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SU-E-J-229: Quantitative Assessment for Timely Adaptive Re-Planning Using Weekly Dose Monitoring for Head and Neck Cancer.

PURPOSE
For patients with head and neck (HN) cancer, mid-course adaptive radiation therapy (ART) is a common practice in our institution to accommodate anatomic changes. The aim of the study is to evaluate whether dose re-calculation on weekly verification images can provide quantitative assessment for timely adaptive re-planning with daily image-guided intensity modulated radiotherapy (IMRT).


METHODS
We retrospectively selected sixty daily verification images acquired on CT-on-rail/CBCT from ten HN patients. These image sets were typically a week apart. Among these patients, six patients received a mid-course ART. Contours of the tumors and organ-at-risks (OARs) were manually delineated by a physician on each verification CT. After placing the treatment iso-center on the verification CTs according to the recorded clinical shifts, daily dose was re-calculated with the same beam configuration as the original plan. For the purpose of this study, electron densities for both verification CTs and planning CTs were set to 1.0 g/cm3.


RESULTS
Two patients had D99 of the CTV < 97% of the planned dose for more than three fractions due to remarkable tumor volume shrinkages. D_max of the spinal cord exceeded a tolerance of 45 Gy for four fractions in additional two patients. D_mean of the parotid increased within 25% of the planned value. D_max of the brainstem and D_mean of the oral cavity did not show significant variation. If the re-planning criteria included D99 of the CTV < 97% of the planned dose and D_max of the spinal cord > 45 Gy, two out ten patients required ART at week 2 and two patients required ART at week 3, respectively.


CONCLUSION
Weekly dose monitoring with re-calculation on verification images can provide quantitative dose guidance for timely adaptive re-planning. Future work will include accumulative dose analysis for the decision of adaptive re-planning. The study is supported in part by Siemens Medical Solutions.