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Re-establishing the photon absorbed dose primary standard on the NPL clinical linac
D. Shipley, J. Pearce, S. Duane, R. Nutbrown
Figure 1: Schematic of the current NPL primary standard graphite calorimeter (left) and close up of the inner jacket and core (right).
INTRODUCTION
A new state-of-the-art clinical linac facility was opened in November 2008 at the NPL in addition to the existing research linac facility. The new machine is an Elekta Synergy Digital Linac with iViewGT portal and XVi 3D x-ray volumetric imaging that can be confi gured to deliver seven x-ray beam energies (instead of the usual maximum of three in any one hospital machine). This feature, together with the ability to provide up to ten electron beam energies, will enable NPL to provide absorbed dose calibrations for the full range of energies currently in therapeutic use in the UK.
The NPL is responsible for maintaining the UK primary standards of absorbed dose to water in both high-energy photon and electron beams. For photons, the primary standard is a graphite calorimeter [1] that directly measures absorbed dose to graphite, that is, the energy deposited in a small graphite core at the centre of the calorimeter, in 60Co gamma ray and MV x-ray beams, divided by the mass of the core (Figure 1).
METHOD
Dissemination of the absorbed dose primary standard is done via calibration of a set of reference standard ionisation chambers. These chambers are placed in a graphite phantom and their response compared with that of the primary standard in terms of absorbed dose to graphite, Ng. These calibrations are then converted to absorbed dose to water, Nw, using a suitable dose conversion method [2,3] (Figure 2). Secondary standard instruments are then calibrated against these reference standards at either 5cm or 7cm depth in a water phantom.
Chamber calibrations are normally given in terms of kQ, the ratio of the chamber calibration factor in a given quality, Q, to that in a reference quality, Q0 (
60Co γ-rays at NPL) where the radiation quality is defi ned in terms of tissue-phantom ratio (TPR20/10).
In this work, a series of 2611 reference ionisation chambers were calibrated against the existing primary standard in all seven x-ray beams on the new NPL clinical linac using the above procedure. Validated Monte Carlo models of the linac source were also developed for each energy using EGSnrc (V4-r2-2-5) [4] and BEAMnrc (2007) [5] packages (Figure 3). These were used to determine a number of corrections in support of these measurements, in particular,
• a correction to account for the presence of non-graphite materials and air/vacuum gaps in the primary standard calorimeter,
• a correction for the different geometrical confi guration of the calorimeter and graphite chamber phantom, and
• water-to-graphite dose ratios at the chamber measurement point in the water and graphite phantoms.
The required absorbed dose to water calibration factors were re-evaluated for each new clinical x-ray energy and compared with previously determined data for the x-ray beams on the existing research linac.
Figure 2: Graphite to water dose conversion measurements on the NPL clinical linac (equidistant dose ratio method) using an external parallel plate monitor chamber.
10MV: Inline 5.0cm in water (10x10cm field, 95cm SSD)
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Mon te C ar lo ( 9 .8 MV :0 .3 MV ,e- s pot :0 .5 mm)
Figure 3: Linac source model for 10 MV x-rays (top). Comparison of calculated depth dose and profi le with ion chamber measurements in a water phantom under reference conditions (bottom). Agreement was typically better than 1-2%.
RESULTS
The presence of gaps and non-graphite materials in the primary standard calorimeter under reference conditions, 10x10cm at 100 cm source-chamber distance (SCD), were found to reduce the dose to the calorimeter core by typically 0.6% for 4MV x-rays and 0.2% for 25 MV x-rays (consistent with earlier experimental investigations of this correction carried on the existing NPL research linac [6])
Changes in the scatter dose component due to the different geometrical confi guration of the primary standard calorimeter and graphite phantom were found to be 0.2-0.3% up to 10MV x-rays and negligible at higher energies predominately due to increased backscatter from extra material present at the back of the phantom (and in good agreement with comparable scatter measurements).
Water-to-graphite dose ratios in scaled phantoms required for the dose conversion at an SCD of 100cm varied from 1.081 for 4MV x-rays to 1.115 for 25MV x-rays and again were consistent with previous values determined on the existing research linac [3]. They were found to be mostly independent of source-chamber distance for a given energy (apart from distances very close to the source where back-scatter becomes an issue in the measurements).
The ratio of absorbed dose calibration factors in water to graphite, Nw/Ng, obtained by combining charge measurements and calculated dose ratios at the chamber positions in the two phantoms (using the equidistant dose ratio method) is shown in Figure 4. Also shown are the results determined in previous work [2,3] on the existingresearch linac.
The mean values for the absorbed dose to water calibration factors, in terms of kQ, for at least six 2611 type chambers at each energy are shown in Figure 5 and show good agreement (within 0.3%) with the kQ curve fi t currently in use. The estimated standard uncertainty in these values is 0.55% (k=1). The typical chamber-to-chamber spread at each point is better than 0.1%.
CONCLUSIONS
Models and methods have been developed for simulating the new clinical linac facility at NPL. The models for seven x-ray energies have been matched within suitably small tolerances with experimental measurement and these sources have been successfully used to determine the required corrections and dose ratios in order to realise absorbed dose to water under reference conditions.
Values for the absorbed dose calibration factor in water, in terms of kQ , on the NPL clinical linac are consistent with those currently in use determined on the existing research linac. For now, 2611 ionisation chamber calibrations will continue to be reported by normalisation of the existing kQ curve. However this will be reconsidered when the new primary standard calorimeter is commissioned and data from this new calorimeter is available.
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0.55 0.6 0.65 0.7 0.75 0.8 0.85Tissue Phantom Ratio (TPR20/10)
Nw/N
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Pe a r c e a n d S h ip le y 2 0 1 0
B u r n s a n d Da le 1 9 9 0
Nu tb r o w n e t a l 2 0 0 0
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Tissue Phantom Ratio (TPR20/10)
kQ (=
NQ
/NQ
0)v a lu e s d e te r m in e d in th is w o rk
kQ c u r r e n tly in u s e a t N PL
Figure 5 Absorbed dose to water calibration factors, in terms of kQ , as a function TPR20/10 (points) compared with the fi t to the data currently in use.
10MV: Depth Dose in water (10x10cm field, 95cm SSD)
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M eas u r ed ( B inned )
M on te Ca rlo ( 9 .8M V :0 .3M V ,e - s po t :0 .5 mm )
Figure 4: Nw/Ng as a function of TPR20/10 for all MV x-ray energies in this work compared to previous work. The estimated standard uncertainty in this work is 0.3%.
National Physical Laboratory, Hampton Road, Teddington,Middlesex, United Kingdom, TW11 0LW
REFERENCES
1] DuSautoy A R, The UK primary standard calorimeter for absorbed dose measurement, NPL Report RSA(EXT) 25, 1991.
[2] Burns J E and Dale J W G 1990 Conversion of absorbed-dose calibration from graphite into water, NPL Report RSA (EXT) 7
[3] Nutbrown R F, Duane S, Shipley D R and Thomas R A S, Evaluation of factors to convert absorbed dose calibrations in graphite to water for mega-voltage photon beams, NPL Report CIRM 37, 2000.
[4] Kawrakow I and Rogers D W O, The EGSnrc code system: Monte Carlo simulation of Electron and Photon Transport, NRCC Report PIRS-701, 2006
[5] Rogers D W O, Ewart G M, Faddegon B A, Ding G X, Ma C-M, We J and Mackie T R, BEAM: a Monte Carlo code to simulate radiotherapy treatment units Med. Phys. 22 503-24, 1995.
[6] Owen B and DuSautoy A R, NPL absorbed dose graphite calorimeter correction for the effect of the gaps around the core, NPL Report RSA(EXT) 12, 1990.
