2.3 Radiation Therapy Modalities & Radiation Toxicities

2.3 Radiation Therapy Modalities & Radiation Toxicities

Radiation therapy (RT) utilizes high-energy ionizing radiation to destroy malignant cells by causing direct and indirect double-stranded DNA damage. Approximately 50% of all cancer patients receive radiation therapy during their disease trajectory—either as definitive curative primary therapy, neo-adjuvant treatment prior to surgery, adjuvant therapy post-resection, or palliative symptom management. Because radiation damage is anatomically restricted to the designated target radiation field, toxicities and nutritional sequelae are strictly site-specific.

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Radiation Physics & Delivery Modalities

Unit of Radiation Dose

Radiation dosage is quantified in Grays (Gy), where 1 Gy represents the absorption of 1 Joule of ionizing radiation energy per kilogram of target tissue (1 Gy = 100 rads).

• Fractionation: Standard radiation regimens divide the total cumulative dose into small daily fractions—typically 1.8 to 2.0 Gy per day, 5 days per week, over 5 to 7 weeks. Fractionation allows normal tissue stem cells to repair sub-lethal DNA damage, reassort into radio-resistant cell cycle phases, and reoxygenate between fractions, exploiting repair capacity differences between malignant and healthy tissues.

Primary Delivery Modalities

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|                                    RADIATION THERAPY MODALITIES                                         |
| 1. External Beam Radiation Therapy (EBRT): Photons (X-rays) generated by linear accelerators (linacs). |
| 2. Intensity-Modulated Radiation Therapy (IMRT): Advanced EBRT using computer-controlled beams to match  |
|    3D tumor contours tightly, sparing adjacent critical normal structures (e.g., parotid glands).      |
| 3. Proton Beam Therapy: Uses heavy charged protons exhibiting a Bragg Peak (zero exit dose).            |
| 4. Brachytherapy: Sealed radioactive sources (e.g., I-125, Ir-192) placed directly within or adjacent   |
|    to tumor tissue (e.g., prostate, cervical carcinoma).                                               |
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The Bragg Peak in Proton Therapy

Unlike photon beams (X-rays), which deposit maximum radiation dose near the skin entry surface and continue delivering energy along their entire exit path through healthy tissue, protons deposit low energy upon entering tissue, release their maximum destructive energy at a specific depth (the Bragg Peak), and instantly drop to zero exit dose. This physical property dramatically reduces collateral radiation damage to surrounding normal organs at risk (e.g., sparing spinal cord, heart, or bowel loops).

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Cellular Mechanisms: Acute vs. Late Toxicities

Ionizing radiation generates reactive oxygen species (hydroxyl free radicals via water radiolysis) that cause single- and double-strand DNA breaks. Cellular death primarily occurs when damaged cells attempt mitosis (mitotic catastrophe).

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Site-Specific Radiation Toxicities & MNT Interventions

1. Head & Neck Radiation (Field: Oral Cavity, Pharynx, Larynx, Salivary Glands)

• Key Toxicities:
  • Mucositis: Painful mucosal erythema, pseudomembrane formation, and ulceration. Graded on WHO scale (Grade 1: soreness; Grade 4: complete ulceration requiring enteral/parenteral nutrition). Begins at cumulative doses >20 Gy.
  • Xerostomia (Dry Mouth): Ionizing radiation damage to parotid and submandibular acinar cells. Cumulative doses >20–30 Gy cause severe dysfunction; doses >50 Gy cause permanent acinar destruction. Leads to loss of salivary amylase, thick viscous mucus, dental caries, altered taste, and severe dysphagia.
  • Dysgeusia & Ageusia: Destruction of taste bud microvilli occurring at >20 Gy. Manifests as metallic or bitter taste, high urea threshold, or total ageusia.
  • Osteoradionecrosis (ORN): A devastating late toxicity occurring at cumulative doses >60 Gy, characterized by bone death, avascular necrosis, and loss of mandibular structural integrity.
• MNT Interventions:
  • Prophylactic placement of enteral feeding tubes (PEG/RIG) prior to RT when anticipated weight loss is >5% or severe mucositis/xerostomia is expected.
  • Salivary substitutes, artificial saliva sprays, salt and soda oral washes (avoid alcohol-containing rinses), and prescription sialagogues (pilocarpine 5 mg TID, cevimeline).
  • Moisten foods with gravies, sauces, or extra virgin olive oil; encourage soft, smooth textures.
  • Avoid acidic, spicy, salty, rough, or hot-temperature foods.

2. Thoracic Radiation (Field: Esophagus, Lung, Mediastinum, Breast)

• Key Toxicities:
  • Acute Esophagitis: Begins at cumulative doses of 20–30 Gy (typically weeks 2–3 of therapy). Manifests as severe odynophagia (painful swallowing), substernal burning, retrosternal chest pain, and globus sensation.
  • Radiation Pneumonitis & Fibrosis: Acute inflammatory cough and progressive exertional dyspnea.
  • Esophageal Strictures: Late toxicity occurring months post-RT due to submucosal fibrosis, causing mechanical dysphagia.
• MNT Interventions:
  • Soft, smooth, non-irritating bland diet (smoothies, puddings, room-temperature textures).
  • Viscous lidocaine topical solutions prior to meals to suppress odynophagia.
  • Avoid hot temperature foods, extreme cold, caffeine, citrus, and carbonated beverages.
  • If luminal narrowing occurs: Endoscopic dilation or enteral feeding tube placement (nasojejunal or J-tube if esophageal access is compromised).

3. Abdominal & Pelvic Radiation (Field: Stomach, Pancreas, Colon, Rectum, Cervix, Prostate, Bladder)

• Key Toxicities:
  • Acute Radiation Enteritis / Proctitis: Manifests at cumulative doses of 30–40 Gy. Radiation damages rapidly dividing crypt cells of the small and large intestine, causing loss of villous height, mucosal blunting, decreased disaccharidase activity (lactase deficiency), watery diarrhea, tenesmus (painful straining), abdominal cramping, and rectal bleeding.
  • Late Radiation Enteritis: Fibrotic thickening of the intestinal wall, intestinal ischemia, non-healing ulcerations, malabsorption, strictures, partial mechanical bowel obstructions, and enterocutaneous/enterovaginal fistulae.
• MNT Interventions:
  • Acute Enteritis: Low-fat, low-residue, lactose-free, low-FODMAP diet. Soluble fiber supplementation (psyllium, pectin) to bulk stool. Isotonic fluid replacement.
  • Avoid insoluble fiber, alcohol, caffeine, sugar alcohols (sorbitol, xylitol), and high-fat foods.
  • Late Enteritis / Strictures: If severe malabsorption or chronic bowel obstruction develops, long-term Home Total Parenteral Nutrition (HPN) is indicated for bowel rest and maintenance of nutritional status.
  • Elemental or semi-elemental enteral formulas (containing hydrolyzed proteins and MCT fats) if small bowel function is partially preserved.