1. Introduction

In the current state of external photon beam radiation therapy, the "small fields" are generated by collimating the photon beam, flattened or unflattened. This is done with the help of collimating system available on medical electron linear accelerators (linacs), which includes secondary jaws, multi-leaf collimators (MLCs), tertiary collimators, etc. [1]. The small photon beams differ from traditionally used

radiation fields (4 cm 4 cm up to 40 cm 40 cm) in terms of their size. Due to their small size, the penumbra region generated from the edges of the fields overlap, resulting in apparent field widening of the fields [2]. As a result the traditional detectors used for dosimetry become large relative to the size of the field, and this may lead to unintended errors while performing measurements for small field [3].

verified by FWHM measurement of the beam profile along the lateral direction. However, in the small beam condition due to the partial blockage of primary source of photon and reduction in the head scatter along the central axis, the output of radiation is reduced. As a consequence the condition of lateral charged particle equilibrium (LCPE) is not fulfilled. Hence, due to the reduction in the radiation output along the central axis, the value of maximum dose also gets reduced, and agreement between the geometrical and irradiation fields is lost. Figure 1 illustrates the overlapping of penumbra region with decrease in field size, as it leads to decrease in output and increase in penumbra width. Hence, the parameters like FWHM breaks down for small-field sizes [3, 10]. In case of small beams due to the overlap of penumbra region, the FWHM of the profile gets broader relative to the

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

collimator settings, and this effect is called apparent field widening.

Any radiation beam which fulfills at least one of the following conditions can be

ii. The partial blockage of the radiation source by the collimating devices along

iii. The detector size is equal to or larger than the size of the radiation beam (see

Conditions I and II are related to the size of the radiation beam, whereas condition III refers to the size of the detector. If all the above mentioned conditions are fulfilled, then the penumbra region overlaps with the volume of the detector.

Schematic illustration of the definition of geometrical and irradiation field size using the concept of geometrical projection and FWHM of radiation beam profile for both broad and small beam conditions: (a) for large field sizes, where condition of LCPE is fulfilled and radiation source is not blocked, the full width at half maximum (FWHM) of the lateral dose profiles is equal to the opening of the collimating jaws at the isocenter. Hence, for large field sizes at isocenter, geometrical field size and irradiation field size are in agreement with each other; (b) for the field sizes of the order of the range of secondary charged particles, the penumbra region of opposite jaws. It results in small error in determining the field size from the FWHM of lateral beam profiles; (c) however, for small field sizes due to the reduction in the radiation output along the central axis, the value of maximum dose is reduced. Hence, the FWHM of lateral beam profile is pushed outward and agreement

i. The absence of LCPE along the central axis of the radiation field (see

2.1 Conditions of small beam

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

2.1.1 Definition of small field

named as small field:

Figure 1).

Figure 3).

Figure 1.

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the central axis (see Figure 2).

between the geometrical and irradiation fields is lost.

The widely accepted Code of Practices (COPs) reported in the Technical Reports Series No.398 of IAEA have procedures to determine absorbed dose to water from measurements made with an ionization chamber in photon, electron, proton, and heavy-ion beams [4–7]. The ionization chambers are used to perform the measurement using the calibration coefficients obtained from primary standard dosimetry laboratories (PSDL) in terms of absorbed dose to water under reference conditions. However, these COPs do not consider the conditions deviating from reference conditions in detail [8].

As a result of technological improvements and introduction to new radiation therapy techniques, the small static radiation beams are rapidly used, which is achieved by using standard or add-on MLCs or by the design of the radiation equipment. Consequently, the uncertainties related to the clinical dosimetry based on traditionally used COPs have been considerably increasing, and errors related to dosimetry have been growing larger. The main causes of this increase in the size of dosimetric errors are that it is not possible to achieve the reference conditions as recommended by traditional COPs on some radiation equipment and nonstandardization of dose measurement procedures in small and composite radiation fields. Hence, many accidents have been reported that have occurred due to the use of recommendations of traditional COPs in dosimetry of small fields [8, 9].

The dosimetry of small fields is quite important. The beam data once configured during commissioning will be used for treatment in the future, so there should be high accuracy in the dosimetry of these small fields. To achieve high accuracy in beam data measurement in small fields and to be able to measure the dose in small fields with high precision, it is quite important to understand the physical aspects of the small fields. The measurement of output factors, beam profiles, and depth dose data is highly influenced by the beam energy, scattering, and field dimensions at the level of detector. The knowledge of the important characteristics of small fields is required to measure the dose parameters and to collect data for treatment. Hence, in 2017 a joint working group from the International Atomic Energy Agency (IAEA) and American Association of Physicists in Medicine (AAPM) proposed a new COP, Technical Report Series (TRS) No. 483 (Dosimetry of Small Static Fields Used in External Beam Radiotherapy). [9] This COP provides recommendations related to the relative and reference dosimetry of small and composite fields. Hence, this chapter discusses the concepts related to the dosimetry of small and composite field sizes.
