gms | German Medical Science

48th Meeting of the Particle Therapy Co-Operative Group

Particle Therapy Co-Operative Group (PTCOG)

28.09. - 03.10.2009, Heidelberg

An evaluation of the reproducibility of CBCT images and the feasibility of using CBCT images for adaptive cranial proton radiation therapy

Meeting Abstract

  • N. Schreuder - ProCure Treatment Centers, Bloomington, IN, USA
  • A. Kapur - ProCure Treatment Centers – Oklahoma, Oklahoma City, OK, USA
  • W. Hsi - UFPTI, Jacksonville, FL, USA
  • Z. Li - UFPTI, Jacksonville, FL, USA
  • O. Zeidan - MD Anderson, Orlando, Orlando, FL, USA

PTCOG 48. Meeting of the Particle Therapy Co-Operative Group. Heidelberg, 28.09.-03.10.2009. Düsseldorf: German Medical Science GMS Publishing House; 2009. Doc09ptcog179

DOI: 10.3205/09ptcog179, URN: urn:nbn:de:0183-09ptcog1790

Published: September 24, 2009

© 2009 Schreuder et al.
This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( You are free: to Share – to copy, distribute and transmit the work, provided the original author and source are credited.



Purpose: The use of in-room cone-beam computer tomography (CBCT) images for the verification of the patient position and target localization has been demonstrated in the past. To expand the use of CBCT images into the field of proton radiation therapy, the reproducibility of CBCT image data sets was studied to evaluate its use for adaptive treatment planning in proton radiation therapy. The reproducibility of the Hounsfield Units (Hus) for repetitive CBCT data sets of two patients treated for head & neck lesions was investigated in this study.

Materials and method: Ten CBCT image data sets for two patients (five each) were acquired on different days using an Elekta XVI CBCT system at the UFPTI facility as well as a Varian CBCT system at the MD Anderson, Orlando facility. Reference axial CT images for both patients were acquired with Phillips CT scanners at the respective facilities. Frequency distributions (FDs) were calculated for all CT and CBCT data sets by means of counting the number of voxels having the same HUs. Since the voxel resolution of the CBCT and axial data sets was not the same, a scale factor derived from the pixel size and scanning thickness differences between CBCT and reference axial CT, was applied to the CBCT FDs. A mathematical non-linear least squares fit, using a combination of Gauss distributions, was applied to the frequency distribution curves for all the CT data sets. This allowed for a method to locate the peaks in the FDs. The variation of peak locations in the soft tissue and bone regions between the five CBCT data sets for each patient was used to evaluate the HU reproducibility of the CBCT data sets.

Results: The number of voxels with HUs in the soft tissue region is much larger than for the bone region for CT and CBCT. The peaks in the FDs of the axial scanner images are much narrower than those in the CBCT images while the areas under the peaks are similar. The standard deviation of the soft tissue peak for the axial scans is 9 HUs while for the CBCT scans the average standard deviation for the 5 sets of CBCT scans is 39.8 HUs. The total area under peaks in the soft tissue regions for both the CBCT and the reference axial images are almost the same. The standard deviation of the peak locations between the CBCT scans are 2.4 HUs and 6 HUs for the soft tissue and bone peaks respectively.

Conclusions: The differences in widths of the peaks in the axial and CBCT images are due to the fact that the noise in the axial CT images is much lower than for the CBCT images. The fact that the area under the peaks in the soft tissue of CBCT is close to the reference CT indicates that the distortion of geometry is minimal in CBCT images. The Small variation in the peak locations shows that a good reproducibility of HUs of CBCT cranial images was achieved. We conclude that based on the presented information, CBCT images can be used to investigate actual daily variation of delivered doses of proton cranial therapy.