Radiochromic film for individual patient QA in extracranial stereotactic lung radiotherapy

Introduction: Modern radiotherapy is characterised by increasingly complex radiation delivery such as Intensity Modulated Radiation Therapy (IMRT) or extracranial stereotactic radiotherapy (ESR). It has become common practice to verify the delivery for each patient in IMRT, however, no such methods have been defined to date for ESR. It was the aim of the present work to develop a method to verify the dose distribution for ESR in a moving phantom using radiochromic film. Methods: Radiochromic film (ISP EBT2) was used in a cylindrical film cassette fitted into a QUASAR phantom (Modus Medical). The cassette can be moved forwards and backwards with motion patterns that can mimic the breathing of individual patients. The radiotherapy treatment plans of four patients were re-planned for the phantom. Between 8 and 10 radiation fields of 6 MV photons from a Varian Trilogy linear accelerator were used to deliver target doses between 18 and 26 Gy per fraction. In order to allow for measurements with EBT2 film all monitor units were divided by 3 or 4 resulting in maximum doses not exceeding 10 Gy. The film was evaluated using a transmission scan on an Epson Perfection V700 scanner with 50 dpi spatial resolution (3 colors, 48bit). A calibration curve was used to convert either the red or green component of the scan to dose. Results: Qualitative film analysis found no discernible discrepancies from planned isodose distributions in the stationary images. The dose distribution in the moving phantom was used successfully to confirm the appropriateness of the ITV construction in the planning process. Conclusions: We have developed a QA procedure that accounts for breathing patterns of individual patients in ESR for lung cancer. Radiochromic film was found to be easy to adapt to this process. © 2011 Elsevier Ltd. All rights reserved.