Prostate brachytherapy

Features of Low-Dose-Rate (LDR) Brachytherapy

Low-Dose-Rate Brachytherapy (Low-Dose-Rate Brachytherapy) is widely used for low-risk prostate cancer. Microscopic iodine-125 isotope seeds are implanted into the prostate tissue as a radiation source. This isotope has a maximum X-ray energy of 35 keV and a half-life of 59 days. Due to these physical parameters, the therapeutic effect is protracted: continuous irradiation of the prostate occurs over several months following a single seed insertion procedure.

Imaging and the Seed Migration Problem

Surgical tissue trauma during implantation inevitably causes local bleeding and pronounced edema. Due to changes in organ geometry, uncontrolled migration of radioactive seeds frequently occurs.

The imaging protocol includes two important stages:

1. Plain pelvic radiography — performed immediately after implantation for an initial assessment of the situation.
2. Fused CT and MRI (Fused CT and MRI) — performed 28 days after source insertion.

It is precisely because of seed migration against the background of edema that the final radiation dose distribution must be determined based on delayed imaging results (after a few weeks), and categorically not from the initial position of the sources immediately after surgery.

Definition of Irradiation Volumes

Precise radiation therapy planning requires strict definition of Target Volumes (Target Volumes):

• Clinical target volume (Clinical target volume, CTV): Encompasses the visible tumor and accounts for its microscopic spread beyond what can be discerned using modern diagnostic imaging methods.
• Planning target volume (Planning target volume, PTV): Formed by further geometric expansion of the CTV. This expansion is necessary to compensate for set-up errors (set-up errors) related to equipment geometry variability and beam alignment, as well as to account for physiological tumor movement during the irradiation fraction (intrafraction tumour movement).

The goal of treatment optimization is to reduce toxicity to healthy tissues and increase the dose to the tumor itself by reducing the safety margin (safety margin). This is achievable only with advanced imaging methods that ensure high accuracy in volume definition and allow tracking of tumor movement directly during radiation delivery.

Computed Tomography Simulation

To date, Computed Tomography Simulation (Computed Tomography Simulation) has almost completely replaced traditional treatment simulators in radiological practice. The main disadvantage of traditional simulators was that they were based on simple kilovoltage X-ray images. Under such conditions, the treatment area was determined exclusively by bony anatomical landmarks, as the technology did not provide soft-tissue definition (no soft-tissue definition). CT simulation completely solved this problem.

Alternative Techniques: MRI-Guided and Non-Imaging Methods

Imaging approaches and source selection can differ radically for other localizations:

• Magnetic Resonance–Guided Brachytherapy for cervical cancer (Magnetic Resonance–Guided Brachytherapy): Sagittal MRI projection is used for clear visualization of the urinary bladder (with a catheter), tumor, and applicators. Unlike prostate LDR therapy, high-dose-rate (HDR) iridium-based therapy is used here. The system includes an intrauterine tube (Intrauterine tube), a vaginal ring (Vaginal ring), and interstitial catheters (Interstitial catheters) inserted at the tumor periphery for adequate dose coverage.
• Clinical Volume Definition - Non-Imaging Based (Clinical Volume Definition - Non-Imaging Based): Used in palliative treatment or for visually identifiable formations where extreme precision is not critical. For example, in skin cancer, the visible tumor border is simply outlined with a marker with the addition of a margin (margin) for microscopic spread. For deep structures, treatment field boundaries are formed using anatomical landmarks.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Assessment of image fusion quality (Fused CT and MRI) during planning in planning
• Isotope physics: comparison of iodine-125 (LDR) and iridium (HDR)
• Applicator placement technique in cervical cancer brachytherapy
• Methods for compensating respiratory and physiological tumor motion during irradiation

Other topics in the "Diagnostic Radiology in Oncology" section

Prepared by WhiteCoat Pro faculty in accordance with the medical school curriculum. For educational purposes only.