A protocol to extend the longitudinal coverage of on‐board cone‐beam CT

Dandan Zheng,Shiyu Song,Jian Wu,Ariel Jefferson,Jeffrey Williamson,Cheng Zhang,Jun Lu,William Sleeman,Nesrin Dogan,Elisabeth Weiss

doi:10.1120/jacmp.v13i4.3796

Abstract

The longitudinal coverage of a LINAC‐mounted CBCT scan is limited to the corresponding dimensional limits of its flat panel detector, which is often shorter than the length of the treatment field. These limits become apparent when fields are designed to encompass wide regions, as when providing nodal coverage. Therefore, we developed a novel protocol to acquire double orbit CBCT images using a commercial system, and combine the images to extend the longitudinal coverage for image‐guided adaptive radiotherapy (IGART). The protocol acquires two CBCT scans with a couch shift similar to the “step‐and‐shoot” cine CT acquisition, allowing a small longitudinal overlap of the two reconstructed volumes. An in‐house DICOM reading/writing software was developed to combine the two image sets into one. Three different approaches were explored to handle the possible misalignment between the two image subsets: simple stacking, averaging the overlapped volumes, and a 3D‐3D image registration with the three translational degrees of freedom. Using thermoluminescent dosimeters and custom‐designed holders for a CTDI phantom set, dose measurements were carried out to assess the resultant imaging dose of the technique and its geometric distribution. Deformable registration was tested on patient images generated with the double‐orbit protocol, using both the planning FBCT and the artificially deformed CBCT as source images. The protocol was validated on phantoms and has been employed clinically for IRB‐approved IGART studies for head and neck and prostate cancer patients.PACS number: 87.57.nj

Highlights

142 Zheng et al.: A protocol to extend cone-beam computed tomography (CBCT) coverage of motion or setup errors;(14-16) and CBCT images were utilized to perform image-guided adaptive radiation therapy (IGART).(17-19)These tasks often require an image volume that encompasses the whole treatment field
To extend the imaging coverage in the transverse plane, a “half-fan” imaging geometry in which the flat-panel detector (FPD) is laterally displaced to project slightly over half of the field of view (FOV) was introduced, extending the transverse FOV up to 48 cm.[20,21,22] While this solves the problem for CBCT localization problems by providing the full transverse cross sections for most patients, the longitudinal coverage is still limited to about 14 cm and 16 cm with reconstruction software versions On-Board Imager (OBI) 1.3 and 1.4, respectively
The planning target volumes (PTVs) displayed in color watershed go beyond the CBCT longitudinal FOV, with the treatment field and dose calculation box even more substantially beyond it

Summary

Introduction

142 Zheng et al.: A protocol to extend CBCT coverage of motion or setup errors;(14-16) and CBCT images were utilized to perform image-guided adaptive radiation therapy (IGART).(17-19)These tasks often require an image volume that encompasses the whole treatment field. To extend the imaging coverage in the transverse plane, a “half-fan” imaging geometry in which the FPD is laterally displaced to project slightly over half of the field of view (FOV) was introduced, extending the transverse FOV up to 48 cm.[20,21,22] While this solves the problem for CBCT localization problems by providing the full transverse cross sections for most patients, the longitudinal coverage is still limited to about 14 cm and 16 cm with reconstruction software versions OBI 1.3 and 1.4, respectively This image length is often shorter than the total treatment field lengths, such as in typical head and neck or prostate treatment fields where nodal coverage is needed.

Methods

Results

Conclusion