Determination of in-phantom field output factors

In-phantom field output factors for clinical beams f_clin are measured at the same reference depth used for measurements in the msr field f_msr. Section 5.3.2 provides the methodology for determining the absorbed dose at z_ref under machine specific reference conditions. According to Ref. [1], z_ref is 10 g/cm² for high energy photons. However, at the time of writing this COP, data for CyberKnife machines are only available referenced or defined at the depth of maximum dose FIG. 18. Possible orientations of an ionization chamber for measurements of lateral beam profiles (arrows indicate scanning directions in the paper plane while circle and crossed circle symbols refer to scanning directions perpendicular to the paper plane).

z_max. Therefore, CyberKnife machines will be considered a special case until more data are available. Note that this COP does not include guidance for the measurement of in-air output factors. Guidance for this purpose can be found in Refs [12, 166, 168, 178].

6.5.2. Determination of the equivalent square small field size

For the purpose of selecting output correction factors for each small field size, the in-plane and cross-plane dosimetric field widths, defined as the FWHM at the detector measurement depth, are derived from the lateral beam profiles obtained as described in Section 6.3.

For rectangular small fields with uneven in-plane and cross-plane FWHMs, the equivalent square small field size is given by the geometric mean [19], i.e.:

clin=

S A B (45)

FIG. 19. Possible orientations of a solid state detector (diode, diamond) for measurements of lateral beam profiles (arrows indicate scanning directions in the paper plane while circle and crossed circle symbols refer to scanning directions perpendicular to the paper plane).

where A and B correspond to the in-plane and cross-plane dosimetric field widths, defined as the FWHM at the measurement depth. Outside the condition 0.7 < A/B < 1.4, which is usually not violated except for the smallest equivalent square small field sizes (below 0.6 cm), a larger uncertainty on the output correction factor than that specified in Table 37 should be considered.

For circular small fields with a FWHM radius r:

clin= = 1.77

S r p r (46)

r corresponds to the radius of the circular field defined by the points where, on average¹³, the dose level amounts to 50% of the maximum dose at the measurement depth.

Note that this guidance is based on equal area of field sizes, which is different from the rule used for equivalent square msr field sizes in broad beams, based on equal photon scatter contributions.

6.5.3. Determination of field output factors

Field output factors, relating the absorbed dose to water of a clinical field f_clin to that of a reference field, f_msr or f_ref, are derived from a measured ratio of detector readings multiplied by an adequate correction that converts the ratio of measured readings into a ratio of values of absorbed dose to water. In this section expressions are given for the case of a clinical field relative to a machine specific reference field. They can also be used for a clinical field relative to a conventional 10 cm × 10 cm reference field, in which case ‘msr’ in the text and equations is replaced with ‘ref’.

As already shown in Section 6.1.1.1, the field output factor, WQ^{fclin msr}_clin^,_,^fQ_msr, relative to the f_msr is defined by:

clin quantities) in the clinical field and the msr field, respectively. Values of the output correction factor kQ^{fclin msr}_clin^,_,^fQ_msr for a range of detectors are given in Tables 23 to 27,

13 To account for potential slight polar asymmetries and/or the effects of measurement fluctuations.

valid at 10 cm depth in water (except for the CyberKnife, for which they are valid at z_max). These factors include generic values for the volume averaging effect.

If the field output factor is determined with the intermediate field method using two detectors, i.e. an ionization chamber down to an intermediate field f_int, as small as possible but without small field conditions (which means that the outer edge of the detector is at least a distance r_LCPE away from any field edges), and a suitable small field detector such as a diode for smaller fields, thereby limiting the effect of energy dependence, then the field output factor is obtained as follows:

clin int

clin int

clin msr clin int int msr

clin msr int clin int msr int msr

int msr

where ‘det’ refers to the small field detector and ‘IC’ to the ionization chamber.

The output correction factor ^clin_clin^,_,^int_int

det f f

Q Q

ék ù

ê ú

ë û is obtained from the tabulated output correction factors with respect to the msr field as follows:

clin msr

In the absence of small field conditions for the intermediate field f_int, the output correction factor for the ionization chamber ^{int msr}_int^,_{, msr}

IC unity for the ionization chambers recommended in this COP.

6.5.4. Some practical considerations

While the determination of the field output factors is based on a relative measurement, it is still important to correct all readings of ionization chambers for influence quantities. Since measurement sequences are often long, atmospheric conditions can vary substantially. Recombination and polarity correction factors can also depend on the field size. Note also that the response of many solid state detectors also exhibits temperature dependence, and that recombination (or dose rate dependence) may also be considerable and thus vary as the field output changes.

The overall measurement sequence consists of individual field size measurements interleaving the reference field measurements. This means that the reference field measurement is done before and after the measurement for each non-reference field. This procedure is time consuming and may not be practical in all clinical situations, but the number of different field sizes interleaved between two reference field measurements is limited based on the known stability characteristics of the beam. This enables correction for drifts of the beam output and can help ensure that the reading for the reference field does not vary beyond acceptable tolerance levels.

In the clinical treatment delivery sequence the collimator setting can be approached in different ways (i.e. from a smaller or from a larger field size).

If the collimator control system allows for it, it is worth characterizing field output factors for the treatment planning system as averages of the measurements taken in the following two situations: (i) after the collimator is moved to a larger field size and then back to the correct field size, and (ii) after the collimator is moved to a smaller field size and then back to the correct field size. In this manner, the influence of the hysteresis of the collimator is minimized by averaging the effect.