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Cauda equina tolerance to high-dose fractionated irradiation
Richard S Pieters1, Andrzej Niemierko, Barbara C Fullerton
1Department of Radiation Oncology, Massachusetts General Hospital and Harvard Medical School, Boston, MA 02114, USA.
Summary
High dose radiation to the cauda equina can cause late neurologic toxicity, with risk increasing significantly with dose. Females show higher tolerance to radiation than males, with toxicity often occurring years after treatment.
Area of Science:
- Radiation Oncology
- Neurology
- Medical Physics
Background:
- Late neurologic toxicity is a significant concern in radiation therapy.
- Understanding dose-response relationships is crucial for optimizing treatment plans.
- Combined proton and photon therapy presents unique challenges for dose assessment.
Purpose of the Study:
- To evaluate late neurologic toxicity rates and clinical outcomes in patients treated with high-dose fractionated radiation therapy.
- To analyze the impact of combined proton and photon beams on the cauda equina (L2-coccyx).
- To establish dose-tolerance limits for the cauda equina.
Main Methods:
- Retrospective review of 53 patients treated to the cauda equina.
- Dose normalization to equivalent dose in 2-Gy fractions (CGE).
- Analysis of neurologic symptoms onset, local recurrence, and survival rates with a median follow-up of 87 months.
Main Results:
- 13 patients (25%) experienced neurologic toxicity, with 54% occurring 5+ years post-treatment.
- Cauda equina dose and gender were significant predictors of neurotoxicity (p=0.002 and p=0.017).
- Estimated 5-year tolerance doses (TD 5/5 and TD 50/5) were 55/72 CGE for males and 67/84 CGE for females.
Conclusions:
- Neurotoxicity probability is a steep function of cauda equina dose (gamma50 ≈ 3).
- Cauda equina radiation tolerance is approximately 11 CGE higher in females than males.
- Extended follow-up is essential for accurate assessment of neurologic damage, as 5-year evaluations are insufficient.