title: Computed tomography number changes observed during computed tomography-guided radiation therapy for head and neck cancer.
abstract: OBJECTIVE
  To investigate CT number (CTN) changes in gross tumor volume (GTV) and organ at risk (OAR) according to daily diagnostic-quality CT acquired during CT-guided intensity modulated radiation therapy for head and neck cancer (HNC) patients.
  
  
  METHODS
  Computed tomography scans acquired using a CT-on-rails during daily CT-guided intensity modulated radiation therapy for 15 patients with stage II to IVa squamous cell carcinoma of the head and neck were analyzed. The GTV, parotid glands, spinal cord, and nonspecified tissue were generated on each selected daily CT. The changes in CTN distributions and the mean and mode values were collected. Pearson analysis was used to assess the correlation between the CTN change, organ volume reduction, and delivered radiation dose.
  
  
  RESULTS
  Volume and CTN changes for GTV and parotid glands can be observed during radiation therapy delivery for HNC. The mean (±SD) CTNs in GTV and ipsi- and contralateral parotid glands were reduced by 6 ± 10, 8 ± 7, and 11 ± 10 Hounsfield units, respectively, for all patients studied. The mean CTN changes in both spinal cord and nonspecified tissue were almost invisible (<2 Hounsfield units). For 2 patients studied, the absolute mean CTN changes in GTV and parotid glands were strongly correlated with the dose delivered (P<.001 and P<.05, respectively). For the correlation between CTN reductions and delivered isodose bins for parotid glands, the Pearson coefficient varied from -0.98 (P<.001) in regions with low-dose bins to 0.96 (P<.001) in high-dose bins and were patient specific.
  
  
  CONCLUSIONS
  The CTN can be reduced in tumor and parotid glands during the course of radiation therapy for HNC. There was a fair correlation between CTN reduction and radiation doses for a subset of patients, whereas the correlation between CTN reductions and volume reductions in GTV and parotid glands were weak. More studies are needed to understand the mechanism for the radiation-induced CTN changes.
authors.affiliation: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, Wisconsin; Department of Radiation Oncology, Sichuan Cancer Hospital, Chengdu, China.
authors.forename: Mei
authors.initials: M
authors.lastname: Feng
authors.affiliation: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, Wisconsin.
authors.forename: Cungeng
authors.initials: C
authors.lastname: Yang
authors.affiliation: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, Wisconsin.
authors.forename: Xiaojian
authors.initials: X
authors.lastname: Chen
authors.affiliation: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, Wisconsin.
authors.forename: Shouping
authors.initials: S
authors.lastname: Xu
authors.affiliation: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, Wisconsin.
authors.forename: Ion
authors.initials: I
authors.lastname: Moraru
authors.affiliation: Department of Radiation Oncology, Sichuan Cancer Hospital, Chengdu, China.
authors.forename: Jinyi
authors.initials: J
authors.lastname: Lang
authors.affiliation: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, Wisconsin.
authors.forename: Christopher
authors.initials: C
authors.lastname: Schultz
authors.affiliation: Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, Wisconsin. Electronic address: ali@mcw.edu.
authors.forename: X Allen
authors.initials: XA
authors.lastname: Li
mesh_terms: D000328:Adult; D000368:Aged; D002294:Carcinoma, Squamous Cell / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D005260:Female; D006258:Head and Neck Neoplasms / Q000000981:diagnostic imaging* / Q000473:pathology / Q000532:radiotherapy*; D006801:Humans; D008297:Male; D008875:Middle Aged; D058958:Organs at Risk / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects*; D010306:Parotid Gland / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects*; D056888:Patient Positioning; D061089:Radiotherapy, Image-Guided / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods*; D013116:Spinal Cord / Q000000981:diagnostic imaging / Q000473:pathology / Q000528:radiation effects*; D014057:Tomography, X-Ray Computed / Q000379:methods*; D047368:Tumor Burden / Q000528:radiation effects*