Practical IMRT QA dosimetry using Gafchromic film: a quick start guide

Abstract

This work outlines a method for using Gafchromic film for dosimetry purposes, by scanning it with currently available commercial scanners. The scanners used were: Epson V800, Epson V700, Epson V37 series, specifically a V370 and a Canon multi-function office printer/scanner. The Epson scanners have 16 bit RGB resolution, the Canon has 8 bit RGB (Red Green Blue) resolution, and the V800 and V700 allow scanning in transmission mode. The V700 uses an Epson White Cold Cathode Florescent Lamp; the recently released V800 uses an Epson light emitting diode (LED) light source, while the V37 series uses a reflective mode and the Epson LED light source. The Epson V37 series scanners are designed for non-professional use so the cost has been kept at a low “entry level” point, so they would be a suitable option for a department wanting to use Gafchromic film or with limited needs that did not justify a more sophisticated and expensive unit. Note that the V800 or V700 scanners are not expensive in context, costing approximately the same as a 25 sheet box of Gafchromic film. The Canon was included to demonstrate that a scanner with 8 bit RGB resolution can be used for dosimetry. These general multi-function units are available in most departments, and they would allow Gafchromic film to be evaluated as a dosimetry tool without a significant investment. Furthermore, they are generally capable of scanning large format film (425 × 350 mm) in one part. Although this is not necessary for dosimetry, it is often useful for machine QA, where dividing the film into two parts to ensure accurate measurements is not practical. Moreover, this analytical method uses software that is freely or commonly available, particularly the image processing package ImageJ. Note ImageJ v1.48 was the version used. The results demonstrate that this method used with the scanners evaluated is a practical method of using Gafchromic film as a dosimeter for IMRT QA.

Appendices

Appendix 1

Use of a scanner with 8 bit dynamic range in reflective mode

The basic problem of using 8 bit resolution is that it only allows 256 values to cover the range of interest. If, for example, we were interested in a dose range from 0 to 250 cGy, this could possibly allow 1 cGy per division as a best case. However, the Canon office scanner returned a pixel value (in the Green channel) of 0 cGy as 136 and for 250 cGy the pixel value was 72. Therefore there were only 64 divisions to cover 250 cGy, which resulted in 4 cGy per division. Furthermore, because the response was non-linear the step per division will be greater at parts of the range. Note that this was the raw scanner values, not the derived function. This resulted in an amplitude resolution of approximately 2 % of the range. A system resolution of better than 1 % is generally required for IMRT QA, in order to discern dosimetric errors of the order of 3 %.

Acceptable amplitude resolution can be achieved by scanning at a high spatial resolution and then down filtering to the required spatial resolution (generally 1 mm/pixel), while enhancing the amplitude resolution in the process. Note that aliasing was addressed as the light scattering properties of Gafchromic film led to a distribution of values for any measurement. This distribution acted effectively as an anti-aliasing filter which, for the Canon printer, worked well in the range from 0 to 3 Gy.

Appendix 2

Using an Epson scanner and the Epson Scan software application

The specific models used were the Epson V800, V700, and V370.

It was assumed that the scanner had been set up according to the manufacturer’s recommendations and connected to a PC (Windows 7), and the Epson Scan software had been loaded.

A 25 × 20 cm film area guide or template should be prepared for the V800 and V700 scanners such that the piece of film may be placed accurately in the active area of the scanner. An area guide was supplied with the V800 and V700 scanners, but a simple right angle template was found to be more practical.

For reflective mode, the scan area can extend to the scanner bezel. The film can be accurately located against the edge of the bezel so a separate area guide is not required.

Activate the Epson Scan software application and ensure that the acquisition parameters are as specified, that all the colour corrections are switched off, then acquire RGB 48 bit, and save as file type.TIF.

Acquire the full area of the film (254 × 203 mm) and specify a high spatial resolution such as 508 dpi (20 dpmm).

Save the pre-exposure “Blank” and the post-exposure “Exposed” scans in a suitable working directory. Since these files can be large, it is recommended to save them on the local hard drive of the scanner workstation.

Using the Canon office scanner

The Canon scanner works in reflective mode and the scan area extends to the scanner bezel. The edge of the bezel can be used to accurately locate the film so a separate area guide is not required. However, ensure that the film is re-positioned accurately between the Blank scan and Exposed scan. Since these are general office scanners, the scanner should be replicated between scans as far as possible, so be aware that other users may leave unwanted marks on the scanner.

To acquire a scan:

Select maximum spatial resolution (for the Canon this was 600 dpi)

Select Colour

Select file type JPG (note several other file types are acceptable to ImageJ, JPG is for Canon)

Select save file option. On the Canon the file is sent to the users email address.

The Canon does not save the file in RGB format, so to prepare the file for processing, open in ImageJ.

Use the Rectangle select tool to mark the extent of the film.

Use Image → Crop. This extracts the specified area.

Use Image → Type → RGB stack. This converts the image to Type RGB

Save as.TIF to the working directory

This should be done for both the Blank and Exposed.

Note regarding JPG file format used by the Canon scanner. JPG is the image format used by the Canon multi-function scanner. Taken in context that software is part of the system, the use of JPG by Canon is a component of the resolution that can be achieved by the scanner. The operator has no control over the compression level used. It could be assumed that it is optimised by Canon, however the end point is the resolution that is achieved considering the unit as a system including the software.

Process data using application ImageJ

Data processing in ImageJ is done using ImageJ Macros. These are scripts which list the commands. They also form a summary documentation of the required steps. They are listed below.

To run a macro in ImageJ, use:

File → Open and select the macro (it should be saved with extension.ijm)

The Macro window should open, then from the Menu bar on the Macro window:

Select Macros → Run Macro

It is assumed the scanned image files (Blank & Exposed) are in the working directory.

Using ImageJ:

Open image file (RGB) of the Pre-Exposure image (Blank).

Run macro: IJmacro_PreProcess_BLANK.ijm

Open image file (RGB) of the Post-Exposure image (Exposed).

Run macro: IJmacro_PreProcess_EXPOSED.ijm

The pre-processed but uncalibrated image of the exposed film should be open at this point.

If there are faults in the image such as pin holes from the phantom, these can be edited. To remove pin holes, draw a small rectangle around the pin hole, and then apply the Median filter with a radius 20. When complete, proceed to next step.

Run macro: IJmacro_Calibrate_V800_EBT3_LowRange.ijm

The calibrated data is now saved as a Text Image in file: Dose_Cal_Text_254x203.txt

This file can be converted to a format that can be read in conjunction with software application SNC Patient using the following steps:

Convert the file to a.csv file using MS Excel. Open the Text Image file using Excel, specify Tab delimited data. Then save file as type.csv (comma separated variable).

Pre-pend a header, such that the application SNC Patient recognises the data as type XiO.

Use Batch file: Add_Header_to_csv_file.bat. This batch file can be run from a CMD prompt with the name of the file to be converted as 1st parameter. See below. Or right click the file to be converted and select Open with, then select batch file.

C:\!Data\254data > Add_Header_to_csv_file.bat Dose_Cal_Text_254x203.csv

This batch file is listed below. The converted file can now be read into the application SNC Patient. The data is now in units of Dose (cGy). The file name is: Dose_Cal_Text_254x203.csvX