Gel Dosimetry for Radiotherapy Patient Planning

Introduction

Intensity-modulated radiation therapy (IMRT) is an advanced method of high-precision radiotherapy that utilises computer-controlled X-ray linear accelerators (Linac), Fig. 1, to deliver precise radiation doses to a malignant tumour or specific areas within the tumour.

Fig. 1. Varian Clinac 2100C/D Linear Accelerator.

The radiation dose is designed to conform to the three-dimensional (3-D) shape of the tumour site by modulating or controlling the intensity of the radiation beam to focus a higher radiation dose to the tumour, while minimizing radiation exposure to the surrounding normal tissue. Treatment is planned carefully using 3-D computed tomography (CT) images of the patient, Fig. 2, in conjunction with computerised dose calculations to determine the dose intensity pattern, which will best conform to the tumour shape.

Fig. 2. CT image of prostate.

Typically, combinations of several intensity-modulated fields from different beam directions produce a tailored radiation dose that maximizes dose to the tumour while also protecting adjacent normal tissues, Fig. 3.

Fig. 3. IMRT patient plan.

As the ratio of normal tissue dose to tumour site dose is reduced to a minimum with the IMRT approach, higher and more effective radiation doses can safely be delivered to tumour sites with fewer side effects in comparison with conventional radiotherapy techniques. IMRT also has the potential to reduce treatment toxicity, even when doses are at relatively high levels.

Currently, IMRT is being used to treat cancers of the prostate, head and neck, breast, thyroid and lung, gynaecologic tissue, liver, brain tumours, lymphomas and sarcomas. IMRT is also beneficial for treating paediatric malignancies.

Conventional methods of dose verification in radiotherapy (e.g. ionisation chambers, films) are no longer practical for use with highly conformal delivery methods such as IMRT. Polymer gel dosimeters are an ideal dose verification system, where ionising radiation generates a 3-D image within the gel, which is representative of the 3-D distribution of the dose of the radiant energy. This 3-D image is optically and magnetic resonance imaging (MRI) detectable due to changes in relaxation times of the gel solvent.

Fig. 4. Optical and MRI scanned gel with irradiated cube design.

Research

The work has been a collaboration between York University Chemistry Department, Hull University MRI Centre and Hull and East Yorkshire NHS Radiation Physics Department. The Liquid Crystal Group have used their knowledge of polymer chemistry and processing techniques to achieve a stable and reproducible gel system, removing artefacts and voids from within the gel seen in many previous samples, Fig.5.

Fig.5. MRI scans of gel showing dosimeters with artefacts and voids, and a fully processed homogenous gel.

Hull University MRI Centre have state-of-the-art MR imaging, including 1.5 Tesla and 3.0 Tesla scanners, Fig. 6. Software has also been developed by the MRI Centre, Fig. 7, to provide full analysis of the scanned gel, including calibration, data analysis, R2 and dose maps, plan comparisons and DICOM file outputs

Fig. 6 and 7. Hull University 3.0 Tesla MRI scanner and in-house developed software solution.

Hull and East Yorkshire NHS Radiation Physics Department have carried out all irradiations and test procedures using three Varian IMRT-capable Linacs. Initial research concentrated on calibration and stability tests with simple beam patterns. Various calibration techniques have been assessed, including multi-flask and in-situ (electron beam) calibrations, Fig. 8. Dose response has been shown to be linear up to 20 Gy, Fig 9.

Fig. 8 and 9. Calibration using multi-flask and in-situ method and linear dose response produced.

Research progressed to multi-beam irradiations and spatial resolution studies as previous gel dosimeters had issues with image stability over time. Beam arrangements including quadrant and star patterns were produced, Fig. 10, and image stability was assessed. It was shown that accurate and stable spatial resolution was present for 10 weeks at cooled (+5 °C) and room temperature.

Fig. 10. Quadrant and star beam arrangements

Following the successful results, the gel dosimeter was irradiated with real patient plans. The plans were far more complex than previous beam arrangements, an example being a plan of a tumour wrapped around a spinal column, Fig. 11.

Fig. 11. An MRI scan of full irradiated patient plan, the dark area representing the dose and the lighter area representing the unirradiated spinal column, and a dose map slice of the plan, with the blue area representing low irradiation and the dark red area showing high irradiation.

Once the irradiated gel dosimeter has been scanned, a complete range of important information can be accessed using the MRI software. Dose maps and R2 maps can be viewed as slices, Fig. 12, isodose maps can be generated, Fig. 13, 3-D dose maps, Fig. 14, and full comparison with real patient plans.

Fig. 12. Dose maps, 1cm apart slices showing boost dose within the patient plan.

Fig. 13. Dose map and corresponding isodose map.

Fig. 14. 3-D dose maps.

Fig. 15. Real plan comparison

The gel dosimeters have been trialled in most of the UK's leading IMRT centres, including Clatterbridge, Christie's, Leeds, Addenbrooke's and Ipswich. A spin-out company, Imagel Ltd., was set up in November 2006 with funding from IP Group.