QA requirements in radiotherapy

With the increased complexity of radiation treatments, more frequent and detailed quality checks are required. Although the main concern is the maintenance of accurate output, it is clear that, if the overall accuracy of treatment needs to be within the tolerances expected by radiation oncologists, other parameters require regular checking. Typical tolerances are (as recommended by the International Commission on Radiation Units and Measurements, ICRU, [18]):

Accuracy of delivered dose to the specification point ±3% (1 standard deviation (SD));

Accuracy of delivered dose at all other points in the target volume ±5% (1 SD);

Accuracy of positioning beam edges and shielding blocks in relation to the planning target volume ±4 mm (1 SD).

Taking into consideration the many steps involved in delivering a dose to a target volume in a patient, each step must be performed with an accuracy that is better than those specified to achieve these recommendations.

The QA program for machines which deliver the radiation exists to assure that their characteristics do not deviate significantly from their baseline values acquired at the time of acceptance and commissioning. Many of these baseline values are entered into treatment planning systems (TPS) to characterize and/or model the treatment machine. They can, therefore, directly affect treatment plans calculated for every patient treated on that machine.

Deviation from the baseline values could thus result in suboptimal treatment of patients.

Machine parameters can deviate from their baseline values as a result of many reasons. There can be unexpected changes in machine performance due to machine malfunction, mechanical breakdown, physical accidents, or component failure.

A number of quality assurance protocols have been written (see introductory part of this section), and these often differ in the test frequencies that they recommend. General guidelines have to be adapted to specific needs anyway. For example, if a machine is being regularly used for stereotactic single fraction high dose treatments, the quality checks relating to the mechanical alignment and stability with arc rotation will need to be carried out frequently, perhaps even before each treatment. On the other hand, for treatments being given over a six-week period, a dose inaccuracy of 3% for two or three fractions can be easily compensated for in subsequent fractions. Thus, less frequent checks may be appropriate.

Table 1.1 shows examples of recommended QA procedures taken from AAPM Task Group (TG) 142 [19], a comprehensive guideline for quality assurance of medical accelerators in MV X-rays external beam radiotherapy. AAPM Task Group 224 is expected to publish the equivalent report for proton machines in 2017 (a preliminary overview can be found in [20]).

Since the publication of the ICRU report 78 [21], there has been no dedicated report dealing with proton therapy quality assurance. Nowadays, the majority of these procedures in clinical activity are either adopted from or modified versions of procedures outlined in the AAPM TG 40 report [22, 23].

The last important step before allowing the patient to be treated is to check the whole chain and perform a quality assurance control of the plan being delivered. Especially for intensity-modulated and stereotactic radiation therapy, individual patient QA is of great importance in detecting possible errors that can result in erroneous treatments. For instance, when calculating an IMRT or SBRT/SRS dose distribution, a number of factors, such as small field dosimetry and MLC leaf modelling, become much more important than they are in standard therapy. Many parameters involved in the treatment are difficult to measure, resulting in potential sources of errors which cannot be controlled with simple machine QA. Therefore, pre-treatment patient QA is needed alongside machine QA to ensure correct treatment delivery.

It is standard practice to check individual plans with a pre-treatment comparison between the measurements and the treatment planning system computation. The most accurate and widespread solution is to measure the dose with a detector inserted in a phantom with a simple geometry. Patient-specific QA based on this practice is generally considered to be the most reliable, and it is mandatory in many countries (e.g. US), although it requires extensive resources. In many countries, a tendency exists to only perform patient-specific QA for the most complex treatment plans and to verify the ‘standard’ ones with independent calculations (e.g. by using specific class solutions for each tumor site). The Netherlands Commission on Radiation Dosimetry provides a good example of this in the Code of Practice for QA and Control for Intensity-Modulated Radiotherapy [24].

Similar procedures have been adopted for plan verification of IMPT irradiations. The accuracy required in the delivery of scanned pencil beams makes the verification of each individual plan essential. The measurements to be compared with planning system computation are typically performed with a detector being placed in a water tank and the beam being shot with a fixed angle of incidence. This configuration allows comparison of planar dose distribution at different depths.

There are two aspects which are equally important in the verification of a patient plan: the absolute dose and the dose distribution. To perform a reliable analysis of plan dose distributions, the concept of gamma index has been proposed by Low et al. [25]. Here, the

tolerance is expressed as a combination of the maximum distance to a point of agreement (∆ ) and the maximum percentage dose difference (∆ ). The measure of acceptability is the multidimensional distance between the measurement and calculation points in both the dose and the physical distance, scaled as a fraction of the acceptance criteria (∆ , ∆ ). In a space composed of dose and spatial coordinates (Figure 1.6), the acceptance criteria form an ellipsoid surface, the major axis scales of which are determined by individual acceptance criteria and the center of which is located at the measurement point in question. When the calculated dose distribution surface passes through the ellipsoid, the calculation passes the acceptance test for the measurement point.

Figure 1.6. Geometrical representation of the dose distribution evaluation based on the gamma index.

The ellipsoid surface that represents the acceptance criteria is defined by the equation:

,

∆

,

∆ , where , | | and , .

The minimum radial distance in the dose-distance space between the measurement point and the calculation points is defined as quality gamma ( ) index:

γ Г , ∀ (1.1)

where

Г , " , _#

∆

, _#

∆ (1.2)

, _# | _# | (1.3) and

, _{# # #} (1.4)

is the difference between dose values on the calculated and measured distributions, respectively. Regions where > 1 correspond to locations where the calculation does not meet the acceptance criteria. The gamma index, as described by Low et al. [25], quantifies the point-by-point difference between measured and calculated bi-dimensional dose distributions. Recently, a 3D gamma metric [26] has been introduced in the field of radiation physics as an extension of the 2D gamma index into another dimension, allowing for consideration and evaluation of the entire volumetric patient dose distribution. A comparison of the results of 2D and 3D gamma analysis for clinical treatment plans can be found in literature [27].

Applications with which to compare the dose grid calculated by the planning system with measurements from detectors are available. A number of technologies have been developed to accomplish this task; an overview of them can be found in Chapter 2 of this thesis. The goal of this work is to characterize a new technology aimed to perform quality assurance tests in modern external beam radiotherapy.

Table 1.1. Examples of machine QA procedures for conventional LINACs from AAPM TG 142

X-Ray flatness annual 1% change from baseline

Electron flatness annual 1% change from baseline

X-Ray symmetry annual ±1 % change from baseline

Electron

symmetry annual ±1 % change from baseline

X-Ray/electron

output calibration annual ±1 % (absolute)

Output factors

isocenter annual ±1 mm from baseline

Coincidence of