Quantitative vs. subjective portal verification using digital portal images

Area of Science:

• Medical Physics
• Radiation Oncology
• Image Processing

Background:

• Accurate patient positioning is crucial in radiation therapy.
• Image registration aligns prescription and treatment fields to ensure accuracy.
• Chamfer matching is a technique used for image registration.

Purpose of the Study:

• To evaluate the performance of interactive anatomy and field edge extraction for image registration.
• To compare observer perceptions of field accuracy with measured discrepancies after anatomical registration.

Main Methods:

• 48 patient cases were analyzed using digital prescription and on-line portal images.
• Images underwent adaptive histogram equalization and edge extraction for anatomical registration.
• Chamfer matching quantified field conformity, comparing quantitative measures with oncologists' subjective evaluations.

Main Results:

• Registration achieved high conformity, with mean field coverage of 95% and overcoverage of 7%.
• Mean translational displacements were 2.9 mm (transverse) and 3.4 mm (cranio-caudal), with 2 degrees rotational displacement.
• Unacceptable portal placements correlated significantly with higher transverse translational errors and lower field coverage.

Conclusions:

• Transverse translational error most strongly correlated with oncologists' field placement assessments.
• Field coverage and cranio-caudal translational error showed variable correlation with oncologists' evaluations.
• Future automated portal verification systems must model the varying significance of different localization errors in clinical practice.