Purpose: Portal measurement is becoming an important tool for in vivo dosimetric verification, and Calypso provides real‐time tracking capability; however, when both work simultaneously, large interference arises for portal measurement. The purpose of this study is to investigate the interference of Calypso on portal measurement and mitigation of the interference by applying aluminum shielding over portal panels. Methods: For the same IMRT field and phantom setup, we acquired portal measurements at every 15 degree gantry angle, without and with Calypso, and for those measurements with Calypso, we also acquired portal measurements without and with aluminum shielding over the portal panels. The aluminum shielding consists of a layer of aluminum foil of 0.1 mm thickness covering the portal panel. The measurements without Calypso and without aluminum shielding were regarded as the reference images. All other measurements were regarded as the test images. We measured the deviation of the test images from the reference images by the amplitude difference and using the Gamma Index (3%, 3 mm). Results: With Calypso interference and without aluminum shielding, the signals are larger than the reference, and in some unfavorable gantry angles, the signals can be as much as 15% larger. With aluminum shielding, the interference was much reduced to ∼3% for those unfavorable angles, and the Gamma passing rate achieves 95% for most of the angles. Conclusion: The Calypso interference on portal measurements is gantry angle dependent due to panel orientation and proximity with respect to the Calypso transducer. Shielding on the portal panel can largely reduce electronic interference, and it is anticipated that with an improved complete shielding over the entire portal panel, the interference could be further reduced.
ASJC Scopus subject areas
- Radiology Nuclear Medicine and imaging