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Oncology-Brachytherapy
I. Definition and Indications:
I. Definition and Indications:
- Brachytherapy: Radiation therapy where sources are placed within or near the tumor. Crucially, precise tumor extent is vital because treatment targets a small volume, and missing the tumor (geographic miss) significantly increases recurrence risk. Accessibility for source insertion and removal, along with accurate source positioning, is also crucial.
- High Localized Dose: Delivers a high dose to a small area, maximizing local tumor control while minimizing damage to surrounding normal tissue due to the sharp fall-off of radiation dose.
- Short Treatment Duration (2-7 days): Uses low-dose-rate irradiation, leveraging the differing repair/repopulation rates of normal and malignant cells for enhanced therapeutic ratio. This also allows for reoxygenation of initially resistant hypoxic cells, increasing their radiosensitivity.
- Compensation for Hypoxic Cells: Higher doses in the tumor center (near sources) help overcome the radioresistance of hypoxic cells often found in avascular/necrotic areas.
- Irregular Tumor Treatment: Allows for treatment of irregularly shaped tumors by precise source placement, avoiding critical normal tissues.
- Staff Radiation Exposure: γ-emitting sources expose staff to low but significant radiation. Afterloading techniques and low-energy radionuclides mitigate this risk.
- Unsuitable for Large Tumors: Typically not suitable for large tumors; may be used as a boost after external beam radiotherapy (EBRT) and/or chemotherapy.
- Accurate Source Positioning Crucial: Dose falls off rapidly (inverse square law), requiring precise source placement. This demands specialized skill and is not universally available.
- Limited Treatment Volume: Surrounding structures (e.g., lymph nodes) are not irradiated.
- Intracavity: Radioactive material placed within body cavities (e.g., cervix, bronchus, esophagus, bile duct).
- Interstitial: Radioactive material inserted into tissues (e.g., prostate, breast, head & neck, anal).
- Surface: Radioactive material placed on the tumor surface (e.g., skin, eye).
- Manual Insertion: Should be avoided due to radiation hazards to staff.
- Afterloading: Radioactive material loaded into pre-inserted applicators (needles, catheters). Reduces staff exposure significantly, allowing for optimal source placement.
- Manual Afterloading: Radioactive material manually loaded into applicators.
- Remote Afterloading: Machines (e.g., Selectron, Microselectron, Cathetron) control source placement, eliminating staff exposure. High-dose-rate remote afterloading often involves multiple outpatient fractions.
- γ Emitters:
- Radium: Obsolete; radon gas is a hazard.
- Cesium-137: Replaces radium; longer half-life (30 years), less penetrating γ rays.
- Iridium-192: Used in wires or seeds; flexible, advantages in interstitial brachytherapy; shorter half-life (74 days).
- Iodine-125: Used for permanent prostate implants; short half-life (59.6 days), low-energy γ rays allow for early discharge.
- β Emitters: Primarily used in eye tumor treatment (strontium-90, ruthenium-106/rhodium-106 plaques).
- Treatment Planning Systems: Various systems (Paris system, Parker-Paterson, Quimby) are used to plan source distribution. The Paris system, common for iridium wire implants, uses parallel, equidistant wires. Computer calculations and graphs (Oxford cross-line curves) assist in dose calculation.
- Dose Prescription: The basal dose rate (mean of minimum values between sources) is calculated. Treatment dose is prescribed to a reference dose line (often 85% of basal dose). Prescription points vary depending on the treatment type (e.g., Manchester A point for gynecological treatments). The ICRU Report 38 recommends reporting dose based on the volume enclosed by a 60Gy isodose line in gynecological treatments.
- 3D Planning: Sophisticated 3D planning using CT or MRI scans for precise dose calculations.
- Biological Effective Dose: Incorporating biological effects in dose calculations.
- High-Dose-Rate Remote Afterloading: Continues to reduce staff exposure and improve treatment outcomes. High-dose-rate pulsed insertions are increasingly replacing continuous low-dose-rate implants for greater homogeneity.
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