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      Dosimetric influence of pitch in patient positioning for radiotherapy of long treatment volumes; the usefulness of six degree of freedom couch

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          Abstract

          Objective:

          Pitch, the rotation around the transversal axis of the patient during radiotherapy has little impact on the dose distribution of small spherical treatment volumes; however it might affect treatment of long volumes requiring a correction with a six degree of freedom couch.

          Methods:

          We included 10 patients each with nasopharyngeal carcinoma (NPC) and esophageal cancer, treated with volumetric modulated arc therapy. Pitch was simulated by tilting the planning CT in ventral and dorsal direction by ± 1.5° and ± 3°. Verification plans were calculated on the tilted datasets and were compared to the original plan and the dose constraints of the organs at risk (OAR).

          Results:

          The deviation in dose to the planning target volume is increasing with the degree of pitch with mean changes of up to 2% for NPC and 1% for esophageal cancer. The most affected OAR in NPC patients are brainstem (max. dose +6.0%) and spinal cord (max. dose +10.0%) when tilted by 3° dorsally and lenses (max. dose +3.3%), oral mucosa (mean dose +2.6%) and parotid glands (mean dose +4.3%) when tilted by 3° ventrally. For esophageal cancer patients, there was no significant change in dose to any OAR. Whereas for esophageal cancer, all tilted treatment plans were still clinically acceptable regarding OAR, 5 NPC plans would no longer be acceptable with a pitch of 1.5° ventral ( N = 1), 3° ventral ( N = 2) and 3° dorsal ( N = 2).

          Conclusion:

          Planning target volume coverage in both tumor entities was only slightly affected, but pitch errors could be relevant for OAR in NPC patients.

          Advances in knowledge:

          A correction with a six degree of freedom couch is recommended for NPC patients with a pitch mismatch of more than 1.5° to avoid exceeded doses to the OAR.

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          Author and article information

          Contributors
          Journal
          Br J Radiol
          Br J Radiol
          bjr
          The British Journal of Radiology
          The British Institute of Radiology.
          0007-1285
          1748-880X
          November 2018
          25 July 2018
          : 91
          : 1091
          : 20170704
          Affiliations
          [1 ] org-divisionDepartment of Radiation Oncology, University Hospital Zurich and University of Zurich , Zurich, , Switzerland
          [2 ] org-divisionInstitute of Diagnostic and Interventional Radiology, University Hospital Zurich and University of Zurich , Zurich, , Switzerland
          [3 ] org-divisionInstitute for Radiation Oncology, Cantonal Hospital Lucerne , Lucerne, Switzerland
          Author notes
          Address correspondence to:. E-mail: sonja.stieb@ 123456usz.ch
          Article
          PMC6475952 PMC6475952 6475952 BJR-D-17-00704
          10.1259/bjr.20170704
          6475952
          30004794
          a31782e7-bf54-4152-bf01-1777137b3eec
          © 2018 The Authors. Published by the British Institute of Radiology
          History
          : 14 September 2017
          : 5 July 2018
          : 9 July 2018
          Categories
          Full Paper
          bjr, BJR
          rt-onc, Radiotherapy and Oncology
          h-n, Head and neck
          gas-abd, Gastrointestinal/Abdominal
          oth, Other

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