Accuracy of manufacturer integrated quality control for helical radiotherapy

Background and Purpose

Manufacturer-integrated quality control (MIQC) systems are often used but not considered standard in codes-of-practice (COP), such as TG148/306 or NCS27, for helical radiotherapy. MIQC can lead to false positive results and generally lacks external validation. Energy quality control (QC) conditions are defined in COPs, manuals, or MIQC using various field sizes, phantoms, and indicators assuming equal response functions to energy changes. This study investigated the accuracy of MIQC for helical radiotherapy.

Materials and Methods

A clinical helical treatment unit was detuned in terms of energy, dose rate, field width, and air pressure. The reproducibility/precision and response/trueness of MIQC, conventional QC methods, and patient-specific quality assurance were evaluated. Monte Carlo calculations were performed to identify differences in responses of depth dose ratios DD10/1.5, DD20/1.5, DD20/10, Tissue-Phantom Ratio TPR20/10, and ratio to max DD10(x) for various field sizes and phantoms.

Results

The accuracy of MIQC for underlying causes was determined; precision was often excellent, but trueness required proportionality correction: e.g., 1 % DD10(x, 5 × 10 cm², H₂O) showed almost equal response to TPR20/10, DD20/10 and exit detector flatness in most conditions but a 2 % DD20/1.5(1x40cm²) and step-wedge MIQC response. Exit detector output constancy was not significantly sensitive to field size changes but was sensitive to energy and dose rates. A guiding table containing response functions and reproducibility coefficients was established.

Conclusions

The MIQC metrological accuracy assessment can be used to define action/tolerance limits for COPs as well as to easily analyze out-of-bound results in routine practice.