Analysis of setup errors and determination of planning target volume margins for thorax (lung, oesophagus and breast) cancers: From Regional Radiotherapy Centre, Kashmir (North India)

Abstract

The focus of this study was to determine the set-up errors so as to estimate the margin between the Clinical Target Volume (CTV) and the Planning Target Volume (PTV) and to suggest optimum margins for planning target volume (PTV) coverage in thorax cancers.

In the present study data from 51 patients was incorporated. A total of 1308 portal images were examined. Set up errors were estimated by superimposing a digitally reconstructed radiograph (DRR), using an electronic portal image device (EPID) as a reference image. The Medio-Lateral (ML), Cranio-Caudal (CC), and Antero-Posterior (AP) directions were subsequently evaluated. According to the shifts obtained, systematic and random errors were computed. The van Herk formula was employed to determine the values for the clinical-to-planning target volume (CTV-PTV) margins.

The systematic error was found to be between 1.0 mm and 1.7 mm, 1.0 mm and 1.8 mm, and 2.1 mm and 3.1 mm along the x, y, and z axis. In the x, y, and z axis, the random error varied from 0.5 mm to 0.7 mm, 0.4 mm to 0.8 mm, and 0.7 mm to 1.7 mm, respectively. Based on the Van Herk equation, the PTV margin following our findings was estimated to be 4.7 mm, 3.3 mm, 8.8 mm for lung, 3.6 mm, 2.7 mm, and 5.7 mm for oesophagus, and 3.0 mm, 4.9 mm, and 8.6 mm for breast in the x, y, and z dimensions respectively.

This study demonstrates that an 8.8 mm extension of CTV to PTV margin for the lung, 5.7 mm for the oesophagus, and 8.6 mm for the breast, serving as an upper limit, is sufficient to guarantee that 90% of patients diagnosed with thoracic cancers will receive a cumulative CTV dose that is at least 95% of the prescribed dose.