An attenuation integral digital imaging technique for the treatment portal verification of conventional and intensity-modulated radiotherapy.

Purpose: To propose an attenuation integral digital imaging (AIDI) technique for the treatment portal verification of conventional and intensity-modulated radiotherapy (IMRT).

Methods: In AIDI technique, an open in air fluence image Io and a patient fluence image I were acquired under the same exposure. Then after doing the dark field correction for both the Io and I, the AIDI image was simply calculated as log(Io/I), which is the attenuation integral along the ray path from the x-ray source to a detector pixel element. Theoretical analysis for the low contrast detection and the contrast to noise ratio (CNR) of AIDI was presented and compared to those for the fluence imaging. With AIDI, the variation of x-ray fluence and the variation of individual detector pixel's response can be automatically compensated without using the flood field correction.

Results: The AIDI image for a contrast detail phantom demonstrated that it can efficiently suppress the background structures such as the couch and generate better visibility for low contrast objects with megavoltage x rays. The AIDI image acquired for a Catphan 500 phantom using a 60 degrees electronic dynamic wedge field also revealed more contrast disks than the fluence imaging did. Finally, AIDI for an IMRT field of a head/neck patient successfully displayed the anatomical structures underneath the treatment portal but not shown in fluence imaging.

Conclusions: For IMRT and high degree wedge beams, direct imaging using them is difficult because their photon fluence is highly nonuniform. But AIDI can be used for the treatment portal verification of these beams.