Role of Positron Emission Tomography and Computed Tomography Imaging for Detecting Disease Recurrence Following Adjuvant Radiation Therapy in Oropharyngeal Cancer

based on daily contours. Statistical significance was determined by the 2-tailed t test, while correlations were assessed by the Spearman coefficient. Results: Primary GTVs decreased significantly in volume over the course of IMRT (median % volume loss, 38.7%; range, 29.5-72.0%; P<.05) at a median rate of 1.2% per fraction (range, 0.92%-2.2% per fraction). Both ipsilateral and contralateral parotid glands underwent significant volume loss (P<.05). Median percentage of volume loss of ipsilateral parotid glands was 31.1% (2.3%-43.9%), and median rate of volume loss was 0.97% per fraction (0.07%-1.4% per fraction). Contralateral parotids experienced a median percentage of volume loss of 21.8% (4.0%-40.5%) and median shrinkage rate of 0.68% per fraction (0.13%-1.3% per fraction). Both ipsilateral and contralateral parotids shifted medially during IMRT. Weight loss correlated significantly with parotid gland volume loss and medial COM shift (P<.05). Dose received by the primary GTVs and parotids did not significantly change during treatment. Median difference between administered mean dose at fraction 33 and planned dose per fraction was 0.008 Gy (-0.060.18 Gy) for ipsilateral and 0.01 Gy (-0.12-0.27 Gy) for contralateral parotids. Conclusion: Integrated on-board MRIs from a tri-Co teletherapy system can be used to accurately contour and analyze primary GTVs and parotid glands over the course of IMRT. COM shifts and significant volume reductions of the primary tumors and parotids were observed, confirming the results of CT-based studies. The enhanced resolution of the MRIs acquired may facilitate online adaptive replanning in the future. Despite anatomic changes during IMRT, significant dosimetric shifts were not always apparent. Author Disclosure: G. Raghavan: None. A.U. Kishan: None. M. Cao: None. Y. Yang: None. A.M. Chen: None.