Prostate Brachytherapy

Selected Readings

AAPM TG-64: Permanent Prostate Seed Implant Brachytherapy (external link)

Overview

Prostate cancer has been found to have a low alpha/beta ratio (~1.5) making it a good candidate for low fraction treatments. Modern prostate treatments involve 15-20 perineal needles guided by transrectal ultrasound.

Treatment Regimen

LDR Prescriptions

The prescription varies significantly based on the radionuclide used. This is because differences in half life influence integral dose deposition and the radiobiology.

I-125: 145Gy
Pd-103: 125Gy

HDR Prescriptions

HDR alone: 13.5Gy x 2 fractions = 27Gy

HDR boost to external beam: 15Gy single fraction

LDR Prostate Brachytherapy

Treatment Process

A volume study is conducted prior to treatment to assess the number of seeds required.
- Volume commonly assessed via CT or transverse ultrasound imaging.
A treatment plan is generated either by computer or standard template.
- Seed spacing ~1cm
- 80-120 seeds are commonly used!
- Sources
  - I-125 with an activity of around 0.3mCi
  - Pd-103 with an activity of around 1.7mCi
Implantation is performed under anesthesia and guided with a transrectal ultrasound probe.
- A Mick Applicator is commonly used to place correctly spaced seeds.
  - Applicator uses a seed magazine and spacing indexer to simplify placing process.
- 21cm long 18 gauge needles are used to place the seeds.
- A template may be used to assist in uniform needle placement.
A cystroscopy is performed at the end of the procedure removing any seeds that may be been placed in the bladder of the urethra.

I-125 seeds

Key Point: The urethra runs centrally through the prostate with a slight curve in the anterior-posterior direction. To prevent blockage or urethral penetration, the mid line of the prostate may not be seeded.

HDR Prostate Brachytherapy

Prostate HDR brachytherapy is typically accomplished using 15-25 interstitial needles inserted through the perineum. Needles are indexed for planning using a template and are guided by rectal ultrasound imaging. Once the needles are inserted, CT imaging is most often used to produce a 3D planned dose distribution.

Prostate interstitial needle placement guided by ultrasound

SpaceOAR is a useful tool in the treatment of prostate cancer using high dose rate brachytherapy.

SpaceOAR

Space OAR is a gel that may be inserted via needle between the prostate and rectum. It is useful in HDR prostate planning as a method of reducing rectal dose by moving the rectum away from the treated volume.

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