Key Facts

Radiotherapy is a cornerstone of cancer treatment, using ionising radiation to cause lethal DNA damage in cancer cells. Understanding the principles of radiobiology, fractionation, and treatment planning is essential for postgraduate medical exams.

Epidemiology

• Approximately 50% of cancer patients receive radiotherapy
• Contributes to 40% of all cancer cures (second only to surgery)
• Used with curative intent in approximately 20% of patients and palliative intent in 30%

Aetiology

• Not applicable (treatment modality)

Pathophysiology

• Direct action: ionising radiation directly damages DNA (double-strand breaks are the key lethal lesion)
• Indirect action: radiation generates free radicals (from water radiolysis) that damage DNA; enhanced by oxygen (oxygen effect)
• Differential sensitivity: tumour cells have impaired DNA repair mechanisms compared to normal tissue
• 4 Rs of radiobiology: Repair (sublethal damage repair in normal tissue), Reassortment (redistribution of cells into radiosensitive phases), Reoxygenation (previously hypoxic tumour cells become oxygenated), Repopulation (accelerated repopulation during treatment)
• Linear-quadratic model: describes relationship between radiation dose and cell kill (α/β ratio); high α/β (e.g. 10 for most tumours) = more sensitive to fractionation; low α/β (e.g. 3 for late-responding tissues) = more sensitive to dose per fraction

Indications for Radiotherapy

• Curative: as primary treatment (cervical, head & neck, prostate) or adjuvant (breast, rectal, brain)
• Palliative: bone metastases (pain relief), brain metastases, SVCO, spinal cord compression, haemostasis
• Emergency: SVCO, spinal cord compression (urgent RT within 24 hours)

Acute Side Effects (during and up to 6 weeks post-RT)

• Skin: erythema, desquamation, pigmentation
• Mucositis: oral, oesophageal, rectal (depending on site)
• Fatigue: most common; affects 80-90% of patients
• Nausea/vomiting: if abdominal/pelvic RT
• Bone marrow suppression: if large fields

Late Side Effects (months to years)

• Fibrosis (lung, bowel, subcutaneous tissue)
• Stricture (oesophageal, rectal, ureteric)
• Secondary malignancy (typically 10-20 years later)
• Lymphoedema
• Infertility
• Cardiac toxicity (if heart in field)
• Radiation myelopathy (spinal cord)

Red Flags

• Spinal cord compression: urgent RT needed within 24 hours
• Radiation pneumonitis: cough, dyspnoea 2-6 months post-RT
• Radiation enteritis: severe diarrhoea, bleeding
• Osteoradionecrosis of jaw: pain, exposed bone (dental assessment before head & neck RT)

Bedside

• Clinical assessment of treatment site
• Skin assessment (CTCAE grading)
• Swallowing assessment if head & neck RT

Bloods

• FBC (myelosuppression if large fields)
• U&Es, LFTs (baseline)
• TFTs (if neck irradiated - hypothyroidism risk)

Imaging

• CT simulation: planning scan for treatment field design
• MRI fusion: improved target delineation (brain, pelvis)
• PET-CT: for target volume definition and response assessment
• Follow-up imaging: response assessment per cancer type

Special Tests

• DVH (dose-volume histogram): evaluates dose distribution to target and organs at risk
• Pulmonary function tests: before lung RT
• Audiometry: if cochlea in field
• Dental assessment: before head & neck RT (prevent osteoradionecrosis)

Non-pharmacological

• Patient education about expected side effects and timeline
• Skin care: gentle washing, aqueous cream, avoid sun exposure
• Nutritional support: dietitian input for head & neck / GI RT
• Oral care: fluoride trays, saliva substitutes
• Psychological support

Pharmacological

• Skin reactions: emollients (aqueous cream), moderate potency steroid cream (betamethasone 0.1%) if symptomatic
• Mucositis: benzydamine mouthwash, gelclair, systemic analgesia (opioids if severe)
• Nausea: ondansetron 8mg, dexamethasone 4mg
• Radiation pneumonitis: prednisolone 40-60mg tapering over 4-8 weeks
• Radiation proctitis: sucralfate enemas, topical steroids
• Amifostine: radioprotectant (limited use; reduces xerostomia in head & neck RT)

Surgical/Interventional

• Fiducial marker placement for image-guided RT
• Brachytherapy applicator insertion
• SpaceOAR hydrogel injection (rectal spacer for prostate RT)

Referral Criteria

• All radiotherapy prescribed by clinical oncologist within MDT
• Emergency RT: spinal cord compression, SVCO (immediate referral)
• Late radiation toxicity: specialist management

• Curative RT: 5-year local control rates depend on cancer type (e.g. cervical 80-90%, prostate 85-95%, early laryngeal 90%)
• Palliative RT for bone metastases: pain response in 60-80% (8 Gy single fraction as effective as 20 Gy in 5 per NCRI Bone Pain Trial)
• Late toxicity risk: generally <5-10% for serious complications with modern techniques
• Secondary malignancy risk: approximately 1-2% at 10-20 years
• IMRT and proton therapy reduce late toxicity compared to conventional techniques

Advanced Radiotherapy Techniques

Landmark Radiotherapy Trials