A multi-centre dosimetry audit on advanced radiotherapy in lung as part of the Isotoxic IMRT study

Background and purpose In lung cancer patients, the accuracy of dose calculation by radiotherapy treatment planning systems may be reduced by the presence of tissue inhomogeneities. Intensity-Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT) commonly include the use of small field segments which can exacerbate these issues. A multicentre dosimetry audit was undertaken to verify the accuracy of treatment delivery at participating centres. Materials and methods An inhomogeneous phantom with regions of lung equivalent materials was used. The audit consisted of two parts: basic beam model tests and clinical trial plan measurements. An ArcCheck diode array was used to measure fluences from the treatment delivery. Ion chamber and film planar dose readings near the interface were acquired. Results Nine centres were visited, with a total of eleven distinct combinations of planning and delivery system. Significant differences were found in ion chamber dose measurements in basic beam model tests between treatment planning system algorithms. For the clinical trial plan test, gamma analysis of the entry and exit dose fluence showed good agreement, with mean pass rates of 97.2% (95% CI: 95.3–99.1) and 99.0% (95% CI: 98.3–99.7) for tolerances of 3%/2 mm and 3%/3 mm respectively. No significant differences were found between treatment planning system algorithms, delivery techniques and linac manufacturers. Conclusion This multi-centre dosimetry audit of complex IMRT/VMAT delivery provides confidence in the accuracy of modern planning and delivery systems in inhomogeneous tissues. The findings from this study can be used as a reference for future dosimetry audits.