2 Brachytherapy Physics-Sources and Dosimetry

Brachytherapy Physics: Sources and Dosimetry

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THE GEC ESTRO HANDBOOK OF BRACHYTHERAPY | Part I: The basics of Brachytherapy Version 1 - 01/12/2014

introduced and defined as the product of the air kerma rate in free space at a measurement distance r from the source centre and the square of the distance r . The reference air kerma rate K . ref and air kerma strength S K are thus mutually related through the inverse square law to the reference distance r 0 as

ufacturer. The medical physicist is, however, always responsible for the validation of the source strength. The measured value should be traceable to a primary standards dosimetry laboratory (PSDL) such as PTB, NPL, or the National Institute of Standards and Technology (NIST) (35). In sections 2.3 and 2.4 on source dosimetry and dose calculation in brachytherapy, the two quan- tities, reference air kerma rate K . ref and air kerma strength S K , will be used in more detail in the given formalisms. For ß-emitters, the AAPM report (37) recommends that the source strength should be expressed in terms of dose rate in wa- ter at a reference distance of 2 mm. In a few reports, the most recent scientific views and recommendations for brachytherapy beta source dosimetry are discussed (28, 41). A recent project, initiated in the joint group of European Associ- ation of National Metrology Institutes (EURAMET, project TP2. JRP6: Increasing cancer treatment efficacy using 3D brachythera- py ; see http://brachytherapy.casaccia.enea.it/), aimed to develop primary standards for brachytherapy source strength specifica- tion in terms of dose-to-water at 1cm from the transversal axis of a source. Clinical introduction of this new concept has not yet been established by the professional societies. The introduction of such standards will need a reconsideration of the procedures for in-house calibration and for implementation in dose calcu- lation algorithms. Research groups working with or in contact with vendors and the standards laboratories must be included in the working groups and task groups of the professional societies to avoid any disparities occuring either from the dose-to-water concept for calibration or from new consensus algorithm types, that might otherwise distract and confuse individual users. In practice the new procedure would mean that for a given source, the reference air kerma rate calibration and the conversion factor of air kerma rate to dose rate will be replaced by a direct dose-to- water calibration. Coordination for this transition from reference air-kerma to dose to water across the entire field of brachythera- py is key. A distinct requirement from the clinical user’s point of view is that such a new approach should at least give the same, but preferably, lower source calibration uncertainties. 4.2 Calibration of sources The manufacturers of sources issue some form of calibration test certificate. This certificate is usually not directly traceable to a Standards Laboratory. However, all sources should have cali- brations traceable to a national or international standard before clinical use (27). As indicated above, and in accordance with the

2

S K =

r 0

(2.4)

K

ref

The recommended unit for air kerma strength is μGy m 2 h -1 . It has been denoted by the symbol U in such a way that

(2.5)

1 U = 1 µGy m 2 h -1 = 1 cGy cm 2 h -1

For elongated rigid sources, the direction from the source centre to the reference point at 1 m is defined in the transversal plane of the source, i.e. at right angles to the long axis of the source (Fig. 2.5). For low dose rate (LDR) brachytherapy applications it is convenient to use the unit μGy h -1 . The current and world-wide adopted approach for brachythera- py dose calculation is based on the AAPM TG-43 dosimetry for- malism (35, 44), which relies on superposition of single-source dose distributions obtained in a liquid water phantom with a fixed volume for radiation scattering. Note that, although the air kerma strength S K and the reference air kerma rate K . ref are dimensionally different, the numerical values are equal. A useful overview of conversion factors for reference air kerma rate and air kerma strength is presented in Table 2.6. The reference air kerma rate is determined for specific sources either by the clinical medical physicist or provided by the man-

Fig. 2.5 Schematic representation of the geometry as defined in AAPM Report 21 of the AAPM task group 32 (1) for defining the brachytherapy source strength. A cylindrical source is assumed, with L s being the length of the active part of the sealed source (active core). The radial distance r used for the measurement is taken in the transversal plane of the source, determining the air kerma rate in free space, K . a (r) . Figure from (3). (Courtesy: D. Baltas)

Table 2.6 Units and unit conversion factors for reference air kerma rate and air kerma strength (3).

Source Strength Quantity

Symbol

SI

Current unit

Conversion factor

μGy h -1

(1/3600) x 10 -6 Gy s -1 = 2.778 x 10 -10 Gy s -1

K .

Gy s -1

Reference air kerma rate

mGy h -1 mGy h -1

(1/3600) x 10 -3 Gy s -1 = 2.778 x 10 -7 Gy s -1

ref

1.0 x 10 3 μGy h -1

U, 1U = 1 μGy m 2 h -1 = 1 cGy cm 2 h -1

(1/3600) x 10 -6 Gy m 2 s -1 = 2.778 x 10 -10 Gy m 2 s -1

S

Air kerma strength

Gy m 2 s -1

K

(1/3600) x 10 -12 Gy s -1 Bq -1 m 2 = 2.778 x 10 -16 Gy s -1 Bq -1 m 2

μGy h -1 MBq -1 m 2

J kg -1 or Gy s -1 Bq -1 m 2

Γ

Air kerma rate constant

δ

μGy h -1 MBq -1 m 2

1.0 U MBq -1