SU-E-J-47: Evaluation of the Mismatches Between DRR's and Port Films for IMRT Fields

To quantify the frequency and magnitude of mismatches between the MLC shape on the DRR created by the Pinnacle planning system and the corresponding port film MLC shape generated by the Mosaiq record-and-verify system for IMRT fields.

A retrospective review was made of the most recent 60 patients to receive IMRT at our clinic. The MLC shape on the DRR created by the Philips Pinnacle planning system for each treatment field was reviewed (573 fields total) and compared with the MLC shape of the port film image, which was generated by the Mosaiq treatment planning system.

Of the 60 patients studied, 20 had at least one leaf mismatched between the MLC shape on the DRR and port film (142 of the 573 fields). Of the affected cases, on average 59% of the fields had a mismatch. The affected fields had an average of 6.7 leaves mismatched with a mean discrepancy of 27mm. The average maximum discrepancy for each affected patient was 69mm. Discrepancies were most common for head and neck cancers.

The MLC shape Mosaiq generates for the port film is the CIAO, or Complete Irradiated Area Outline, which is the area that is actually treated. The Pinnacle DRR displays the maximum leafmotion, which can be different for larger fields in which the MLC leaves abut within the collimator jaw opening. The discrepancy can create substantially different MLC shapes. The problem can be solved by not filming the MLC shape and only using the films for isocenter placement; however, displaying the area receiving treatment can be a useful safety check, possibly preventing a treatment error. The persons assessing the films must be aware of this issue and evaluate the films carefully.