Purpose: This study's intent is to test if first 3 fractions of a new IGRT protocol consisting of 3 CBCTs followed by daily kV pair and weekly CBCT are able to represent of all remaining treatment fractions. In this study, we are focused on discussing the setup errors of CBCTs. We will discuss neither the setup errors of kV pairs or between kV pairs and CBCT.
Methods: We would like to compare a new IGRT protocol includes 3 CBCT followed by daily kV pair and weekly CBCT with the current IGRT protocol includes daily CBCT. We randomly selected 10 prostate patients who have daily CBCT in the entire treatment course (total 350 set of CBCT). Our evaluation criterion is the fraction of 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 'new protocol eligible’ (eGroup) the group of patients for whom the setups with daily CBCT images match within the pre‐selected thresholds of 5, 10, and 15 mm those of the consecutive CBCTs for at least 50% of the fractions. A new protocol ineligible (iGroup) is defined as the group of patients other than eGroup.
Results: The fractions of patients in the eGroup were 10%, 80% and 90%, when the setup thresholds selected were 5, 10 and 15mm respectively. The sensitivities of determining iGroup from the first 3 fractions of CBCT were 0.88, 1.0, and 1.0, and the specificities were 1.0, 0.75 and 1.0 with thresholds 5, 10 and 15 mm respectively.
Conclusion: The method suggested here well categorized cases for our new IGRT protocol. More patients are eGroup eligible if larger setup threshold is used. CBCT frequency in the daily kV plus weekly CBCT protocol may need to increase for protocol ineligible patients.
Not Applicable / None Entered.