SU-F-J-33: On-Line Daily KV Image Protocol of Lung Patient Setup with Minimal CBCT.

PURPOSE: This study's intent is to test if the first 3 fractions of an IGRT protocol consisting of a kV orthogonal pair and subsequent CBCT are representative of all remaining treatment fractions.

METHODS: At our institution the standard IGRT protocol includes CBCT after kV imaging for the first 3 fractions followed by daily kV and weekly CBCT. We randomly selected 20 lung patients who have at least 8 CBCTs in the entire treatment course (total 187 sets of orthogonal kV images plus CBCT). Our evaluation criterion is the fraction of kV-vs-CBCT position mismatches in the first 3 fractions. An additional CBCT is acquired if only 1 fraction mismatches among the first three. We define as 'protocol eligible' (eGroup) the group of patients for whom the setups with kV images match within the pre-selected thresholds of 3, 4 and 5 mm those of the consecutive CBCTs for at least 60% of the fractions. A protocol ineligible (iGroup) is defined as the group of patients other than eGroup.

RESULTS: The fractions of patients in the eGroup were 70%, 85% and 95%, when the setup thresholds selected were 3, 4 and 5mm respectively. The sensitivities of determining iGroup from the first 3 fractions of kV and CBCT were 0.83, 1.0, and 1.0, and the specificities were 0.93, 0.94 and 0.89 with thresholds 3, 4 and 5 mm respectively.

CONCLUSION: The method suggested here well categorized cases for our kV-based on-line IGRT protocol. More patients are eGroup eligible if larger setup threshold is used. CBCT frequency in the on-line daily kV plus weekly CBCT protocol may need to increase for protocol ineligible patients. Also, our IGRT CBCT frequency could be decreased when position differences between kVs and CBCTs are less than 1.5mm, the average positional standard deviation of out kV-CBCT sets.