A review of recent advances in optical fibre sensors for in vivo dosimetry during radiotherapy

This article presents an overview of the recent developments and requirements in radiotherapy dosimetry, with particular emphasis on the development of optical fibre dosemeters for radiotherapy applications, focusing particularly on in vivo applications. Optical fibres offer considerable advantages over conventional techniques for radiotherapy dosimetry, owing to their small size, immunity to electromagnetic interferences, and suitability for remote monitoring and multiplexing. The small dimensions of optical fibre-based dosemeters, together with being lightweight and flexible, mean that they are minimally invasive and thus particularly suited to in vivo dosimetry. This means that the sensor can be placed directly inside a patient, for example, for brachytherapy treatments, the optical fibres could be placed in the tumour itself or into nearby critical tissues requiring monitoring, via the same applicators or needles used for the treatment delivery thereby providing real-time dosimetric information. The article outlines the principal sensor design systems along with some of the main strengths and weaknesses associated with the development of these techniques. The successful demonstration of these sensors in a range of different clinical environments is also presented.Radiation dosimetry deals with methods for quantitative determination of energy deposited in a given medium directly or indirectly by ionizing radiations. A dosemeter can be defined generally as any device that is capable of providing a reading that is a measure of the average absorbed dose deposited in its (dosemeter) sensitive volume by ionizing radiation. There are commonly agreed codes of practice in the UK that define how dosimetric calibration of treatment beams should be performed,^1,2 in addition to defining the types of ionization chambers that may, or may not, be used for these measurements.Radiotherapy is in a period of rapid scientific and clinical development. The introduction of novel treatment techniques, for example, stereotactic ablative radiotherapy and volumetric modulated arc therapy (VMAT), delivered through the use of technologies such as flattening filter free (FFF) beams and dynamic multi-leaf collimation, is driving the requirement for increasing levels of accuracy and precision in dosimetry. These treatment delivery options are causing existing, well-established, dosimetric equipment to be extended to the limits of its capability. Other recent developments in treatment options include the use of protons and heavy ions, and the availability of small animal irradiation platforms provides additional scope for novel dosimetric systems. Furthermore, the increased use of image-guided radiation therapy, including the use of kilovoltage cone beam CT, MR and positron emission tomography, provides a different set of problems to existing technologies employed within traditional radiotherapy dosimetry.There are also recommendations for comprehensive quality assurance (QA) programmes^3,4 to assess the performance of all types of radiotherapy treatment equipment, including the treatment planning system (TPS), against known tolerances and for comparison with baseline measurements. The increasing complexity of modern treatment modalities has also introduced a more comprehensive patient-specific QA programme to verify, pre-treatment, an individual patient delivery. Radiotherapy also includes multiple layers of checking from the simple cross-checking of work, through independent monitor unit calculations, and on to independent audits of treatment centres'' planning and dosimetry performance.In addition to ensuring the correct calibration of treatment beams, and verification of the delivery pre-treatment, it is important to monitor dose delivery during treatment (in vivo), rather than verification of the treatment to a phantom. In an ideal scenario, the dose delivered directly within the tumour volume, and/or dose to specific organs at risk (OARs), would be measured while the patient is receiving their treatment. However, this is currently generally carried out by measuring the dose at a “surrogate” position, usually by placing a radiation detector directly on, or near to, the patient''s skin surface to provide either an entrance or exit dose value, rather than directly within the tumour itself. There is a growing interest in the need to perform such in vivo measurements in part owing to increasing awareness of the potential risks associated with incorrect delivery or planning of radiation treatments, and because of the use of increasing complex delivery techniques such as intensity-modulated radiation therapy (IMRT) and VMAT, and the move towards more hypofractionated treatments delivered with large doses per fraction.The importance of in vivo dosimetry has been further highlighted in recent years as a result of a number of major radiotherapy incidents,^5–7 and whilst the vast majority of radiotherapy sessions are performed without incident, an international review of radiotherapy incidents identified >7000 incidents over three decades (1976–2007). The incidents range from underdosing, leading to a recurrence risk, to overdosing, causing toxicity and even death.⁵ The investigations following major incidents have generally recommended that some form of in vivo dosimetry measurement would be beneficial,⁸ and professional bodies such as the American Association of Physicists in Medicine have recommended that clinics “should have access to TLD or other in vivo systems”.⁹There are a number of different options available for use as an in vivo dosemeter, with the most commonly used being thermoluminescent detectors (TLDs), diodes, metal-oxide semi-conductor field effect transistors (MOSFET), film and electronic portal imaging devices. These options each have relative strengths and weaknesses, and a number of review articles^10–12 have highlighted the merits of each. For a detailed summary of in vivo dosemeters, not restricted to optical fibre sensors, see table 1 from Mijnheer et al¹⁰ for dosemeters in external beam radiotherapy and table 3 from Tanderup et al¹² for dosemeters in brachytherapy. Methods to infer the full three-dimensional dose distribution are also being developed primarily by the use of back-projected electronic portal imaging images to reconstruct the dose within the CT volume used to plan the patient''s treatment^13–15 or through the analysis of the treatment log files to recreate the multileaf collimator (MLC) positions used during the treatment.^16,17In recent years, there has been some interest¹⁰ in investigating alternatives to the established in vivo detectors, such as plastic scintillation detectors (PSDs), optically stimulated luminescent detectors, radiophotoluminescent dosemeters and implantable MOSFETs. Some of the main reasons for the development of these alternatives to the existing options include the increasing interest in combining a MRI scanner with a radiotherapy linear accelerator (linac), the development of heavy ion and particle beams in radiotherapy and the introduction of new small animal irradiation platforms for radiobiological investigations. These new technologies present different problems from the effect of magnetic fields on dosemeters,¹⁸ the response of dosemeters in different types of treatment beams,^19,20 the miniaturization of treatment fields^21,22 and the associated complexity of radiation dosimetry at very small field sizes.^23,24Optical fibres offer a solution for in vivo radiotherapy dosimetry with many advantages over currently employed clinical dosimetry systems. An optical fibre radiation dosemeter is a photonic system based on optical fibre technology, whereby radiation introduces a modification or modulation in some of the characteristics of the optical signal. The optical fibres can be directly affected by the radiation, in which case it is called an intrinsic sensor, or it can be used for the sole purpose of transmitting the optical signal, and is known as an extrinsic sensor. There are a number of different dosimetry techniques that can incorporate optical fibres to further improve the overall system, and these techniques are discussed in turn, together with examples of such optical fibre-based systems.