Acheiving this D90 value is best achieved by aiming for a D90 of 120% in the pre-implant setting (about 180 Gy).
  • Post-implant planning is performed at some point after the seeds have been inserted. This planning is used to calculate the actual dose distribution that was delivered from the implanted seeds.
    • A CT scan is used to accurately identify the location of the implanted seeds
    • Post-implant planning may be immediate (days 0-1 after insertion) or delayed (30+ days after insertion). Early post-implant planning has the benefits of rapid assessment while the patient's case is still fresh in the minds of the oncologist, therapist and physicist but may be less accurate than late planning due to the presence of oedema so soon after insertion.

Planning at the time of seed insertion is not possible with current technology due to limitations of TRUS at accurately identifying seed position.


Dose Constraints

The three critical tissues for prostate brachytherapy are the urethra, the rectum and the nerves and vessels supplying the penis.

  • The rectal dose is usually reported as the rectal volume receiving the prescription dose (RV100); this should be 1 cm3 or less.
  • The urethral dose is usually reported as U30 or U5, representing the dose received by 30% or 5% of the urethra. The U5 should be less than 150% of the prescribed dose and U30 should be less than 125%.
  • There are no guidelines for limiting dose to the neurovascular supply of the penis. Identifying these structures and avoiding placement of seeds within them is recommended

Insertion of Seeds

The Seattle Method is the most commonly used approach for insertion of seeds. This method, developed in Seattle, uses an TRUS probe attached to a 'stepper' which moves the probe in and out of the rectum at 5 mm intervals. The stepper device is fixed to a perforated grid. The holes on this grid are aligned to the ultrasound image, allowing the operator to determine through which holes the seeds are to be placed. In most circumstances the seeds are placed 1 to 1.5 cm distant to each other to prevent hot spot formation.
The patient is anaethetised (either general/spinal/epidural) for insertion of the seeds as the process is quite painful.
Once the seeds are inserted, post planning takes place either immediately or from 1-30 days later.
The type of implantation device can vary:

  • Stranded seeds include several seeds loaded onto a single wire, attached to a deployment device. The seed to be deployed is placed in the appropriate position with TRUS guidance, and then the operator can manipulate the device to deposit the seed at the desired location. This can be repeated for as many seeds are on the stranded wire.
  • Loose seeds can also be deployed. A common applicator is the Mick Applicator, see here for a presentation on the Mick Applicator in use. The main drawback of loose seeds is that they can migrate more easily through vascular channels and may be deposited in the lungs.

Toxicities and Seed Migration

Lower Urinary Tract Symptoms are common and can last for several months (longer with 125I implants). This can sometimes take years to resolve (< 20% of patients). Urinary retention at the time of insertion occurs in less than 20% of patients and is correlated to the IPSS and peak urine flow rate.

Seed migration occurs when a seed moves from its implanted position. This can be local migration (rarely above 5 mm) or distant migration (haematogenous). Distant migration usually occurs to the lungs.


Combination of LDR seeds and external beam radiotherapy

In some cases where the risk of extraprostatic extension is high (eg. Gleason 7 or high PSA tumours) LDR seeds alone do not provide adequate dosage to the surrounding periprostatic tissue. In these situations some centres employ an additional treatment with external photon beams.


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