The history and principles of chemical dosimetry for 3-D radiation fields: Gels, polymers and plastics

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Abstract

Over recent decades, modern protocols of external beam radiotherapy have been developed that involve very steep dose gradients and are thus extremely sensitive to errors in treatment delivery. A recent credentialling study by the Radiological Physics Center at the MD Anderson Cancer Center (Texas, USA) has noted potentially significant inaccuracies in test treatments at a variety of institutions. 3-D radiation dosimetry (often referred to as “gel dosimetry”) may have an important role in commissioning new treatment protocols, to help prevent this type of error. This article discusses the various techniques of 3-D radiation dosimetry, with a focus on the types of radiosensitive samples used and on the optical computed tomography readout technique.

Introduction

Over the last few decades, external beam radiotherapy has developed into an extremely precise modality for delivering high curative or palliative doses of ionising radiation to the human body in a safe fashion that attempts to minimise radiation-induced side effects. Conventional photon therapy can be accurate to within ±3% and it is universally accepted that a high degree of accuracy in dose delivery is essential for the success of radiotherapy (ICRU, 1993). Confidence in its delivery is such that the rare case of an overdose exceeding 10% is a reportable incident in the UK (IPEM, 2002).

Improved methods of conforming the dose in such a way as to enable dose escalation in the target volume, whilst simultaneously sparing organs at risk, have seen widespread introduction over the last 10 years. However, these modern protocols inherently involve steeper dose gradients than methods they supersede and are thus more sensitive to errors in delivery.

By analysing the performance of a variety of institutions, one can investigate globally the extent to which inaccuracies in delivery compromise the quality of treatment. The Radiological Physics Center (RPC) at the MD Anderson Cancer Center runs a radiotherapy credentialing service and one of the modalities evaluated is intensity-modulated radiotherapy (IMRT) of the head and neck. An anthropomorphic phantom is shipped to the participating institution and irradiated with a fixed test protocol. The phantom is shipped back to the RPC and the experimental deviations from the predetermined plan are evaluated. Relative to the known abilities of external beam therapy, the criteria for passing the credentialing are relatively generous, viz. a 7% dose deviation and 4 mm distance-to-agreement. Despite this, it was, perhaps, something of a shock to discover that out of a total of 254 irradiations, carried out between 2001 and 2007, 71 (28%) failed the credentialing (Ibbott, 2007).

A number of questions arise out of the RPC findings:

  • How does one properly commission a complex radiotherapy treatment modality? This may include not only IMRT, but image-guided radiotherapy (IGRT), proton and heavy-ion therapy, brachytherapy and radioisotope therapy. Is it sufficient to perform quality-assurance (QA) checks of individual beams and devices (e.g., radioactive implants), or do we need to measure experimentally a complete 3-D distribution?

  • How does one properly commission each new treatment protocol with a given device? What measurements need to be performed at every institution to avoid failing the credentialing test? How often do these measurements need to be repeated?

  • What experimental measurements are appropriate for each individual patient?

Given this backdrop, 3-D radiation dosimetry—often previously referred to as “gel dosimetry”—should see an important and increasing role in this area. At present, however, despite a great deal of ongoing research, uptake of the technology for routine clinical applications has been slow, for a variety of technical reasons that will be discussed below.

This article will provide some of the background to the field. Section 2 will describe the development of chemical dosimetry, focusing particularly on the techniques leading to 3-D measurements, with other chemical dosimeters such as alanine and radiochromic film mentioned in passing. Section 3 will outline the history of optical computed tomography (CT), whilst Section 4 will show an example of the technique in action.

Section snippets

Techniques of 3-D dosimetry

Previous methods of dosimetry use either single point (e.g., ion chambers, TLD's, diodes, alanine pellets) or 2-D (e.g., film or diode array) detectors. However, it is becoming increasingly clear that complex radiotherapy treatments require true 3-D verification. In IMRT, for example, it is not enough to confirm in 2-D the dosimetry of each of the separate fields, because part of what needs to be verified is the overlap of the fields and the alignment procedure. If fields overlap where they

A brief history of optical CT

The development of optical CT occurred contemporaneously in a number of separate geographical locations and in at least two completely different fields, which had similar requirements. This is perhaps illustrative of the idea that when the underlying technology on which a measurement relies becomes cheap and widely available, an “invention” will arise naturally. In this case, the consumer imaging boom of the 1990s gave the impetus for the production of CCDs, whilst high-performance photodiode

Conclusions

3-D dosimetry is a field that has emerged steadily over the last 20 years to a position where there is now a real possibility of useful application in the clinic. Methods of delivery of highly conformed radiation dose currently far outstrip our abilities to measure those same doses routinely in the clinic and it is certain that 3-D methods of measurement will be needed in the future. However, further refinements in technique and improvements in formulation of the dosimeter materials used will

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