Using three-dimensional equieffective dose mapping to audit a methodology for calculating permitted doses for head...

Area of Science:

• Radiation Oncology
• Medical Physics

Background:

• Reirradiation is becoming more common in clinical practice.
• Accurate dose calculation for the spinal cord and brainstem during reirradiation is complex.
• Prior radiation dose must be carefully considered to ensure patient safety.

Purpose of the Study:

• To compare a simple isodose-based method with voxel-wise equivalent dose in 2 Gy fractions (EQD2/2) mapping for reirradiation dose compensation.
• To assess the accuracy and safety of the isodose-based method for spinal cord and brainstem dose calculation.

Main Methods:

• Retrospective review of ten head and neck reirradiation cases.
• Mapping of original dose distributions onto reirradiation CT scans.
• Comparison of cumulative doses calculated by the isodose-based method and 3D EQD2/2 mapping.

Main Results:

• The isodose-based method was found to be safe, with all cumulative doses within tolerance.
• A maximum difference of 9.7 Gy was observed between the two methods.
• In two cases, spinal cord and brainstem doses approached tolerance limits.

Conclusions:

• A simple, isodose-based method for calculating spinal cord and brainstem reirradiation tolerance is safe and effective.
• This practical method can be implemented in standard planning systems without advanced software.
• It offers a viable alternative for managing reirradiation doses when sophisticated tools are unavailable.