Dose calculation accuracy using cone-beam CT (CBCT) for pelvic adaptive radiotherapy.

Area of Science:

• Medical Physics
• Radiotherapy Technology
• Image-Guided Interventions

Background:

• Adaptive radiotherapy requires accurate dose calculations based on daily imaging.
• Cone-beam CT (CBCT) offers volumetric imaging but requires precise Hounsfield Unit (HU) to electron density (ED) calibration for dose accuracy.
• Standard quality assurance (QA) phantoms may not be optimal for CBCT HU calibration.

Purpose of the Study:

• To evaluate the dose calculation accuracy of Varian's CBCT for pelvic adaptive radiotherapy.
• To assess the accuracy of Hounsfield Unit (HU) to electron density (ED) calibration using different phantoms and scan ranges.
• To determine the dosimetric impact of CBCT HU calibration on various radiotherapy treatment plans.

Main Methods:

• Calibrated HU to ED for CBCT using a mini CT QA phantom integrated into an IMRT QA phantom.
• Evaluated Catphan 500 with an annulus for HU calibration, assessing its suitability for bone and air inserts.
• Investigated HU dependence on scatter by varying scan ranges (6 cm, 12 cm, 20.8 cm).
• Acquired and registered CBCT and planning CT for a pelvic phantom.
• Generated single field, 3D conformal, and IMRT plans on both CT datasets.
• Calculated dosimetric differences with and without inhomogeneity correction.

Main Results:

• The combined CT QA and IMRT phantom provided accurate HU calibration.
• Catphan 500 with an annulus yielded inaccurate HU calibration due to unsuitable materials and small inserts.
• Dosimetric differences, particularly with inhomogeneity correction, were primarily attributed to HU calibration discrepancies.
• Maximum dosimetric differences for 6 MV varied: 6.7% (single lateral field), 3.3% (3D conformal), and 2.5% (IMRT).
• 12 cm and 20.8 cm scan ranges showed improved accuracy for specific plan types, with 12 cm optimal for 3D and IMRT.

Conclusions:

• Accurate HU to ED calibration is critical for dose calculation accuracy in CBCT-based adaptive radiotherapy.
• A combined mini CT QA and IMRT phantom is recommended for CBCT HU calibration in pelvic treatments.
• The Catphan 500 phantom is not suitable for CBCT HU calibration due to material and insert limitations.