Room: Exhibit Hall | Forum 6
Purpose: To evaluate the adequacy and future improvement of the current CTV-to-PTV margin and imaging frequency in HN treatment at our institution.
Methods: Two regimens of HN treatment are currently clinically used: 3mm CTV-to-PTV expansion margin with daily CBCT, or 5mm CTV-to-PTV margin with CBCT daily at the first three fractions, then twice per week. Under the second regimen, the worst-case-scenario setup error of individual patient while treating without CBCT was approximated as the average CBCT shiftsÂ±two standard deviations. Eleven patients were selected and the delivered dose distribution was approximated as the sum of planned dose as fractions with IGRT and perturbed dose due to setup uncertainty. The approximated dose distributions were compared to the treatment plans to evaluate changes in target coverage and OAR sparing. To evaluate the adequacy of a smaller margin, a third regimen was tested: 3mm CTV-to-PTV expansion with CBCT daily at the first three fractions, then twice a week. Eleven patients of the first regimen were used and setup errors were approximated with daily CBCT shifts. The delivered dose was approximated similarly as described above.
Results: For the worst-case-scenario plans under second regimen, the normalized D95 of the prescribed doses to the CTVs and GTV were 101.62Â±1.06% and 101.54Â±0.68%, respectively. The changes in Dmax of spinal cord, brainstem and esophagus were 0.92Â±2.18Gy, 1.22Â±3.83Gy, and -0.14Â±2.50Gy, respectively. In the simulation of third regimen, the normalized D95 of CTVs and GTV were 101.42Â±3.69% and 102.95Â±4.56%. The Dmax variations of OARs were similar to the previous case.
Conclusion: The 5mm CTV-to-PTV expansion margin is sufficient to compensate the setup uncertainty without daily CBCT imaging. The CTV-to-PTV expansion margin could potentially be reduced in clinic with lower CBCT frequency.