Verification of lung attenuator positioning before total body irradiation using an electronic portal imaging device.

PURPOSE: We report the first clinical experience with an electronic portal imaging device for lung attenuator positioning before delivery of total body irradiation. We demonstrate a technique for lung attenuator placement which reduces the dose to the patient during setup, reduces the patient setup time, and increases the accuracy of lung attenuator positioning. METHODS AND MATERIALS: Patients are treated with total body irradiation using a dedicated dual source irradiation facility prior to receiving bone marrow transplantation. The dose rate to the patient's midline is limited to 0.10 Gy/min, and partial transmission lung blocks are used to minimize radiation induced pneumonitis while delivering adequate dose to the regions under the blocks. Lung blocks are placed on the patient's back and chest wall, and portal images are used to verify proper block placement before the remaining treatment dose is delivered. RESULTS: We report the use of a liquid ionization chamber matrix electronic portal imaging device for imaging total body irradiation patient setups. CONCLUSION: The dose to the patient using the EPID for portal imaging is a factor of 7.5 lower than that needed for film. Image quality is superior to that of film due to digital processing. Since less time and dose are needed for imaging, it is demonstrated that better and more efficient final placement of the lung blocks can be achieved.