6. CODE OF PRACTICE FOR RELATIVE DOSIMETRY OF
6.1. Equipment
6.1.1. Detectors for relative dosimetry
The guidance regarding detectors for relative dosimetry is given in Section 4.2.1. It must be emphasized that no ideal detector exists for measurements in small fields. For the determination of both field output factors and lateral beam profiles, the use of two or preferably three different types of suitable detectors is therefore advised so that redundancy in the results can provide more confidence and assurance that no significant dosimetry errors are being made. An example could be a combination of detectors with correction factors above and below unity (so that the product of these factors is close to one), such as a small air filled ionization chamber, radiochromic film and an unshielded diode, or a diamond, liquid ion chamber and an organic scintillator.
6.1.1.1. Detectors for measuring field output factors
A full discussion on detectors used for measurements of dosimetric parameters for relative dosimetry is given in Chapter 4. As discussed in Section 3.2.3, field output factors (see also the definition in Section 2.3.2.1) are derived from a ratio of detector readings according to:
clin a directly measured value, an experimental generic value or a Monte Carlo calculated generic value. Data for kQfclin msrclin,,fQmsr as a function of field size are given in Section 6.6 for different detectors and machines.
The minimum field size recommended for measurements with real time detectors (those providing an instantaneous and potentially continuous signal readout) and for off-line detectors (those that provide a readout after post-processing) is such that the detector specific output correction factor is not greater than ±5% for a particular machine. For this reason Tables 23–27 do not include kQfclin msrclin,,fQmsr values outside this interval. It is understood that detectors or machine configurations not included in the tables require an experimental or Monte Carlo determination, but extrapolation of the tabulated values is to be avoided.
As an example, according to the tabulated values, the PTW 60008 and 60016 shielded diodes are not to be used for field sizes smaller than 1 cm (equivalent square) in WFF and FFF machines with 6 MV (Table 26) or 10 MV (Table 27).
For the determination of field output factors, the volume averaging effect will be one of the limiting issues for the choice of a detector. The detector size is such that the volume averaging correction factor ( )clin
vol clin
The volume averaging correction factor ( )clin
vol clin f
k Q is calculated using:
( )
( )where w(x,y) is a weighting function specific to the ionization chamber geometry, described in Appendix I, where examples of the calculation of the volume
12 Note that no procedure is provided to determine the beam quality Qclin of the clinical field. It should be understood as the beam quality of a small field at a radiotherapy machine for which the beam quality of the reference field is Qref or Qmsr. For the user, the only relevant difference from the reference field is the field size, but the beam quality Qclin is explicitly used to indicate that the charged particle spectrum at the measurement depth will be different from the charged particle spectrum in the reference field.
averaging correction factor are given. It is advised that the field size dependence of the detector’s response be smaller than 2% over the range of field sizes measured. This number is a typical variation of the change in the response of unshielded diodes for an increase of the equivalent square field size S by 5 cm.
As discussed be Section 4.2.1, a number of specialized detectors (e.g. radiophotoluminescent detectors, plastic and organic scintillators) and techniques to use them are available. Experience on their use is limited to workers with specialized training and access to specialized equipment. It is advised that users of these detectors develop significant expertise before using them for measurements of clinical dosimetric parameters.
Dosimetry of small fields in non-water and heterogeneous media is beyond the scope of this COP, but it is important to be aware that these conditions may introduce significant energy and material dependent perturbations, and using generic data for such conditions can result in significant clinical errors [169–172].
6.1.1.2. Detectors for measuring beam profiles
It is advised that for the experimental determination of lateral beam profiles, detectors have high spatial resolution (requiring the use of detectors with a small area perpendicular to the beam axis or quasi-continuous detectors such as radiochromic film), limited energy dependence in their response and limited dose rate or dose-per-pulse dependence (see Table 6 for limits).
Liquid ion chambers, unshielded diodes, microdiamonds and organic scintillators have a small sensitive volume and are suitable for profile measurements using a scanning system. Given the asymmetries in their construction and the influence of stem irradiation effects, the orientation is always such that the stem is parallel to the beam axis. It is advised that the effect on the profiles of irradiating the stem and parts of the cables always be investigated, minimized and, if possible, corrected for. Also, the effect of charge recombination needs to be assessed and, if necessary, corrected for.
Radiochromic film is very suitable for lateral profile measurements, but needs adequate readout and calibration procedures. Any other detector with a dispersed radiosensitive agent (such as a gel dosimeter) needs to be thoroughly investigated, characterized and benchmarked against other detectors.
6.1.2. Phantoms
Guidance regarding phantoms is given in Section 4.2.2.