Why is Dosimetry Important?
In radiation therapy, a treatment plan is carefully created to deliver ionizing radiation to the tumour while minimizing the dose deposited in healthy tissue. Accurately delivering radiation according to the plan is important for the outcome of the cancer treatment; ensuring this and making necessary adjustments to the treatment requires reliable dosimetry. Dosimeters are used to measure the radiation dose at a particular point in space, monitoring dose used to treat cancer and creating a feedback loop to control treatment delivery. Dosimetry is also important for continuous development and innovation in radiation therapy.
What is the Optical Fibre Dosimeter?
Developed by UHN and INO, the optical fibre dosimeter is a device for measuring radiation in real-time during therapy. The small, MRI compatible dosimeters can be inserted into the body to provide dose measurements from within the tumour or sensitive neighbouring normal tissues. At its core is a radiochromic material, a substance that instantly changes colour and transparency when exposed to ionizing radiation. A tiny mirror is coated with a thin film of this material, and the mirror is attached to an optical fibre. Light sent down the fibre passes through the radiochromic film, reflects off the mirror, and can be detected back down the fibre—a signal that changes with the radiation dose.
How is it different?
There is a need for robust radiation monitoring in brachytherapy—where a small radioactive source is placed inside the body—due to the difficulty in modeling the dose distribution, and uncertainties from applicator placement and patient organ motion. Indeed, verifying the delivered dose may be more important in brachytherapy than in multifraction external-beam radiotherapy because a single deviation from plan can have a larger effect on treatment outcome. Conventional dosimeters have shortcomings in brachytherapy applications as many cannot be inserted into tissue due to size, and the ones that can require regular calibration procedures.
Just 0.5 mm across, the optical fibre dosimeter can be placed in a catheter to take measurements from within the body. Fully MRI-compatible and with real-time read-out, the probes are ideally suited for use with MR-guided brachytherapy, an important emerging trend in treatment. The small size and low cost of these fibre-based dosimeters means multiple detectors can be used at once to monitor the radiation dose at several points within the patient.
A further advantage of this technology is that the fibre and radiochromic probe interact with radiation similar to how water does, thus eliminating the need for time-consuming calibrations to detect radiation of different energies. Its water-equivalent composition also means that the probe does not significantly modify the dose delivered to surrounding tissue when inserted in patient.