Room: AAPM ePoster Library
Purpose: To evaluate the sensitivity of an integral quality monitor system (IQM) (iRT Systems), consists of a wedge-shaped, single and large-area ionization chamber, for systematic MLC position error with 4 MV X-ray beam.
Methods: Three conformal radiation therapy (breast, orbit, and spine), 1 IMRT (sinus) and 2 VMAT (prostate and pelvis) plans with 4 MV X-ray beam were used to investigate the MLC error sensitivities. The errors for all leaves of both sides were added position error, ±0.1 to 1.5 mm as systematic open/close and ±0.3 to 3.0 mm as systematic shift for conformal plans, and ±0.2 to 1.0 mm as systematic open/close for IMRT and VMAT plans. Dose measurement with a farmer type chamber (PTW GmbH), inserted into water phantoms, was also performed in the same way to be compared with the IQM results. The dose with/without the MLC errors for each plan was delivered and the cumulative signal counts or doses were measured with the IQM or farmer chamber, respectively. The difference of both was calculated based on the results of original delivery.
Results: For conformal plans, the cumulative signal count error of the IQM had linear response for the magnitude of the MLC error increased and decreased, whereas the farmer chamber could hardly detect the MLC positions of systematic open/close errors. In addition, the IQM had much stronger correlation to systematic shift errors than the farmer chamber. For IMRT and VMAT plans, the IQM detected same range error as larger difference than the farmer chamber.
Conclusion: This study indicates the IQM is more sensitive to the MLC error than the dose measurement with the farmer chamber for 4 MV X-ray beam because of a large ionization volume.
Funding Support, Disclosures, and Conflict of Interest: This study was supported in part by a research grant from APEX Medical, Inc., Tokyo, Japan.