SU‐GG‐T‐144: IMRT Pre‐Treatment Plan and In‐Vivo Verification with 2D‐ARRAYs and Multi‐Wire Ionisation Chambers

Abstract

Purpose: A concept for quality assurance and IMRT plan verifications with 2D‐ARRAYs and the DAVID‐ System is presented. Method and Materials: A 2D‐ARRAY (PTW‐Freiburg) and a multi‐wire ionization chamber (DAVID‐system, PTW Freiburg) are used in this work. The DAVID‐system is a translucent, multiwire transmission‐type ionisation chamber, placed in the accessory holder of the accelerator. Each detection wire is positioned in the projection line of a MLC leaf pair. The signal of each wire is proportional to the line integral of the ionisation density along this wire. The 2D‐ARRAY is used for daily dosimetrical checks (dose on central axis, MLC calibration, symmetry, flatness, energy) and for pretreatment IMRT plan verification. During the dosimetric verification of an IMRT plan reference values are measured with the DAVID‐system and stored in a patient specific database. During daily treatment the signals are re‐measured and compared to the reference values. Thus a direct connection to the IMRT plan verification is possible. A warning occurs if a deviation beyond a chosen threshold is detected. In an “expert” mode the physicist can analyse each single segment of the plan and detected errors are related to MLC pair. If necessary a re‐verification with 2D‐ARRAYs can be performed. Results: The application of the concept for standard IMRT cases and examples for error detection capabilities (e.g. de‐calibrated MLCs, neglected segments) are shown. For typical IMRT plans with field sizes beyond 10 cm × 10 cm the DAVID‐ system is able to detect positioning errors of MLC pairs in the sub‐millimeter region. Conclusion: The procedures can be used during daily routine with a minimum of additional time and are assuring a closed dosimetrical QA loop for IMRT.