title: The impact of dosimetric inadequacy on treatment outcome of nasopharyngeal carcinoma with IMRT.
abstract: OBJECTIVE
  This study aims to address the relationship between tumor size and dosimetric inadequacy in treating nasopharyngeal carcinoma (NPC), and how it subsequently affects the local control.
  
  
  METHODS
  444 NPC patients treated with IMRT from 2005 to 2010 were included in the study. The planning aim was to deliver at least 66.5 Gy (i.e. 95% of 70 Gy) to 95% of the primary gross tumor volume (GTV_P) while keeping all the critical neurological organs at risk (OAR) within dose tolerance. Treatment outcome were analyzed according to T stage, GTV_P volume and the degree of under-dosing.
  
  
  RESULTS
  Disease outcome was related to T stage, GTV_P volume and the degree of under-dosing. The 5-year local failure free survival (LFFS), disease free survival (DFS) and overall survival (OS) for T4 disease were 74%, 50.4% and 63.6% respectively. 48 cm(3) was identified as the critical cut-off GTV_P volume, the large volume group (GTV_P ≥ 48 cm(3)) had lower 5-year DFS (50.4% vs. 76.6%) and OS (65.2% vs. 86.3%, p < 0.001). Most T4 diseases (and some T3) were under-dosed (<66.5 Gy) and an under-dosed GTV_P volume of 3.4 cm(3) was found to be prognostically important. Multivariate analyses showed that the effect of GTV_P volume on LFFR and DFS was outweighed by the degree of under-dosing.
  
  
  CONCLUSIONS
  Treatment outcome of locally advanced NPC was significantly affected by the volume of under-dosed (<66.5 Gy) GTV_P due to the neighboring neurological structures. A new set of OAR dose constraint and specification is proposed.
authors: Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong. Electronic address: ngwt1@ha.org.hk.
authors: Wai Tong
authors: WT
authors: Ng
authors: Department of Medical Physics, Pamela Youde Nethersole Eastern Hospital, Hong Kong.
authors: Michael C H
authors: MC
authors: Lee
authors: Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong.
authors: Amy T Y
authors: AT
authors: Chang
authors: Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong.
authors: Oscar S H
authors: OS
authors: Chan
authors: Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong.
authors: Lucy L K
authors: LL
authors: Chan
authors: Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong.
authors: Foon Yiu
authors: FY
authors: Cheung
authors: Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong.
authors: Wai Man
authors: WM
authors: Hung
authors: Department of Clinical Oncology, Pamela Youde Nethersole Eastern Hospital, Hong Kong.
authors: Connie C C
authors: CC
authors: Chan
authors: Clinical Oncology Center, University of Hong Kong-Shenzhen Hospital, China.
authors: Anne W M
authors: AW
authors: Lee
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D003131:Combined Modality Therapy; D005260:Female; D006801:Humans; D008297:Male; D008875:Middle Aged; D009303:Nasopharyngeal Neoplasms / Q000188:drug therapy / Q000532:radiotherapy*; D011879:Radiotherapy Dosage; D050397:Radiotherapy, Intensity-Modulated; D016896:Treatment Outcome; D055815:Young Adult
keywords: Dose specification; Dosimetry; IMRT; Nasopharyngeal carcinoma; Organ at risk