4.3 Measurement of TPR20,10(10)

The determination of TPR20,10(10) from the measurements obtained for TPR20,10(S), where S is the equivalent square fmsr, by using the measurement data in the analytic expression given by Palmans (Eq. (26)) [90]. Figure 14 shows the experimental set-up to be used for measurement of TPR20,10(S), with source-todetector distance of 100 cm and at a depth of 20 g/cm<sup>2</sup> and 10 g/cm<sup>2</sup> .

low-energy electrons reaching the axis of the beam will be reduced. Hence, the mean electron energy is increased, as a result of which stopping power ratio is also

Theory, Application, and Implementation of Monte Carlo Method in Science and Technology

unshielded diodes as a result of reduction in phantom scatter [71, 80].

However, various Monte Carlo-based studies have revealed that the charged particle spectrum generated inside water is not much affected by the change in photon fluence. Hence, the stopping power ratio of water to air does not vary more than 0.5% at 10 cm of depth for 6 MV photon beam for field sizes ranging from 10 cm 10 cm to 0.3 cm 0.3 cm or circular fields of 0.3 cm diameter [78, 79], for depth ranging up to 30 cm maximum variation of 1% has been reported [70]. However, the response of diode detectors is affected by this hardening of the photon beam due to the noticeable change in the mass-energy absorption coefficient ratio of water and silicon. For field sizes ranging from 10 cm 10 cm to 0.5 cm 0.5 cm, the variation of 3–4% has been reported in the response of the

For reference dosimetry in photon beams of high energy and large field sizes of beam quality Q using the air-filled detectors calibrated with respect to beam quality Q0, the radiation quality correction factor is required. There are two methods defined to account for beam quality [1, 3, 7]. First is the tissue phantom ratio at the depth of 20 and 10 g/cm<sup>2</sup> using water as a medium for 10 cm 10 cm beam size and source-to-detector distance (SDD) of 100 cm, TPR20,10(10,10)x [1]. The second method is based on percentage depth dose at a depth of 10 cm to 10 cm 10 cm beam size and source to surface distance of 100 cm, %dd(10,10)x. These beam

For some calibration laboratories, it is possible to provide calibration of air-filled detectors using clinical linear accelerator photon beam from calibration laboratories. This methodology for calibration of measurement equipment is much more realistic as there are small variations on the absorbed dose to water calibration factor for radiation equipment of the same kind, as the quality of beam varies moderately between the modern equipment of the same type. Therefore, it is possible to use the same radiation beam quality correction factor for similar model of air-filled detectors and radiation emitting equipment of the same kind. Hence, the dosimetric measurements on such machines can be performed without correction for beam quality. This methodology has been applied at some level for Gamma Knife® (Elekta AB, Stockholm), Cyberknife®, and TomoTherapy® (Accuray Inc., Sunnyvale, CA) radiation generators. Also, the components of the clinical linear accelerators such as secondary jaws and multi-leaf collimators are employed for having better machine uniformity and accurate small-field size definition [25–27]. It is important to remember that by the above method of calibration of equipment, there is no requirement for beam quality correction factors, and even then beam quality indices are crucial from commissioning and quality check procedure perspectives. Since the nominal photon beam energies used for intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), and stereotactic methods are below 10 MV, and the variation of kQ,Q0 to quality of the beam is small [1, 2]. A large number of add-ons are utilized in IMRT and stereotactic radiotherapy treatment methods, which makes it impossible to prepare tables for beam quality correction factors for each and every combination of radiation emitters, add-ons, and detector types. Hence, kQ,Q0 is not available in all machine/ detector combinations. As a result the beam quality index or beam quality correction factor is required to relate the beam quality used for the detector calibration and the beam quality of the user machine. Since, it is sometimes not possible to

affected.

4.2 Beam quality

60

quality indices are utilized to calculate .

Dependence of TPR20,10(S) on the field size S, for beam size ranging between 4 and 12 cm and photon energies between 4 and 10 MV.

#### Figure 14.

The experimental set-up to be used for measurement of TPR20,10(S).

TPR20,10(10) for 10 cm � 10 cm can be derived using the following relationship:

$$TPR\_{20,10}(10) = \frac{TPR\_{20,10}(S) + c(10 - S)}{1 + c(10 - S)}\tag{26}$$

4.4 Measurement of %dd(10,10)x

DOI: http://dx.doi.org/10.5772/intechopen.89150

Measurement setup for %dd(10,S).

Figure 15.

63

source-to-surface distance of 100 cm.

5. Determination of absorbed dose

Similar to the determination of TPR20,10(10) from measurements of TPR20,10(S), the %dd(10,10)x can be determined from the %dd(10) for S cm � S cm msr field size using the analytic relation provided by Palmans [90]. Figure 15 illustrates the experimental setup for the measurement of %dd(10, S) for S cm � S cm beam size and measurement depths of maximum dose (zmax) and 10 g/cm<sup>2</sup> at a

Prospective Monte Carlo Simulation for Choosing High Efficient Detectors for Small-Field…

beams (below 10 MV). However, the use of lead foil of 1 mm thickness is recommended for %dd(10,S) measurements in FFF radiation beams; in order to remove the electron contamination and analytic expression, Eq. (27) is used to calculate %dd(10,10)Pb. Then %dd(10,10)x can be obtained from %dd(10,10)Pb

The %dd(10,10)x can be calculated using the following expression:

using the relation provided by AAPM Task Group report 21.

where 4 cm <sup>≤</sup> <sup>S</sup> <sup>≤</sup> 12 cm and C = (53.4 <sup>∓</sup> 1.1) � <sup>10</sup>�<sup>3</sup>

Lead foil is not required in the measurements of %dd(10,S) for WFF radiation

In case of radiation-emitting equipment, where field beam of 10 cm � 10 cm

(fref) can be established, the dose measurement is performed using the

.

ð27Þ

This relation is valid when 4 cm <sup>≤</sup> <sup>S</sup> <sup>≤</sup> 12 cm, where C = (16.15 <sup>∓</sup> 0.12) � <sup>10</sup>�<sup>3</sup> . Prospective Monte Carlo Simulation for Choosing High Efficient Detectors for Small-Field… DOI: http://dx.doi.org/10.5772/intechopen.89150

Figure 15. Measurement setup for %dd(10,S).
