Brachytherapy — General Overview & Principles

Definition

"As long as the sources are placed precisely within the tumour, there is minimal exposure to radiation of healthy tissues further away from the sources. This allows very high doses to be administered to the target volume." — Grainger & Allison's Diagnostic Radiology

Classification

By Placement

By Dose Rate

By Duration

• Permanent implants — Radioactive seeds left in the patient; activity decays over time (e.g., ¹²⁵I, ¹⁰³Pd in prostate LDR)
• Temporary implants — Applicators/catheters placed, source delivered, then removed (e.g., HDR ¹⁹²Ir afterloading)

Radiation Sources

Technique: Remote Afterloading

1. Inactive applicators (hollow tubes, needles, or catheters) are placed in or around the tumour — ideally under real-time imaging (ultrasound, CT, or fluoroscopy).
2. The patient is taken to a shielded room.
3. A remote afterloading machine (e.g., Nucletron Flexitron, Varian GammaMedPlus) robotically advances the ¹⁹²Ir source along each applicator.
4. The source dwells for a calculated number of seconds at 5 mm intervals within the tumour volume — the dwell time at each position determines local dose.
5. The source is retracted and the applicators removed (or left for the next fraction).

Dosimetry & Imaging

• Dosimetry is computed by summing contributions from each source position within the volume.
• CT is preferred for accurate applicator localisation.
• MRI provides superior soft-tissue delineation, particularly for prostate and gynaecological cancers.
• Specific MRI sequences (proton-rich) can identify ¹²⁵I seeds in the prostate.
• Because sources move with the tumour (unlike external beam), patient setup errors and tumour motion are less clinically significant.

Clinical Applications

1. Prostate Cancer

• LDR seed implant: ¹²⁵I or ¹⁰³Pd seeds implanted transperineally under transrectal ultrasound (TRUS) guidance. Used for low-to-intermediate risk disease.
• HDR brachytherapy: ¹⁹²Ir afterloading, typically 2+ fractions; used as monotherapy or boost after EBRT for intermediate/high-risk disease.
• Potency preservation rates appear superior to prostatectomy or EBRT alone.

2. Gynaecological Cancers

• Cervical cancer: Tandem-and-ring or tandem-and-ovoid intracavitary brachytherapy is a cornerstone of radical treatment, delivering boost dose after EBRT.
• Endometrial cancer (adjuvant): Vaginal vault brachytherapy (HDR 21 Gy in 3 fractions to 5 mm depth) — vaginal control rates 98–100%. The PORTEC-2 trial demonstrated equivalence of vaginal brachytherapy vs. pelvic EBRT for high-risk early-stage endometrial cancer.

3. Breast Cancer

• Accelerated partial breast irradiation (APBI) via interstitial catheters or intracavitary balloon (MammoSite) device.
• Shorter treatment duration than whole breast EBRT (days vs. weeks).

4. Endobronchial / Lung (Palliation)

• Polyethylene catheter placed via flexible bronchoscopy adjacent to central airway tumour; HDR ¹⁹²Ir delivered in ~3 sessions over 1 week.
• Primary goal is palliation: improves cough (20–70%), dyspnoea (25–80%), haemoptysis (70–90%).
• Not appropriate when a fistula involves the airway, or when the tumour directly invades major vessels.
• Fatal haemoptysis reported in up to 5–10% of patients.

5. Other Sites

• Head and neck (tongue, lip, floor of mouth)
• Skin tumours (surface moulds)
• Bile duct / oesophageal tumours
• Soft tissue sarcoma (interstitial catheters placed intraoperatively)

Advantages Over External Beam Radiation Therapy (EBRT)

Complications

• Early: Local pain/discomfort, urinary symptoms (prostate), vaginal discharge
• Late: Radiation proctitis, urinary incontinence/stricture, fistula, radiation bronchitis/stenosis (endobronchial)
• Endobronchial-specific: Fatal haemoptysis (5–10%), airway stenosis, fistula formation

Key Isotopes at a Glance

¹²⁵I   — 28 keV, T½ 60 days  → Permanent prostate LDR
¹⁰³Pd  — 21 keV, T½ 17 days  → Permanent prostate LDR
¹⁹²Ir  — ~380 keV, T½ 74 days → HDR afterloading (most applications)
¹³⁷Cs  — 662 keV, T½ 30 yr   → Historical gynaecological LDR