Isodose surface differences: A novel tool for the comparison of dose distributions

Area of Science:

• Medical Physics
• Radiation Oncology
• Radiotherapy Quality Assurance

Background:

• Current radiotherapy dose distribution comparisons often rely on statistical methods.
• These methods may not fully capture the clinical significance of detected differences in patient-specific quality assurance (PSQA).

Purpose of the Study:

• To introduce and validate a novel method for comparing dose distributions based on isodose surface differences (ISD).
• To link acceptance tolerances to equipment capabilities and incorporate a clinical perspective into the analysis.

Main Methods:

• Isodose surfaces were represented as histograms based on the distance from the isocenter.
• Isodose Surface Differences (ISD) were calculated as normalized differences between reference and evaluated surface histograms.
• The ISD method was validated against the gamma index using VMAT and IMRT plans with induced errors.

Main Results:

• The ISD method demonstrated sensitivity to all introduced errors in radiotherapy plans.
• Results allowed for evaluation of dose discrepancies per isodose, distinguishing between planning target volume (PTV) and normal tissue regions.
• ISD identified errors missed by conventional gamma analysis.

Conclusions:

• The Isodose Surface Differences (ISD) method offers a clinically relevant, registration-free approach to dose evaluation.
• ISD can serve as a standalone or complementary tool, addressing limitations of existing methods in radiotherapy QA.