Exploration of timing of modifying adaptive radiotherapy plan for esophageal cancer
Objective The influence on dose distribution due to target volume change during radiotherapy for esophageal cancer was analyzed to select the right time to modify the radiotherapy plan.Methods Thirty cases of esophageal cancer undergoing radiotherapy were selected.Images of the patients were collected daily during the first week and before the 6th,9th,11th,14th,16th,19th,and 21st treatment sessions.The new margins of the target area were calculated according to the mean value of the positioning errors which were recorded during the first week.A new plan was created using the newly calculated target margin to evaluate target volume change and its impact on target dose coverage D95%.Based on the changes in target volume observed during the radiotherapy process,the timing for modifying the radiotherapy plan was determined.The actual doses before and after the modification were summed and compared with the dose distribution according to the original plan.Results The expanded margins values calculated by the positioning error were LR:0.47cm、AP:0.47cm、CC:0.84cm.The volume of the target area generated using the newly expanded margins was significantly smaller than the initial target area(P<0.05).However,the target dose coverage D95%of the new plan did not differ statistically compared to the original plan(P>0.05).As radiotherapy progressed,the target volume experienced varying degrees of shrinkage,with the most rapid change occurring between the 11th and 14th treatments,where a shrinkage rate of 5.5%was observed.The radiotherapy plan was modified after the 11th treatment,resulting in a statistically significant reduction in the cumulative dose to the V5、V10、V20、V30、Dmean of lungs,V30、V40 of heart and Dmax of spinalcord in the revised plan(P<0.05).However,there was no statistically significant difference in the target dose coverage D95 between the modified cumulative plan and the original plan(P>0.05).Conclusion The new external margins value are smaller than the conventional external margins value and the new radiotherapy plan developed based on positioning errors observed during the first week didn't compromise the dose distribution to the target.An adaptive plan was developed when the target receding rate reaches its maximum,and that can further decrease the dose to normal tissues without affecting target dose distribution.It is advisable to revise the radiotherapy plan after the 11th fraction.