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:
| Isotope | Sym | Half-life | Energy (MeV) | Type | Uses |
|---|---|---|---|---|---|
| Iridium-192 | Ir-192 | 73.8 d | 0.38 (avg $\gamma$) | LDR/HDR | Interstitial, Intracavitary |
| Iodine-125 | I-125 | 59.4 d | 0.028 (avg $\gamma$) | LDR | Prostate, Eye |
| Palladium-103 | Pd-103 | 17 d | 0.021 (avg $\gamma$) | LDR | Prostate |
| Cesium-137 | Cs-137 | 30.2 y | 0.662 ($\gamma$) | LDR | GYN (hist.), Intracavitary |
| Strontium-90 | Sr-90 | 29 y | Y-90 (2.28 $\beta$) | LDR | Surface (Pterygium) |
Dose Rate Classification: 📌 LDR (Low & Long), HDR (High & Hurry)
| Rate | Dose Rate ($Gy/hr$) | Duration | Examples |
|---|---|---|---|
| LDR | 0.4-2 | Days | Prostate seeds, GYN (old) |
| MDR | 2-12 | Hours | Some GYN, H&N |
| HDR | >12 | Mins/frac | GYN, Bronchus, Esoph, Breast |
| PDR | Pulsed (~1 $Gy/pulse$) | Hrs (pulses) | Mimics LDR radiobiologically |
- 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:
| Site | Isotopes | Technique/Applicator | Key Point / Dose Example |
|---|---|---|---|
| Cervix | Ir-192, Cs-137 | Tandem & Ovoids (Fletcher-Suit, Manchester), Interstitial | Point A: 80-85 Gy (EQD2) |
| Prostate | I-125, Pd-103 (LDR); Ir-192 (HDR) | LDR seeds; HDR catheters | LDR: 145 Gy (I-125) |
| Breast | Ir-192 | APBI (Interstitial, MammoSite) | APBI: 34 Gy/10 fr |
| H&N (e.g. Lip, Tongue) | Ir-192 | Interstitial, Moulds | Boost/Monotherapy |
| Skin | Ir-192, Sr-90 | Moulds, Applicators | Superficial lesions |
| Lung (Endobronchial) | Ir-192 (HDR) | HDR catheter | Palliation, Boost |
| Esophagus | Ir-192 (HDR) | Intraluminal HDR | Palliation |
⭐ 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.
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