Brachytherapy

Introduction & Principles - Close Quarters Combat

Brachytherapy ("short-distance" therapy): Radioactive sources are placed directly into or very near the tumor. Rationale: To deliver a high, concentrated radiation dose to the tumor while minimizing exposure to surrounding healthy tissues, leveraging rapid dose fall-off.

• Advantages:
  • High dose conformity to target.
  • ↓ radiation toxicity to normal tissues.
  • Potentially shorter overall treatment time (in select cases).
• Disadvantages:
  • Invasive procedure.
  • Risk of radiation exposure to medical staff.
  • Limited to localized and accessible tumors.

Underlying Physics: The Inverse Square Law is fundamental. Radiation intensity ($I$) from a point source decreases with the square of the distance ($d$) from the source: $I \propto 1/d^2$. This explains the steep dose gradient.

⭐ Brachytherapy allows for significant dose escalation to the tumor volume, which can lead to improved local control rates, particularly in cancers like cervical, prostate, and some head & neck sites when compared to External Beam Radiation Therapy (EBRT) alone.

📌 Remember: Close contact, High dose to tumor, Sharp fall-off protects normal tissue (CHS).

Sources & Classification - Radioactive Seeds & Systems

Radioactive Sources: Key isotopes:

Dose Rate Classification: 📌 LDR (Low & Long), HDR (High & Hurry)

• Interstitial: In tissue (prostate, breast).
• Intracavitary: In cavity (GYN, esophagus).
• Intraluminal: In lumen (bronchus, bile duct).
• Surface: On skin/eye (skin Ca, pterygium).
• IORT: Surgery, tumor bed.

Loading Techniques:

• Manual: Historical, ↑staff exposure.
• Remote Afterloading: Standard, remote load, ↓staff exposure.

⭐ Ir-192: versatile for LDR/HDR; optimal half-life & gamma energy.

LDR vs. HDR Choice:

Applications & Techniques - Precision Strikes

Brachytherapy delivers highly conformal radiation directly to/near the tumor, sparing normal tissues.

• Temporary Implants: Sources removed after treatment (e.g., HDR for cervix, breast).
• Permanent Implants (Seeds): Sources remain indefinitely (e.g., LDR for prostate - I-125, Pd-103).

Common Brachytherapy Applications:

⭐ Point A (cervical Ca): 2 cm superior to cervical os & 2 cm lateral to tandem. Crucial for dose prescription.

Dosimetry & Safety - Dose, Dangers, Defense

• Dosimetry & Planning Goals:
  • ICRU reference points guide dose reporting.
  • DVH analysis: D90 (e.g., >95% PTV coverage), V100 (target volume receiving 100% dose), OAR dose constraints (e.g., V20 for rectum).
• Brachytherapy Workflow:

• Radiation Safety (📌 ALARA):
  • Time: Minimize exposure duration.
  • Distance: Maximize distance from sources (inverse square law).
  • Shielding: Use barriers (lead, concrete) for staff/public.
• Complications:
  • Acute: Mucositis, dermatitis, pain, swelling. Resolve with supportive care.
  • Late: Fibrosis, necrosis, strictures, fistulae. Site-specific, potentially permanent.

⭐ High Dose Rate (HDR) brachytherapy allows for outpatient treatment and optimized dose distributions due to source stepping ability during treatment delivery for complex target volumes near critical structures.

High‑Yield Points - ⚡ Biggest Takeaways

• Brachytherapy delivers radiation internally, with sources placed close to or inside the tumor.
• Key types include Low Dose Rate (LDR) for continuous exposure and High Dose Rate (HDR) for short, intense treatments.
• Common isotopes: Iridium-192 (most common for HDR), Iodine-125, Palladium-103 (LDR permanent implants like prostate seeds).
• Offers high localized dose to the target volume, significantly sparing surrounding healthy tissues due to rapid dose fall-off.
• Widely used for cervical, prostate, breast, esophageal, and skin cancers.
• The inverse square law (I ∝ 1/d²) is a fundamental principle governing its dose distribution.
• Manchester system and Paterson-Parker rules are historical systems for dose calculation, especially in gynecological applications.