Radiochromic film is superior to ion chamber arrays for IMRT quality assurance

Because of the complexity of fields used for IMRT, it is important to verify that all delivered fields are the same as those that were planned. A number of methods to make the required beam intensity distribution measurements to assure such congruence are in use, including radiochromic film and ionization chamber arrays. The claim that radiochromic film is superior to ionization chamber arrays for IMRT quality assurance is the Proposition debated in this month's Point/Counterpoint. The end of the past and the beginning of the new millennium brought a change in the design of radiation fields used to treat malignant diseases. Radiation oncologists embraced intensity modulated radiation therapy (IMRT) as an alternative to conformal radiation delivery.1 The earlier radiation therapy approach was based on uniform radiation fields, whereas the new one has employed fluence maps built with beamlets.2 As our knowledge of cancer functional substructures using functional imaging expands, the old dogma of delivering uniform dose to the target as a prerequisite of tumor control is slowly vanishing. Instead, the dose painting radiation delivery approach based on biological target volumes is paving the way toward the future.3 As a result of this paradigm shift in radiotherapy, beam design and delivery quality assurance (QA) programs have become more complex. Fluence maps used to build IMRT plans feature high dose gradients, usually extending over fairly small spatial regions. Consequently, questions have arisen as to whether clinical linear accelerators can actually deliver the IMRT dose distributions shown on screens of the treatment planning systems. The natural tool to answer this question at a time of increasing use of IMRT has been radiographic film, which has had a long history of use in radiotherapy QA programs. A particular version of Kodak ready-pack EDR-2 film was developed for this purpose.4 Although radiographic film based dosimetry had disadvantages (temperature of developer, nontissue equivalence of silver halide based sensitive component, etc.), various protocols were developed for the use of radiographic film for IMRT QA. The current trend toward filmless radiology and radiotherapy departments, however, will lead to essentially no access to traditional radiographic film and wet developer systems. Recently, radiochromic film,5,6 which has all the advantages of conventional silver halide film (two-dimensional dosimetry, thinness, ruggedness, and permanent record) but without its numerous disadvantages (need for development and its impact on the readout signal, temperature and chemical composition of developer, nontissue equivalence, sensitivity to visible light, strong energy dependence at low photon energies, etc.), has become an important dosimetric tool. Its high spatial resolution combined with low spectral sensitivity make it ideal for the measurement of dose distributions in regions of high dose gradients. Relatively poor spatial resolution and energy dependent response of alternative dosimeters (including ionization chambers) may introduce uncontrolled uncertainties in dose measurements for dose distributions composed of a large number of beamlets and their accompanying overlapping penumbral regions. The latest development in radiochromic film is external beam therapy (EBT) GAFCHROMIC™ film,7 designed to replace silver halide radiographic film for IMRT QA procedures. In addition to higher sensitivity than its predecessors (MD-55 and HS), this model is available in larger sizes and at much lower cost than previous GAFCHROMIC™ film models. Quality assurance covers a wide range of tasks, but fundamental to any radiotherapy treatment verification is the measurement of absolute dose. Isodose curves, leaf operation, and plan verification are secondary to the need to be able to say accurately what dose was delivered. For that, the ionization chamber is the undisputed gold standard. Ion chambers are stable, reliable, easy to calibrate, and have a well-understood simple physical process underlying the measurement. An ion chamber array, whether it be 1D or 2D, is a straightforward extension of the single ion chamber used for reference dosimetry or beam scanning. One-dimensional arrays have been used for more than a decade,8 and 2D arrays are now available from several manufacturers.9,10 Ion chamber arrays are attractive for IMRT quality assurance for many reasons. From an absolute dose point of view, it is a huge advantage that the array technology is based on that used for reference dosimetry, and therefore the properties of ion chambers apply—accuracy, long-term stability,9 linearity with dose,10 infinite repeatability, and the measurement of both integrated dose and instantaneous dose rate simultaneously. The last two points are essential if one wishes to investigate beam startup, leaf movement, or other linac-related issues. Ion chamber arrays are easy to set up, require little user training, and can be moved easily from one machine to another. All the commercial systems currently available come as integrated array/electrometer/software packages and, therefore, there is little user input into the result and the measurement reflects the dose delivered, not the measurement technique used. That results should be user-independent is a fundamental (but generally ignored) aspect of a QA program. Multiple vendors and models mean users can choose what best suits their needs and that there is competition, which results in continued development. One obvious criticism is that the resolution of all arrays is relatively coarse. Anything less than 3 mm is not practical at present. Several authors8,10 have shown, however, that for the majority of measurements, this apparent coarseness does not have a significant effect on the measurement of dose distributions. If higher resolution is required, it has been shown that simple mechanical systems can be used to make small shifts to the whole array to easily give 1 mm resolution.11,12 Arrays have been extensively validated using both point detector scans8,9 and film techniques,13 and the conclusion of those authors is that ion chamber arrays measure dose distributions correctly. From the data presented, it would seem that an array can also be used for daily output checks with the central ion chamber of the array replacing a separate measurement of ion chamber in phantom. Thus, absolute, 2D beam data can be obtained on a daily basis.14 Equipment is rarely used for only one measurement and arrays have a range of uses beyond IMRT QA including MLC calibration15 and even Monte Carlo beam modeling.16 Considering all these points it is difficult to imagine any dosimeter system that could be superior to the ion chamber array for quality assurance of IMRT. I agree that the ionization chamber is one of the most convenient and well known dosimeters used so far in radiotherapy physics. However, its relatively poor spatial resolution could be a key detriment if used for IMRT QA. As an example, one of the most commonly observed IMRT failures is the hot junction (usually 1–2 mm wide) in the split IMRT field for H&N patients. It might not be prudent to generalize performance characteristics of a certain detector (ion chamber) based on the fact that a particular (even IMRT) delivered plan has given an expected result.10 While the ionization chamber has been the dosimeter of choice for reference and relative dose measurements in the past when large radiation fields have been used for patient treatments, the question here is whether ion chambers provide accurate and reliable dose data for the fields used for IMRT. Conversion of the chamber signal (charge created within the cavity filled with air) into dose in the medium when the chamber is not present, relies on the existence of charged particle equilibrium and requirements imposed by cavity theory. Bragg–Gray cavity theory requires that, for example, the size of the cavity is small compared to the range of the charged particles, and that the energy deposited within the cavity originates only from the charged particles crossing it. All these conditions are readily achieved in large radiation fields and with the presence of one ionization chamber in large homogeneous waterlike phantoms. One should be more careful, however, when conditions are present where a large number of beamlets with overlapping penumbral regions are traversing a detector that consists of a large number of air-filled cavities. To summarize, the poor spatial resolution of ion chambers as well as requirements for charged particle equilibrium and cavity theory signal-to-dose conversion are major concerns that support my contention that radiochromic film is superior to ion chamber arrays for IMRT quality assurance. It is not a recent phenomenon that radiotherapy requires new dosimeters to meet an emerging treatment modality—from the discovery of x rays, treatment technology and practice have run ahead of dosimetry. For techniques such as IMRT, SRS, VMAT, etc., measurement technology and, even more so, dosimetry protocols, are several years behind the routine treatment of patients, and such a gulf spurs development in dosimeters leading to the wide range of options available today. Timeliness of the result—current film protocols require a delay of 24 h between irradiation and read-out of the film. Although this may be acceptable for research and specialized purposes, it is unacceptable for routine QA. By contrast, linac-mounted ion chamber arrays can provide immediate information on the dose delivered to the patient for each irradiation. Noise and accuracy—these are still an issue for radiochromic film. Batch film calibration is required (using a calibrated ion chamber) and high levels of noise in the scanned images means that multiple films are required to yield uncertainties less than 1%. Stability of the system—it was clearly demonstrated in 2009 that this field is far from mature since EBT film was replaced by a new version, EBT-2, requiring a different calibration process. Radiochromic film undeniably has its uses but it is a field in development and the report card states "could do better." For IMRT QA today, ion chamber arrays offer the best combination of accuracy, spatial resolution, and ease of use.

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