A local assessment of the PTV margin calculation in patients treated to the larynx with VMAT

Abstract

Background: The introduction of Volumetric Arc Therapy (VMAT) in the local oncology hospital necessitated a more precise understanding of the true uncertainty and corrections for the Planning Target Volume (PTV) margin. The study's aim was to ensure that the PTV margins for patients treated to the larynx with VMAT were optimised for use in the local oncology hospital. Methods: The PTV margin was calculated based on data from 20 patients who received VMAT to the laryngeal region in the local oncology department. Van Herk’s formula was used for the calculation of the PTV by considering the population systematic and population random errors of the following errors: target volume delineation errors, inter-fraction errors, intra-fraction errors, inter-observer variation in image matching. All data were collected and analysed using both descriptive and inferential statistics using Microsoft Excel and Statistical Package for the Social Sciences (SPSS®). Results: The margin size was most affected by target volume delineation errors and intra-fraction errors. Target volume delineation errors had the most significant influence with population systematic errors measurements of 3.47 mm in the left-right (X) and anterior-posterior (Z) directions, and 6.92 mm in the superior-inferior (Y) directions. If the target volume delineation errors were ignored, the calculated PTV margin would be 4.9, 9.4, and 7.6 mm in the X, Y, and Z directions, respectively. The PTV margin including all the measured errors in the X, Y, and Z directions was calculated to be 10.5 mm, 20.5 mm, and 12 mm, respectively. Conclusion: The PTV margin size obtained in this study was larger when compared to similar studies, however it was the first study to consider target volume delineation errors and inter-observer variation in image matching. Addressing these sources of errors can reduce errors and achieve a smaller margin size.