27.2 Teletherapy vs. Brachytherapy, Patient Immobilization & Side Effect Management

27.2 Teletherapy vs. Brachytherapy, Patient Immobilization & Side Effect Management

1. External Beam Radiation Therapy (Teletherapy)

External beam radiation therapy, or teletherapy, involves delivering ionizing radiation from a source located at a distance (typically 80 cm to 100 cm Source-to-Axis Distance [SAD] or Source-to-Surface Distance [SSD]) from the patient's body.

Primary Teletherapy Energy Sources

1. Megavoltage Linear Accelerators (4 MV to 18 MV Photons):
   • Modern standard of care for teletherapy. High energy photon beams exhibit a pronounced skin-sparing effect, where the depth of maximum dose ($d_{\ ext{max}}$) occurs below the skin surface ($d_{\ ext{max}} \approx 1.5\ ext{ cm}$ for 6 MV, $3.0\ ext{ cm}$ for 18 MV). High MV photon energies provide deep tissue penetration with low surface doses, ideal for deep-seated pelvic, thoracic, and abdominal malignancies.
2. Cobalt-60 Teletherapy Units:
   • Historical teletherapy modality utilizing a sealed radioactive source of Cobalt-60 ($^{60}\ ext{Co}$).
   • Radioactive Decay & Energies: $^{60}\ ext{Co}$ undergoes beta decay into excited Nickel-60 ($^{60}\ ext{Ni}$), which immediately emits two characteristic gamma-ray photons with energies of 1.17 MeV and 1.33 MeV (average energy 1.25 MeV).
   • Half-Life: Radioactive half-life ($t_{1/2}$) of 5.26 years, requiring source replacement or decay corrections of roughly 1% per month.
   • Physical Limitations: Physical source size (typically 1.5 to 2.0 cm diameter) produces a larger geometric penumbra (fuzzy field edge) compared to the small focal spot of a Linac.

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2. Brachytherapy (Internal Sealed Source Therapy)

Brachytherapy ("short-distance therapy") involves placing encapsulated radioactive sources directly inside or immediately adjacent to the tumor volume. Because dose decreases rapidly according to the Inverse Square Law ($I \propto 1/r^2$), brachytherapy delivers an extremely high radiation dose to the tumor while rapidly sparing surrounding normal tissues.

Dose Rate Classifications

• Low Dose Rate (LDR): Delivers dose at 0.4 to 2.0 Gy per hour. Requires continuous inpatient hospital stays lasting 24 to 72 hours with permanent or removable source implants.
• High Dose Rate (HDR): Delivers dose at rates exceeding 12 Gy per hour (> 0.2 Gy/min). Uses a computer-controlled remote afterloader system that drives a high-activity source through specialized catheters for several minutes per fraction on an outpatient basis.

Radioisotopes Utilized in Brachytherapy

• Iodine-125 ($^{125}\ ext{I}$): Half-life of 59.4 days; emits low-energy X-rays (27–35 keV). Widely used as permanent interstitial seed implants for early-stage prostate carcinoma.
• Iridium-192 ($^{192}\ ext{Ir}$): Half-life of 73.8 days; emits average gamma energy of 0.38 MeV. High specific activity makes it the primary radionuclide in modern HDR remote afterloading systems for gynecological, breast, and head and neck cancers.
• Cesium-137 ($^{137}\ ext{Cs}$): Half-life of 30.0 years; emits 0.662 MeV gamma rays. Historically used in LDR manual or remote afterloading for cervical and uterine cancers (Fletcher-Suit applicators).
• Gold-198 ($^{198}\ ext{Au}$): Half-life of 2.7 days; emits 0.412 MeV gamma rays. Used for permanent interstitial implants in localized head and neck tumors.

Anatomic Placement Modalities

1. Interstitial Brachytherapy: Radioactive seeds or needles are inserted directly into the tumor tissue matrix (e.g., prostate seeds, breast interstitial templates).
2. Intracavitary Brachytherapy: Radioactive sources are positioned inside body cavities adjacent to the tumor using specialized applicators, such as tandem and ovoids or tandem and ring for cervical carcinoma.
3. Surface Mold (Epicutaneous) Brachytherapy: Sources are arranged on custom-fit plastic molds applied directly to cutaneous or mucosal surfaces (e.g., skin or hard palate lesions).

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3. Teletherapy vs. Brachytherapy Comparison Table

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4. Patient Immobilization & Positioning Systems

Precise patient positioning and rigid immobilization are imperative to ensure setup reproducibility within < 2 mm tolerance throughout a multi-week course of radiation therapy.

Immobilization Devices

• Thermoplastic Masks: Custom-molded rigid plastic mesh softened in a warm water bath ($65-70^\circ\ ext{C}$) and contoured precisely over the patient's face, head, neck, or shoulders. Used for brain and head/neck radiation therapy.
• Vacuum Cushions (Vac-Lok Systems): Rigid airtight bags filled with polystyrene beads. When positioned under the patient and evacuated with a vacuum pump, the cushion forms a custom, rigid mold supporting the thorax, abdomen, pelvis, or extremities.
• Headrests & Knee Fixators: Standardized foam head cushions (e.g., Silverman headrests A-F) and elevated knee bridges to maintain reproducible spinal curvature and pelvic tilt.
• Laser Alignment Systems: Three-dimensional room lasers (sagittal, bilateral lateral, and overhead ceiling lasers) align with permanent patient skin tattoos or reference marks establishing the treatment isocenter.

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5. Radiation Toxicity & Patient Side Effect Management

Radiation side effects are categorized based on onset timing and underlying cellular pathogenesis into acute toxicities and chronic (late) toxicities.

Acute Side Effects (Occur during or within 90 days of RT)

Caused by rapid radiation-induced cell death in rapidly proliferating stem cell populations (epithelium, mucosal linings, bone marrow):

• Erythema: Radiation dermatitis starting around 20–30 Gy due to cutaneous hyperemia and capillary dilation.
• Dry Desquamation: Skin flaking, pruritus, and dryness caused by depletion of basal layer epidermal stem cells.
• Moist Desquamation: Severe skin peeling, weeping, and exposure of the raw dermal layer occurring in skin folds (groin, axilla, inframammary fold) at doses > 40–50 Gy when basal stem cells are completely depleted.
• Mucositis / Stomatitis: Painful mucosal inflammation, ulceration, and dysphagia resulting from mucosal epithelial sloughing in head and neck RT.
• Alopecia: Hair loss localized exclusively to the irradiated treatment field; temporary at doses < 30 Gy, permanent at doses > 45–50 Gy.
• Fatigue: The single most common universal systemic side effect of radiation therapy, caused by accumulation of cellular debris and cytokine release.

Chronic / Late Side Effects (Occur months to years post RT)

Caused by microvascular damage (endothelial proliferation, ischemia) and irreversible parenchymal replacement by dense connective tissue:

• Fibrosis: Progressive tissue induration, joint stiffness, and loss of organ elasticity.
• Tissue Necrosis: Osteoradionecrosis (especially of the mandible following dental extraction in head/neck fields) and soft tissue necrosis.
• Secondary Malignancy: Radiation-induced secondary neoplasms (e.g., sarcomas or carcinomas) developing in irradiated tissue margins 10 to 30 years post-exposure.

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6. Side Effect Classification & Radiation Nursing Care Table