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Oncology- Radiotherapy Fractionation
I. Core Concepts:
Two main approaches exist:
A. Few Large Daily Fractions:
A. Radical Radiotherapy (Curative Intent):
I. Core Concepts:
- Objective of Fractionation: To determine the optimal combination of total radiation dose (Gy), number of doses (fractions), and overall treatment time to effectively target cancer while minimizing damage to healthy tissues.
- Linear Quadratic (LQ) Model: This model describes how tissues respond to radiation. Early-reacting tissues (e.g., skin) show a linear relationship between dose and damage (α component). Late-reacting tissues (e.g., spinal cord) exhibit a quadratic relationship, meaning damage increases disproportionately with dose (β component). The LQ model supports the use of many small fractions to minimize late-reacting tissue damage.
Two main approaches exist:
A. Few Large Daily Fractions:
- Advantages: Fewer patient visits, less resource consumption, faster tumor response, reduced tumor repopulation during treatment.
- Disadvantages: Limits total safe dose, increases risk of late tissue damage, reduced reoxygenation potential (which improves radiotherapy effectiveness), often insufficient to eradicate all cancer cells.
- Advantages: Less severe acute reactions (due to longer treatment time), reduced late tissue damage, allows for higher total dose, maximizes reoxygenation, potentially eradicates all cancer cells, allows dose reduction if acute reactions are unexpectedly severe.
- Disadvantages: Increased resource and patient demand, potential for tumor repopulation during prolonged treatment, prolonged acute reactions may require supportive care.
- Tumor Radiosensitivity: Varies widely. Some tumors (lymphoma, seminoma) are highly radiosensitive, requiring lower doses. Others (gliomas, sarcomas) are radioresistant.
- Normal Tissue Tolerance Doses (2Gy per fraction): These are maximum doses before significant late damage occurs:
- Testis: 2 Gy
- Lens of the eye: 10 Gy
- Whole kidney: 20 Gy
- Whole lung: 20 Gy
- Spinal cord: 50 Gy
- Brain: 60 Gy
- Inter-fraction Interval: At least 6 hours should separate fractions to allow for tissue repair. With once-daily fractionation, nearly all repair occurs before the next treatment.
- Hyperfractionation: Delivers many small fractions (<2gy) to increase total dose without increasing late tissue damage. may involve weekend treatments or multiple daily treatments.< />pan>
- Accelerated Radiotherapy: Shortens overall treatment time to reduce tumor repopulation during treatment. Often combined with hyperfractionation (e.g., ChaRT regimen). Example: ChaRT (Continuous Hyperfractionated Accelerated RadioTherapy) delivers 54 Gy in 1.5 Gy fractions three times daily for 12 days.
A. Radical Radiotherapy (Curative Intent):
- Goal: Highest tolerable dose for maximal cancer eradication.
- Lower doses used for highly radiosensitive tumors or microscopic residual disease.
- Multiple daily fractions (~2Gy) minimize late damage.
- Significant acute toxicity is acceptable due to potential survival benefit.
- Requires patient fitness for daily attendance.
- Goal: Quick symptom relief, may not impact survival significantly.
- Lowest effective dose and fraction number are preferred.
- Avoid prolonged acute damage; late effects may be irrelevant.
- High-dose palliative radiotherapy may be appropriate in specific advanced cases with durable local disease control as a goal and reasonable life expectancy.
- Understand the LQ model: This is fundamental to understanding why fractionation is used.
- Compare and contrast: Clearly differentiate between few large vs. many small fractions, highlighting advantages and disadvantages of each.
- Know tolerance doses: These are crucial for safe treatment planning.
- Understand advanced techniques: Grasp the principles behind hyperfractionation and accelerated radiotherapy.
- Distinguish treatment intent: Recognize how treatment goals (radical vs. palliative) influence regimen selection.
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