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How valid is the assumption of equal effect per fraction?
M Rezvani1, G Ross, G M Morris
1Research Institute, University of Oxford, Churchill Hospital, UK.
Summary
Fractionated radiotherapy dose per fraction validity was tested using a rat foot model. Findings challenge the assumption of equal biological effect, indicating repair and variable effectiveness in fractionated radiotherapy.
Area of Science:
- Radiation Oncology
- Radiobiology
Background:
- Fractionated radiotherapy is a cornerstone of cancer treatment.
- The assumption of equal biological effect per dose fraction is widely used.
- This assumption underpins dose calculations in radiotherapy planning.
Purpose of the Study:
- To investigate the validity of the equal biological effect assumption in fractionated radiotherapy.
- To examine the acute skin reaction in a rat foot model.
- To assess the impact of varying numbers of 2-Gy fractions followed by top-up doses.
Main Methods:
- Utilized a rat hind foot model for irradiation studies.
- Administered fractionated doses of 60Co gamma-rays with varying top-up doses.
- Assessed moist desquamation as the primary endpoint and analyzed data using probit analysis.
Main Results:
- A single 2-Gy fraction followed by a top-up dose showed reduced effectiveness (58%), suggesting repair.
- Multiple 2-Gy fractions initially increased top-up dose effectiveness.
- Further increases in fraction number led to decreased top-up dose effectiveness, contradicting predictions.
Conclusions:
- The study's findings contradict the assumption of equal biological effect per fraction.
- The linear-quadratic (LQ) model with a single alpha/beta ratio could not explain the observed results.
- Radiotherapy dose fractionation may involve complex biological responses not captured by current models.