IMPLEMENTATION IN QUALITY ASSURANCE PROCEDURES

As already pointed out in the previous paragraphs, area detectors are essential for quality assurance purposes in external beam radiotherapy, especially:

in pre-treatment plan verification, where planar dose distributions need to be measured;

in machine quality assurance, when dynamic measurements in which dose distribution is changing with time are needed. In this case, scanning systems with point detectors cannot be used;

in machine quality assurance and LINAC commissioning, when there is the need to accelerate the measurement of static dose distributions beyond the capability of a scanning system.

Moreover, due to the characteristics of the treatment beams described in Section 1.1, the ideal detector should feature a small volume together with minimal energy and dose rate dependence in order to provide accurate dose information.

Methods based on dosimetry with radiochromic films [48, 49] and polymer gel [50, 51] are valuable especially because they can provide 2D and 3D dose reconstruction. Additionally, a great benefit of these types of detectors is their near water equivalence, resulting in a minimal perturbation of the radiation spectrum. On the other hand, measurements with film and gel might require complex procedures, time-consuming processes and accurate calibration workflows to obtain reliable results. Therefore, these solutions are often intended for reference measurements but not used in routinely (daily, weekly, or monthly) quality assurance controls.

Due to their efficiency, versatility, reliability, and extreme manageability, 2D detector arrays have become one of the most used solution for LINAC quality assurance [52, 53, 54].

Furthermore, they have become the standard devices for patient plan verification in intensity-modulated radiotherapy techniques [55, 56, 57]. A very well-consolidated pre-treatment plan verification procedure consists of measuring the dose with a 2D detector inserted into a phantom with simple geometry, as discussed in Section 1.2.2 [58, 59]. Besides that, there is

also an emerging market which addresses the need for online treatment verification with area detectors [60, 61, 62, 63].

It is also worth mentioning that, for both machine QA and patient QA, there are available solutions based on dose reconstruction from electronic portal imaging devices (EPID) integrated into LINACs machines [64, 65].

Features which are typically taken into account when classifying 2D detectors are the sensor type (ion chamber or semiconductor), the volume of a single sensor, the number of sensors and the spatial arrangement of sensors over the active area. In practice, the dosimetric performance of the detector strongly depends on the materials and the design, and there is always a tradeoff between performance and technical feasibility/costs.

Air-vented ionization chambers are till the gold standard for dose determination because of their low sensitivity dependence on radiation energy and their excellent stability in time.

However, the air-vented ionization chambers used currently in 2D detectors often have their response influenced by volume averaging effects when the sensitive volume is large compared to the field size. It is important to point out here that in modern radiotherapy there is the reasonable trend to increase the conformity of dose distribution to the tumor (and therefore the sparing of organs at risk) as much as possible by using very steep dose gradients. Steep dose gradients can be achieved with beam modulation and/or the use of stereotactic cones. The effect of volume averaging can lead to a smoothing of such steep dose gradients as those found in typical intensity-modulated techniques [66, 67] and SRS, where high doses per fraction are delivered and the margin for errors is reduced.

Furthermore, a poor spatial resolution is responsible for dose distributions being defined inaccurately. On the other hand, the sensitive volume of each chamber should be kept large enough to ensure a good signal to noise ratio and a resultant sufficient sensitivity. Dose gradient in typical SRS beams can be in the order of 10%/mm (cf. Paragraph 4.2); the spatial resolution of commercial air-vented ion chambers detectors is typically greater than 5 mm and the volume of each sensor not smaller than 0.032 cm³ [68].

Liquid-filled ionization chambers may represent a valid option to build detectors with smaller sensitive volume and higher sensor spatial density. This is possible due to the higher density of the sensitive medium compared to air. Detectors based on this technology have been

already introduced in the market and they have been proved to be suitable especially for dose measurements in small photon beams [69, 70]. However, the major disadvantage of liquid-filled ionization chambers is their strong dependence of sensitivity on dose rate as a consequence of the reduced ion mobility and speed, which makes them not ideal for dose measurements in stereotactic beams [69, 70, 71]. In some cases, [72] the collection time is about 9 ms, which is larger than the typical 3 ms (1/PRF) LINAC pulse period. Thus, PRF dependence is superimposed upon dose per pulse dependence.

Detectors based on diode technology have sensors with a small sensitive volume and high sensitivity, which can be arranged in a high-resolution grid. However, they may exhibit dose rate dependence and radiation energy dependence [73, 74]. The dose per pulse dependence of this detector type is due to the filling of carrier traps in the lattice, while the energy dependence is caused by the enhancement of the photoelectric effect, being the atomic number Z higher than the average one of water. The Z-effect in silicon-based detectors plays an important role in the dosimetry of small fields [75].

Moreover, silicon sensors may show a non-excellent radiation hardness (as mentioned in Paragraph 2.1.2.5), although modern construction technology has significantly reduced this weakness [76].

Indeed, detector arrays enjoy numerous advantages over point detectors in a water phantom, over films, and over gel dosimeters. They are efficient because of the simultaneous acquisition of many point doses; they are versatile, reliable and easy to set up. Currently, dose information can be projected in a 3D space through a calculation engine from measured 2D fluences. The increase of the sensors’ spatial density while maintaining high dosimetric performance in the verification of complex radiotherapy techniques has become one of the most challenging tasks in modern detector technology development. Besides the sensor geometry itself, one of the major problems common to silicon-based and IC-based detectors, is the escalation of readout complexity when reducing the pitch and/or increasing the active area. While films and gels are insensitive to this problem, for silicon-based and IC-based detectors, the best compromise has to be evaluated.

Chapter 3 DEVELOPMENT OF A NEW