Dosimetry issues related to the SSRT project at the ESRF

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1 Dosimetr issues related to the SSRT project at the ESRF P. Berkvens 1, J. F. Adam 2, E. Brauer 1, A. Bravin 1, F. Esteve 2, C. Nemo 1, Y. Preado 1, M. Renier 1, H. Requardt 1 and M. Vautrin 2 1 European Snchrotron Radiation Facilit BP22 Grenoble Cede 9 France 2 INSERM-U647/ESRF, Grenoble, France; MRI Unit, Grenoble Universit Hospital, Grenoble, France Abstract The present paper describes the Stereotactic Snchrotron Radiotherap (SSRT) project at the ESRF, emphasiing on the particular radiation safet issues related to this novel technique. The dosimetric characteriation of the monochromatic X-ra s used for therap and for both 2D and tomograph imaging is described. The dosimetr protocol is based on the IAEA 398 protocol, with the special provision to simulate uniform radiation fields b scanning an ioniation chamber and a water phantom through the. The dualit between the scanning ioniation chamber geometr and the standard broad geometr is mathematicall derived. It is shown that the use of a transmission ioniation chamber scanned through the allows the on-line measurement of the integrated dose during patient treatment. 1. Introduction The ID17 line at the European Snchrotron Radiation Facilit in Grenoble, France is dedicated to medical applications. In the past a medical research protocol in the field of snchrotron radiation angiograph has been conducted. Presentl two radiation therap programs, the Stereotactic Snchrotron Radiotherap program (SSRT) and the Mirco Radiation Therap program are being investigated and are progressing towards the clinical phase. The SSRT technique consists in the stereotactic irradiation of a (brain) tumour using a monochromatic, with a dose enhancement effect due to the injection of a contrast agent in the tumour. Snchrotron radiation s are characteried b their reduced height (tpicall of the order of 1 mm). This particular aspect requires special attention concerning the dosimetric characteriation of these s. International dosimetr protocols recommend the use of ionisation chambers to characterie and to calibrate X-ra s used for radiotherap purposes. The most widel used protocol for conventional radiotherap in hospitals is the IAEA 398 protocol [1] and is based on absorbed dose in water. However, other protocols [2], using ionisation chambers calibrated in air in terms of air kerma, are used in reference dosimetr protocols for low- and medium-energ -ras for radiotherap and radiobiolog. These protocols assume broad irradiation geometries, which impl that the transverse dimensions of the X-ra are larger than the transverse dimensions of the ionisation chamber. In the case of radiotherap techniques using snchrotron radiation X-ra s the broad irradiation condition can not be fulfilled, since the X-ra s will alwas have at least one transverse dimension which is smaller that the corresponding dimension of an commerciall available ionisation chamber. The standard dosimetr protocols therefore need to be adopted to take into account this particular aspect. This is the purpose of the dosimetr protocol based on scanning the ionisation chamber through the X-ra, described hereafter. 2. The use of a thimble ionisation chamber in a flat Ionisation chambers used for absolute dosimetr in radiation therap must be calibrated in uniform broad radiation fields, whose transverse dimensions are much larger than the corresponding dimensions of the ionisation chamber. When using the ionisation chamber in a flat, the geometr is completel different from the standard calibration conditions. We must therefore proof that we can nevertheless use the broad calibration factors of the ionisation chamber. When eposing an ionisation chamber to a uniform field, the different parts of the active volume of the chamber respond in a highl non-uniform wa, depending on the chamber s geometr and due to the presence of the central electrode. Figure 1 shows the response of a PTW 312 ionisation chamber to a flat, as a function of the transverse position of the relative to the ionisation chamber [3]. RADSYNCH'9 - Page 25

2 Φ electrode = 1 mm air kerma (mg/ma) position (micron) Fig.1 - Response of a PTW 312 ionisation chamber to a flat. If we denote b S() the relative sensitivit of the ionisation chamber as a function of one of its transverse dimensions, the measured dose rate radiation field with an absolute dose rate with C a calibration factor. 5.5 mm relative transverse position of the flat (µm) D calibration when eposing the ionisation chamber to a uniform calibration D can be epressed as: D calibration = C D S( ) d, (1) active volume Fig.2 - Tpical geometr encountered in snchrotron radiation X-ra dosimetr. Consider the geometr of figure 2. An ionisation chamber is eposed to an X-ra that has one transverse dimension ( ) larger than the corresponding dimension of the ionisation chamber, while the second transverse dimension ( ) is smaller than the corresponding dimension of the ionisation chamber. Assuming that the is uniform over its entire surface, and is characteried b a dose rate D (absorbed dose in water or air kerma), the dose rate measured measured with the ionisation chamber will be given b measured D D (absorbed dose in water or air kerma) D = C S( ) d. (2) Epression (2) shows that the accurate measurement of the ionisation chamber s transverse response S() allows the use of a standard ionisation chamber to carr out absolute dosimetr measurements in tpical snchrotron radiation X-ra s. The real difficult of this method results from the fact that the transverse response function S() of the ionisation chamber depends on the energ spectrum of the X-ra at the point of measurement and from the fact that the measured dose rate will be ver sensitive to the eact = RADSYNCH'9 - Page 26

3 position of the ionisation chamber. These two complications are avoided b scanning the ionisation chamber through the along the direction, at a constant speed v and measuring the integrated dose, instead of doing a static dose rate measurement (see figure 3). h slits v v Fig.3 - Principle of scanning ionisation chamber. fied vertical slits = h slits Ever plane cutting the ionisation chamber at a given position will be eposed to a with a dose rate D (absorbed dose in water or air kerma) during a constant time interval /v. The measured integrated dose D measured (absorbed dose in water or air kerma) will therefore given b. D measured = C ( d (3) v t = active volume v S ) D dt d = C D S( ) active volume or: v D calibration = Dmeasured (4) Epression (4) shows that from the measured integrated dose during the ionisation chamber scan a value for the dose rate can be obtained b multipling the measured integrated dose, using the broad calibration factors, b the factor v /. Epression (4) is theoreticall onl valid if the intensit of the radiation field is such that the ionisation chamber is used in a regime where ion recombination is negligible. Indeed, the space charge distributions inside the ionisation chamber are obviousl different when irradiating the chamber at once in a broad or when irradiating its different parts one after the other and the effects of ion recombination are epected to be different. Measurements using a PTW 312 semifle ionisation chamber seem to indicate that epression (4) would still be valid for ion recombination effect as large as 8% [3]. 3. Relationship between a static uniform and a scanned snchrotron radiation Consider a pencil-tpe, with transverse dimensions d and d and with a uniform distribution, e.g. characterised b its fluence F (at = ). The dose rate in the point (,,) due to the pencil parallel to the -ais, at the transverse position (,), for a given static geometr (infinite water phantom or irradiation on air) can be written as (see figure 4): (,) = F T (, ;, ) d d D, pencil (5) with T pencil (, ;,) the geometr dependent conversion factor between fluence and dose rate (G cm 2 ). (,,) d d (,,) Fig.4 - Irradiation b a pencil-tpe. RADSYNCH'9 - Page 27

4 The dose rate in the point (,,) due to a uniform broad, with field sie,, centred around the - ais is then given b: D broad (,,) = F T (, ;, ) d d = F T (,, ) T flat, pencil (,, ) d = d T (, ;, ) pencil flat d d (6) T flat (,,) d is the dose rate in the point (,,) due to a flat horiontal with width, centred around the -ais, and with a height d, located at a vertical position (see figure 5, left part). d (,,) (,,(t)) (,,) (,,) Fig.5 - Irradiation b a flat (left: static; right: scanning through flat ). v We consider net the configuration used for SSRT, where we scan the object to be irradiated through the flat snchrotron radiation. If denotes the height of the, and its width, the dose rate in the point (,,) is given b (see figure 5, right part): D, = flat (,, t) F T (,, ( t) ). (7) The object is scanned at a constant speed v through the. The dose in the given point, integrated during the scan over a height, centred on the, is given b: D, = flat v v t (,) D(,,, t) dt = F T (,, ) d = D broad (,,). (8) Epression (8) is the theoretical basis for defining the dosimetric characterisation of the flat X-ra and to define the link with the treatment planning software. 4. Comparison between the scanning ionisation chamber protocol and the IAEA 398 protocol The IAEA 398 protocol defines the determination, on the central ais, of the absorbed dose under reference conditions, the determination of the central ais depth dose distributions and the determination of dose profiles at different depths. Epression (8) showed that a broad irradiation is perfectl simulated b scanning the phantom verticall through the flat X-ra, at constant speed. The standard dosimetric quantities can be measured b scanning a thimble ionisation chamber verticall through the, at constant speed. Epression (4) showed that the standard, broad, calibration factors for the ionisation chamber can be used. The broad field sie is obtained horiontall b setting the slits to the required width and verticall b the height of the scan (smmetricall around the ais). The measurement of the absorbed dose under reference conditions is obtained from epression (8) b using the appropriate value for and for =. The central ais depth dose is obtained b measuring the dose, using epression (8), for different values of, alwas for =. An eample of measured percentage depth dose profiles is given in figure 6. These measurements were carried out using a PTW 312 semifle ionisation chamber in a RADSYNCH'9 - Page 28

5 3 cm 2 cm 2 cm water phantom, for a quasi-monochromatic of 8 kev [4]. Finall b measuring the dose, using epression (8) for a given value of and for different values of, horiontal dose profiles are obtained. Figure 6 shows an eample of measured horiontal dose profiles. Fig.6 - Measured percentage depth dose profiles (left) and horiontal dose profiles (right). 5. Dose measurements during the patient treatment Contrar to a conventional radiation treatment, where the integrated dose can be measured online using a calibrated transmission ionisation chamber, epression (8) shows that the use of a fied, calibrated transmission ionisation chamber will not allow an online measurement of the integrated dose, because of the dependence of the integrated dose on the vertical scanning speed v. This would make the implementation of a reliable integrated dose interlock in the patient safet sstem too complicated. A different dose measurement must therefore be implemented. The solution will consist in the installation of a transmission ionisation chamber moving with the medical chair (see figure 7). As shown hereafter, the insertion of a transmission ionisation chamber, behind the 2D mask, moving together with the mask (and the medical chair) verticall through the, will provide a measurement for the integrated dose delivered to the patient for a given orientation. IC IC v Fig.7 - Left: fied ionisation chamber IC in standard radiation therap; right: scanned ionisation chamber IC behind 2D mask in SSRT configuration. The treatment planning software defines, for each individual irradiation orientation, the optimum twodimensional shape of the uniform and the corresponding dose to the tumour. Using the same the treatment planning software gives a value for the corresponding on-ais dose at the reference depth in the standard water phantom (see figure 8). The initial ( = ) transverse shape can be described b min and ma, the minimum and maimum height respectivel and b ( ma min )() the -dependent width. The dose rate in the reference point (,,) is then given b: ma ma ( ) D D ( ) F T pencil ( ) 2,, = d d,;,. (9) ( ) min min RADSYNCH'9 - Page 29

6 F dt T ma D 1 min () D 2 (,,) (,,) min ma () treatment geometr water phantom ma () Fig.8 - Correspondence between dose D 1 delivered to the tumour and the absorbed dose to water D 2 at the reference depth in the water phantom. T F dt ma min () min (,,) v ma () ma min () min Fig.9 - Geometr for dosimetr measurement with moving 2D collimator. The geometr of figure 8 is reproduced b inserting a 2D collimator, with a shape identical to the 2D defined b the treatment planning software, centred on the -ais, fied to the medical chair (therefore moving at the same speed during the vertical scan). If the 2D collimator is sufficientl thick to attenuate completel the, the dose rate in the reference point (,,) for the geometr in figure 9 is given b: ma ( ), pencil d (1) ( ) D(,, t) = F T (,;, ( t) ) ma The integrated dose over the vertical scan (scan interval covers at least the interval min ma ) is given b: D ( ) ma ma, d d D 2D v pencil = v min ma ( ) (,) F T (,;, ( t) ) (,,) =. (11) Epression (11) shows that the correspondence between the integrated dose during a vertical scan and the on-ais absorbed dose rate in water obtained from the treatment planning software still holds. The integrated dose D(,,) can now again be measured using a thimble ionisation chamber, at the condition that the opening of the 2D collimator covers completel the active volume of the ionisation chamber (the minimum sie of the tumours to be treated will be such that this condition will be fulfilled). The measurement of the integrated dose with the ionisation chamber not onl allows obtaining the dose rate in the 2D, using epression (11), but actuall provides a direct measurement of the integrated absorbed dose to water in the reference point (the dose D 2 in figure 8), and which can therefore be related, via the treatment planning software, to the integrated dose delivered to the tumour, scanned with the same speed v through the with the same height (the dose D 1 in figure 8). The integrated dose measured with a transmission chamber, inserted behind the 2D collimator, covering the entire collimator hole and moving together with the collimator during the scan, will be proportional to the integrated absorbed dose to water at the reference depth. It is therefore possible to calibrate this transmission ionisation chamber relative to the thimble ionisation chamber placed at the reference depth in the water phantom. In this wa the transmission ionisation chamber will provide a direct measurement of the integrated absorbed dose in water in the reference point, the latter being directl related to the dose delivered to the tumour under the same scanning conditions. It is important to note that the dose, measured with the thimble ionisation chamber, and the dose, measured with the transmission ionisation chamber, var in the same wa with the scanning speed v and the height. The calibration factor thus obtained for the transmission ionisation chamber will be independent RADSYNCH'9 - Page 3

7 of v and. The online dose measurement with the transmission ionisation chamber, during the patient treatment, and the associated dose interlock will therefore be independent of the scanning speed v and the height. This is a fundamental advantage of the proposed dosimetr sstem. Although the calibration factor of the transmission ionisation chamber is, as mentioned above, independent of the absolute value of the vertical speed of the chair v, it is important to keep this speed constant during the calibration of the transmission ionisation chamber, because of the -dependence of the response function of the thimble ionisation chamber (or more precisel, because of the difference between the -dependence of the response functions of the thimble ionisation chamber and of the transmission ionisation chamber). In the same wa, although the value for the dose delivered to the tumour, derived from the integrated dose measured with transmission ionisation chamber, is independent of the absolute value of the vertical chair speed v,, it is again important to keep this speed constant during the patient treatment, because a non constant speed will result in a verticall inhomogeneous dose deliver. It should be noted that the dualit between the dose D 2 obtained from the treatment planning software and the dose measured when scanning the ionisation chamber is mathematicall onl eact if the geometr used b the software treatment planning to obtain the dose D 2 is at all times identical to the geometr during the scan. This requires that the transmission chamber moves together with the collimator and the water phantom. Indeed, a transmission ionisation chamber, placed behind the 2D collimator, but remaining at the fied height, would create a variable geometr during the scan, which cannot correspond to the static geometr used b the software. 6. Conclusions We have shown that a commerciall available thimble ionisation chamber can be used to characterise flat X- ra s, b scanning it at constant speed through the, using the standard broad calibration factors of the ionisation chamber. The Monte Carlo based treatment planning software relates the dose delivered to the tumour to the dose at the reference depth in a water phantom. The latter can be measured online b inserting a transmission ionisation chamber behind the 2D collimator, fied to the medical chair. References [1] IAEA Technical Reports no. 398, Absorbed dose determination in eternal radiotherap, Vienna, 2. [2] C. M. Ma et al., AAPM protocol for 4-3 kv -ra dosimetr in radiotherap and radiobiolog, Med. Phs. 28(6), , 21. [3] Feasibilit of a dosimetr protocol using a thimble ionisation chamber for MRT eperiments on ID17, internal report, P. Berkvens, 26 April 25. [4] Mathias Vautrin, SSRT Dosimetr Internal eperiment Report, 6 March 29. RADSYNCH'9 - Page 31