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October 17, 2005

Digital Virtuoso — Filmless Facilities Call New Dosimetry Tune
By Arthur J. Olch, PhD
Radiology Today
Vol. 6 No. 21 P. 8

The work of a good physicist is never done.

When Childrens Hospital Los Angeles began offering intensity modulated radiation therapy (IMRT) approximately seven years ago, I realized that—as chief of physics—I had work to do.

Because IMRT delivers high doses of radiation to tumors while leaving surrounding tissues unaffected, I had to develop and implement a top-notch dosimetry process to make sure the treatment was delivered properly and, most importantly, precisely.

Now, as the hospital moves toward eliminating film in all areas—including radiology—I am once again rolling up my sleeves. Like many other physicists across the country who are dealing with the migration toward digital operations, I need to develop new dosimetry procedures and implement new technologies that do not rely on film to ensure high-quality service to patients.

To accomplish this task, it’s important to understand the following:

• the intricacies of IMRT and its quality assurance requirements;

• the effect operating without film will have on radiation therapy dosimetry and quality assurance;

• the advantages and disadvantages of various filmless dosimetry alternatives;

• the potential quality assurance pitfalls inherent in a digital environment; and

• what attributes to look for in quality assurance software.

Upping the Dosimetry Ante
Dosimetry has always been an important element of radiation therapy. However, dosimetry has become especially crucial as Childrens has adopted advanced radiation therapy procedures such as IMRT.

With IMRT, patients receive superior care because the highest doses of radiation are delivered to tumors while surrounding organs and tissues are spared. The advantages of IMRT are numerous and include lower complication rates, better tumor disintegration, and improved cure rates.

In addition, it is especially important to shield growing organs, bones, and brains from unneeded radiation when children undergo radiation treatment. Such protection enables children who survive their diseases to maintain a higher quality of life. So, the need for IMRT and the need for precision become exponentially more important when dealing with a young patient population.

To garner all the benefits, however, IMRT treatment has to be implemented correctly. That means the role of the physicist and the importance of dosimetric quality assurance becomes even more important.

To start, physicists need to understand the unique dosimetric requirements associated with IMRT. The complexity of dose calculations required with IMRT eliminates the option of planning by the more common method, which is used with conventional radiation. Instead of empirically presetting beam directions, weights, and shapes—and then computing the dose distributions—physicians specify treatment objectives that are then translated into the beam configurations that deliver dose histograms to the tumor and normal surrounding tissues.

So, when Childrens Hospital began to offer this radiation treatment several years ago, we had to develop and implement an effective dosimetric quality assurance (QA) solution.

How did we handle dosimetric quality assurance in the film-based environment? With reverse planning in place, the medical physicist generated a treatment plan showing the desired dose distribution in the patient and the beam parameters required for its realization. If the dose distribution was not exactly right, the initial dose constraints were then modified and a new plan was developed.

To make sure that the radiation was delivered in exactly the right doses to exactly the right places, the physicist would then compare a radiation plan from a treatment planning system and a film irradiated to that plan using QA software from Radiological Imaging Technology, Inc. (Colorado Springs, Colo.) The software performs a complex computing process verifying dose optimization and localization.

The film dosimetry method worked well, providing physicists with millions of dose points on each film and the complete picture needed for effective IMRT quality assurance. The software produced high-quality images with high resolutions up to 89 microns or 285 dots per inch where each sampled pixel is adjacent to the next. There were no gaps in the measurement. Additionally, composite images and planar images could be analyzed because the software enabled users to get a complete picture by viewing images from any angle.

The software provided the ability to scan the film in two dimensions and then transform that into a digital image file. As a result, physicists could subtract the planning system’s dose from the measured doses and examine the magnitude of the difference in the 2-D images. The software provided the means to calculate a number of analytical comparisons.

Playing the Next Card
Medicine does not stand still. For the past few years, our hospital has been moving toward a filmless environment in its radiology department.

Although the digital impetus was not born in our therapeutic milieu, there are, indeed, many advantages when working with digital images in the radiation therapy environment. For example, when acquiring images electronically, healthcare providers can offer more accurate patient positioning. When an image is captured electronically and stored in a database, there is virtually no chance of assigning the wrong image to a patient. In addition, it is much easier to retrieve a digital image.

Most important, however, computerized digital images can be easily and precisely compared. Film images, on the other hand, can only be “eyeballed” and thus do not offer the same level of precise comparison.

The move to a completely digital environment, however, means that the diagnostic radiology department will eventually stop purchasing and maintaining film processors. Like many other hospitals across the country, the radiation therapy department at Childrens routinely uses the film processors to perform dosimetric QA.

As a result, I now am hard at work adapting dosimetric QA practices to the emerging digital environment.

The first task? Finding an electronic technology to produce dosimetric measures that can be compared to radiation plans emanating from treatment planning systems.

Instead of relying on films that have been irradiated to mimic treatment plans, our physicists now must rely on images obtained from other electronic technologies. As a result, I had to find and implement a digital technology that enables physicists to perform the dosimetric quality assurance required with IMRT. My search entailed looking at the pros and cons of the following options:

• Computed radiography (CR) is a digital image acquisition and processing system for radiography that uses computers and laser technology. A reusable plate is exposed and placed into a special scanner. Images are captured instantly and do not require film developing and mounting. With CR, images can be recorded on laser-printed film or transmitted and stored digitally.

One advantage with CR is that the technology can be used with photostimulable phosphor plates. The digital filmlike plates have a special coating that, when irradiated, store the intensity of the radiation until readout. A laser scanner scans the plate releasing light with an intensity that is proportional to the radiation intensity. The list is collected at each pixel in the plate and a 2-D image is formed and stored in a computer or PACS.

One of the most important advantages of photostimuable phosor plates is that they can be reused roughly 1,000 times. In addition, the form factor is similar to film and can be used in the same phantoms as film. Plus, with these plates, physicists can acquire high spatial resolution (0.1 millimeter) images at any angle to the beam.

While these plates offer many advantages, the cost—at approximately $100,000 per plate—can be prohibitive for some healthcare organizations.

• Electronic portal imaging devices (EPIDs) are digital imaging systems used to verify patient positioning and geometric treatment accuracy in external beam radiation therapy. EPIDs are now being used by many treatment facilities for treatment simulations. EPID images can help target tumors because the images are viewed immediately after a patient is positioned. With film, by the time the film is exposed, processed, and analyzed, patients often have moved, making it more difficult to zero in on the tumor.

EPIDs use a 2-D detector array that is typically attached to the linear accelerator by a retractable arm. The device directly acquires and stores digital images.

The main advantage with EPIDs stems from the fact that no scanning is required, as images are directly acquired.

On the flip side, EPIDs carry a high price tag and the technology only acquires low-resolution images (0.5 to 0.7 millimeters) that are perpendicular to the beam.

• 3-D dosimetry systems, such as BANG Gel, rely on a gelatinous material stored in a spherical container to produce images. When irradiated, its chemical properties at each point change in proportion to the radiation dose. The container is scanned by an MRI scanner that reads the chemical properties, which are then viewed as images in any plane.

The quality of the spatial resolution (approximately 3 millimeters) is one of the advantages of this technology. However, the systems are costly and cannot be reused. Plus, the systems require an MRI scanner and expensive light scanner, both of which can be cost prohibitive for many hospitals.

• Radiochromic film is a self-developing film that can be used in the same applications as standard film and scanned by the same device used to scan standard film. With this technology, users get all the advantages of film without the processing. However, radiochromic film is more costly than standard film, costing approximately $8 per sheet—thus, defeating the purpose of most digital initiatives.

Moving Forward
After exploring the possibilities, I decided that Childrens would use CR technology and photostimuable phosphor plates for dosimetric QA. Selecting the technology, however, was just the first step in moving toward a digital dosimetric QA environment.

After selecting the technology, I also had to make sure our QA software would perform in the digital environment. I discovered that the RIT QA system does, in fact, work with film and various digital systems, including CR and EPID. When working with these digital systems, the software converts images into RIT format, giving users the complete palette of RIT 113 QA routines and functionality. In addition, the software combines any number of images for a composite analysis, thereby helping to overcome some directional concerns associated with digital technologies.

In addition, the software supports digital equipment from various manufacturers. So, as Childrens moves forward with its foray into a completely digital environment, interoperability will not be an issue if the hospital chooses to try products from various vendors.

Most important, perhaps, is the fact that physicists can access 13 IMRT QA routines, whether they are using film or digital technologies. The routines provide a suite of measurements including differential thermal analysis, gamma function, analysis, profiles, isodose, subtraction, and addition.

As we begin to implement digital dosimetric QA, it’s important to remember that image resolution with digital technologies does not equal film. As a result, our physicists must be aware that their QA may not be as accurate as it was in a film-based environment. As such, in the future we may consider using digital techniques for routine QA and film for commissioning and some patient specific QA tests. To meet this need, iCRco, Inc. (Torrance, Calif.) makes a scanner capable of digitizing both film and CR plates.

In addition to this technology, many others are likely to be introduced to the market. As a result, I will once again roll up my sleeves and work to develop the best dosimetric IMRT QA processes for the patients at Childrens Hospital.

— Arthur J. Olch, PhD, is an associate professor of clinical pediatrics and radiation oncology at the University of Southern California and chief of physics at Childrens Hospital Los Angeles.

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