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Radiation-Induced CT Number Changes Observed During Radiotherapy for Head and Neck Cancer

I Moraru*, C Yang, X Li, Medical College of Wisconsin, Milwaukee, WI

SU-E-J-194 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: To characterize radiation-induced CT number (CTN) changes in tumor from the CT data acquired during IGRT for head and neck cancer and to investigate the potential for using this change as a indicator for response-adapted radiation therapy (RT).

Methods: CTN changes are measured on daily diagnostic-quality CTs acquired during image-guided IMRT using a CT-on-rails (CTVision, Siemens) for 28 patients with head and neck cancers, treated with 70 Gy in 35 fractions and concurrent chemotherapy. Differences in the distribution of CTN in Hounsfield units (HU) for various regions of interest (ROI) between daily CT sets were quantified. These RIOs included gross tumor volume (GTV), the spinal cord irradiated and un-irradiated tissues. Statistical analyses compared histogram mean, standard deviation, CTN of peak and asymmetry, to indentify inter-fraction trends in each ROI. Correlation of the CTN changes in GTV with tumor regression and local control will be evaluated.

Results: Inter-fraction changes in the CTN histograms for the GTV were observed with no ascertainable correlations to tumor regression. Similar, but reduced changes, were observed in the cord. There were no noticeable CT number differences in un-irradiated tissue, indicating that the changes observed in the GTV and cord were radiation- induced. For the GTV, the mean CT number varied as much as +13 HU and -12.5 HU over 30 fractions.

Conclusion: Radiation-induced CT number changes in tumor were observed from the daily CTs acquired during RT delivery for patients with head and neck cancer. The inter-fraction CTN variation was patient-specific and showed no correlation to tumor regression for the patients studied. The correlation of the CTN change in tumor with local control will be presented.

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