Brachytherapy Fundamentals: HDR, LDR, and Clinical Applications for FRCR Part 1

Brachytherapy (from Greek "brachy" meaning short distance) involves placing radioactive sources directly into or adjacent to tumours. This allows high radiation doses to the target while sparing surrounding tissues through rapid dose fall-off. Understanding brachytherapy principles is essential for FRCR Part 1.

Brachytherapy Techniques

Intracavitary Brachytherapy:

• Sources placed within a body cavity adjacent to the tumour
• Uses applicators (tandem and ovoids, tandem and ring, cylinders)
• Primary application: Cervical, uterine, vaginal cancers

Interstitial Brachytherapy:

• Sources placed directly into tumour tissue
• Uses needles or catheters implanted through tissue
• Applications: Prostate, breast, head and neck, soft tissue sarcomas

Intraluminal Brachytherapy:

• Sources placed within tubular structures
• Applications: Bronchus, oesophagus, bile duct

Surface (Plaque/Mould) Brachytherapy:

• Sources placed on tissue surface
• Applications: Skin cancers, eye (choroidal melanoma)

Dose Rate Classification

Low Dose Rate (LDR):

• Dose rate: 0.4-2 Gy/hour
• Treatment duration: Hours to days
• Continuous radiation delivery
• Historically used for cervical cancer (Manchester system)
• Still used for permanent prostate implants (I-125 seeds)
• Requires hospitalisation (temporary implants)

High Dose Rate (HDR):

• Dose rate: >12 Gy/hour
• Treatment duration: Minutes per fraction
• Multiple fractions (typically 2-10)
• Remote afterloading (source travels to treatment position)
• Outpatient treatment possible
• No radiation exposure to staff
• Allows dose optimisation

Pulsed Dose Rate (PDR):

• Hourly pulses using HDR-type source
• Mimics LDR radiobiology
• Requires prolonged hospitalisation
• Limited availability

Common Radioisotopes

Isotope	Half-life	Energy	Application
Iridium-192	74 days	380 keV avg	HDR (most common)
Cobalt-60	5.3 years	1.25 MeV	HDR (some centres)
Caesium-137	30 years	662 keV	LDR gynaecological
Iodine-125	60 days	28 keV	Permanent prostate implants
Palladium-103	17 days	21 keV	Permanent prostate implants

Cervical Cancer Brachytherapy

Standard Approach:

• External beam RT (45-50 Gy) + concurrent cisplatin + brachytherapy boost
• Intracavitary technique using tandem (intrauterine) and ovoids/ring (vaginal fornices)
• HDR: 5-6 fractions of 5-7 Gy to Point A
• LDR: 35-40 Gy to Point A

Point A: Traditional prescription point, 2 cm lateral to the cervical canal and 2 cm superior to the lateral fornix (or mucous membrane of vagina).

Image-Guided Brachytherapy:

• GEC-ESTRO recommendations promote MRI-guided planning
• Target volumes: GTV, HR-CTV, IR-CTV
• OAR dose reporting: D2cc for bladder, rectum, sigmoid
• Improved local control and reduced toxicity

Prostate Brachytherapy

LDR Permanent Implants:

• I-125 or Pd-103 seeds implanted transperineally
• Ultrasound-guided placement
• Monotherapy for low-risk disease
• Typical prescription: 145 Gy (I-125) or 125 Gy (Pd-103)

HDR Prostate Brachytherapy:

• Temporary catheters placed transperineally
• Can be monotherapy or boost after EBRT
• Boost: 10-15 Gy in 2 fractions
• Monotherapy: 27-38 Gy in 2-4 fractions

Breast Brachytherapy

Accelerated Partial Breast Irradiation (APBI):

• Interstitial or intracavitary (balloon catheter) techniques
• HDR: 34 Gy in 10 fractions over 5 days
• Alternative to whole breast RT for selected early-stage patients

The Inverse Square Law

The fundamental advantage of brachytherapy is rapid dose fall-off following the inverse square law:

Dose ∝ 1/r²

Doubling the distance reduces dose to 25%. This allows:

• High doses to the target
• Relative sparing of distant normal tissues
• Steep dose gradients at tumour edges

Radiobiological Considerations

LDR vs HDR:

• LDR allows continuous repair during treatment
• HDR requires fractionation to allow repair between fractions
• HDR fractionation schedules designed to be biologically equivalent to LDR

Converting LDR to HDR:

BED calculations are used to design HDR schedules equivalent to historical LDR prescriptions.

Key Exam Points

• Intracavitary: Sources in body cavity (cervix, vagina)
• Interstitial: Sources directly in tissue (prostate, breast)
• LDR: 0.4-2 Gy/hour; HDR: >12 Gy/hour
• Ir-192: Most common HDR source (half-life 74 days)
• I-125: Permanent prostate implants (half-life 60 days)
• Point A: 2 cm lateral and 2 cm superior to cervical os
• D2cc: Dose to most irradiated 2 cm³ of OAR
• Inverse square law governs dose fall-off
• HDR advantages: outpatient, no staff exposure, dose optimisation