EPID-based in vivo dosimetry for stereotactic body radiotherapy of non-small cell lung tumors: Initial clinical experience.

PURPOSE

EPID-based in vivo dosimetry (IVD) has been implemented for stereotactic body radiotherapy treatments of non-small cell lung cancer to check both isocenter dose and the treatment reproducibility comparing EPID portal images.

METHODS

15 patients with lung tumors of small dimensions and treated with volumetric modulated arc therapy were enrolled for this initial experience. IVD tests supplied ratios R between in vivo reconstructed and planned isocenter doses. Moreover a γ-like analysis between daily EPID portal images and a reference one, in terms of percentage of points with γ-value smaller than 1, P, and mean γ-values, γ, using a local 3%-3mm criteria, was adopted to check the treatment reproducibility. Tolerance levels of 5% for R ratio, P higher than 90% and γ lower than 0.67 were adopted.

RESULTS

A total of 160 EPID images, two images for each therapy session, were acquired during the treatment of the 15 patients. The overall mean of the R ratios was equal to 1.005±0.014 (1 SD), with 96.9% of tests within±5%. The 2D image γ-like analysis showed an overall γ of 0.39±0.12 with 96.1% of tests within the tolerance level, and an average P value equal to 96.4±3.6% with 95.4% of tests with P>90%. Paradigmatic discrepancies were observed in three patients: a set-up error and a patient morphological change were identified thanks to CBCT image analysis whereas the third discrepancy was not fully justified.

CONCLUSIONS

This procedure can provide improved patient safety as well as a first step to integrate IVD and CBCT dose recalculation.