MapCHECK used for rotational IMRT measurements: Step-and-shoot, Tomotherapy, RapidArc

Purpose: To measure patient-specific-QA dose distributions with a 2D array of diodes, the MapCHECK, for the dose delivered, with step-and-shoot and rotational IMRT. Methods: Two MapCHECKs were used that had different styles of diode connection. These MapCHECKs were used in their original manufactured configuration and in a modified configuration. The modification made in the clinic consists of filling air gaps with sheets of Lucite that had custom-machined slots for the diodes and by adding pieces of copper to offset the intrinsic asymmetry of the diodes. The MapCHECKs were housed in a tight-fitting phantom fabricated from solid water. Measurements were made on IMRT treatment plans delivered with Varian linear accelerators with step-and-shoot and RapidArc methods, and a TomoTherapy machine with helical delivery. Patient plans and QA plans were developed with the XiO, Eclipse, and TomoTherapy planning systems. All MapCHECK data were analyzed with its commercially available software. Results: Kilovoltage CT imaging of the MapCHECK in its phantom has streak artifacts from high atomic number components. These artifacts must be corrected in order to obtain accurate calculation of dose. The original MapCHECK is found to have an angular dependence of +/- 20%. A modification of the MapCHECK has been made that reduces the angular dependence to +/- 2%. Proper compensation for attenuation by the treatment couch is also necessary for accurate results. The modified MapCHECK has been successfully used for doing patient-specific QA for IMRT treatments delivered with step-and-shoot, TomoTherapy, and RapidArc methods. Treatment plans that require large amounts of fluence from the 90 degrees and 270 degrees directions have significantly better QA measurements when made with modified MapCHECKs. Conclusions: When properly modified, the MapCHECK response becomes isotropic. It can then be used for patient-specific QA measurements for IMRT treatments delivered with step-and-shoot and rotational techniques such as helical Tomotherapy and RapidArc. (C) 2010 American Association of Physicists in Medicine. [DOI: 10.1118/1.3431994]