PROCEDURE 1.1.2.3 – NON-DOSIMETRIC EVALUATION: IMAGE ACQUSITION, USE, AND DISPLAY (SEMI-ANNUALLY)

I. PURPOSE

The purpose of this procedure is to provide written guidance for the periodic evaluation of CT scanner performance, CT image acquisition, use and display in treatment planning.

II. GENERAL REQUIREMENTS

The requirements for this procedure are derived from the following sources:

AAPM TG-40 “Comprehensive QA for Radiation Oncology: Report of AAPM Radiation Therapy Committee Task Group 40”

AAPM TG-53 “Quality Assurance for Clinical Radiotherapy Treatment Planning”

AAPM Report 39 “Specification and Acceptance Testing of Computed Tomography Scanners”

III. RESPONSIBILITIES

The dosimetry/physics staff is responsible for the performance of this procedure.

IV. INSTRUCTIONS

A. CT Scanner Performance Characteristics

1) Principle. The service representative performs preventative maintenance and systems calibration and quality assurance semi-annually. To ensure comprehensive oversight of quality for radiation oncology, we will review the results of the performance tests. The results must meet generally accepted criterion.

2) Method
a) The service representative will forward a hardcopy of the results of the following test: CT Number accuracy (water) versus location, versus slice position, and versus kVp, and slice thickness accuracy.

B. CT Image acquisition, use, and display.

1) Principle. The use of CT scanners in the Radiology Department requires all images to be transferred to the TPS via the PACS network. The accuracy and consistency of the data is critical to the integrity of radiotherapy treatments. The CT numbers, relative to electron density, must be accurate. The images must be oriented and ordered correctly and displayed in accurate grayscale (window and level).

2) Method

a) The CIRS Electron Density Phantom is scanned (per Form 1.1.2.3 CT QA Scan Protocol) in the CT scanners in use in radiology semi-annually. The images are transferred to the TPS and a hardcopy of the images printed.

b) Create new plan under patient: QA

c) Import the proper image set into the planning environment. Review the order and orientation of the images relative to the hardcopy. Verify the slice thickness and order is correct.

d) Using the ROI tool, contour each of the density plugs in the central slice of the study such that the ROI contains approximately 100 pixels. Record the mean CT number, located under the Statistics tab, for each density plug and compare to the baseline data. In addition, repeat the measurement for the water plug series reviewing only water and air locations.

e) Measure the diameter of the inner ring of the CIRS Phantom using the measurement tool. Determine the deviation from the expected value.

f) Measure the diameter of the entire phantom vertically and horizontally using the measurement tool. Determine the aspect ratio relative to the expected value.

g) Verify windowing and leveling works properly on image set.