A Measuring System with Recombination Chamber for Photoneutron Dosimetry at Medical Linear Accelerators

Transcription

1 A Measuring System with Recombination Chamber for Photoneutron Dosimetry at Medical Linear Accelerators N. Golnik 1, P. Kamiński 1, M. Zielczyński 2 1 Institute of Precision and Biomedical Engineering, Warsaw University of Technology, Sw. A. Boboli 8, PL Warsaw, Poland. golnik@mech.pw.edu.pl 2 Institute of Atomic Energy, PL Otwock-Swierk, Poland Abstract. The aim of this work was to develop a relatively simple measuring system for photoneutron dosimetry at medical electron accelerators. The system is suitable for routine determination of the photoneutron contribution to the organ dose equivalent or to ambient dose equivalent, outside the irradiation field. The accuracy is sufficient for optimisation of radiation protection - e.g. for proper shielding, including the neutron shields, if reasonable. The measuring system contains two ionisation chambers - one is a specially designed in-phantom tissue-equivalent recombination chamber used as a main detector, and the second is a high-pressure C-CO 2 ionisation chamber, which serves as a monitor of X-ray radiation. Both chambers are connected to a two-channel automated system for measuring control and data acquisition. The use of the TE chamber enables to determine both the absorbed dose and the recombination index of radiation quality, Q 4. For this, the measurements of ionisation current at two polarising voltages have to be performed. The value of Q 4 is equal to one for gamma radiation of 137 Cs source and 0.95 for gamma radiation with energy of several MeV. For neutrons, the values of Q 4 range from 1 to about 12. Therefore, any value of Q 4 greater than 1 indicates some presence of fast neutrons or other high-let particles. Tests of the system, were performed in reference fields of neutron and gamma isotope radiation sources. The results showed that 0.5% contribution of photoneutrons to the total absorbed dose would result in an increase of the Q 4 value from 1 to 1.045±0.01. This means, that the dose equivalent due to photoneutrons in the considered medical accelerator field can be directly (on line) determined with the accuracy of about 40%, by automated measurements with single instrument. First results of the measurements at medical accelerator confirmed this expectation. 1. Introduction Radiation fields around medical accelerators, are slightly contaminated with neutrons, generated by photon-neutron nuclear reactions. This concerns practically all the accelerators operating at maximum photon energy of 15 MeV or higher. The level of neutron production and its unwanted whole-body dose to the patient vary around different treatment units due to differences in the construction of the treatment head and the design of the treatment room [1-4]. The International Electrochemical Commission (IEC) recommends certain limits for the neutron absorbed dose in the patient plan [5], but practically almost no measurements are performed in radiotherapy departments, The main reason, to discourage medical physicists from making the measurements is lack of convenient measuring equipment for the routine use. A relatively simple measuring system, with a recombination chamber [6], has been recently proposed [7] for radiation protection measurements along the treatment couch (outside the irradiation field) in the treatment room or in the maze and control room. This paper presents a short overview of the method and the results obtained at the medical accelerator. In case of the medical accelerator fields, some difficulties in applications of recombination chambers are associated with high dose rates and pulsed structure of the radiation field. Therefore, a special recombination chamber was designed and the correction for volume recombination was determined in order to minimise the influence of the volume recombination in the chamber. 1

2 Recombination chambers are high-pressure, usually tissue-equivalent, ionisation chambers operating under a condition of initial recombination of ions. This kind of recombination occurs within tracks of single ionising particles. Therefore, the tracks of ions generated in the chamber cavity have to be relatively distant and the ions should be collected in short time, before they diffuse far enough from the original track and can recombine with ions from another track (volume recombination). The initial recombination does not depend on the dose rate and depends on local ionisation density within the tracks of ionising particles. Generally, the use of recombination chambers makes it possible to determine the total absorbed dose and recombination index of radiation quality, Q 4 [8] in a phantom of interest. Then, dosimetric quantities, like ambient dose equivalent H*(10) or the dose equivalent in a distant organ, can be derived from the measured quantities. It was shown [7] that such measurements can be performed also in the vicinity of a medical accelerator. The measurements can be performed within about 20 minutes and the results can be obtained on-line. Additionally, the neutron absorbed dose can be estimated for some comparison with the recommended limits. The recombination chambers practically do not suffer from saturation and dead-time effects. Therefore, they can be used in relatively intense radiation fields, provided that a proper correction for volume recombination is introduced. Other advantage is that ionisation chambers are common devices in radiotherapy department. This makes it relatively easy to connect the recombination chamber to electronic devices that already exist in radiotherapy centres. 2. Recombination index of radiation quality As mentioned above, the recombination index of radiation quality, RIQ, is measured by recombination chamber, operating under conditions of initial recombination of ions i.e, not only at saturation but also at lower supplying voltages. The condition of initial recombination is essential, because only the initial recombination depends on local density of ions, while does not depend on dose rate. The term local density of ions, µ, means here the density of ions averaged over a short segment of an ionising particle track, scaled in such a way that µ = 1 for reference gamma radiation of 137 Cs. The considered length of the segment or the size of cluster of ions is equivalent to about 70 nm of unit density tissue [6, 9]. In order to determine the RIQ, one has to measure the ionisation current of a recombination chamber at two specially chosen polarizing voltages, denoted as U S and U R. Therefore, a family of different RIQ's can be specified, depending on the choice of the voltages. For radiation protection purposes, U S is usually set as a high voltage, within the saturation range. Then, U R has to be determined for each chamber during the calibration in reference field of 137 Cs gamma radiation source. The calibration involves determination of the whole saturation curve. Most often [8, 9], U R is chosen as a voltage, which ensures 96% of saturation (i.e. 4% of recombination) for the reference radiation. If the voltages U S and U R are chosen like described above, the RIQ is denoted as Q 4 and defined as: ( 1 fr ) Q4 = (1) 0.04 f R = f(u R ) ion collection efficiency (the ratio of the ionisation current at voltage U R to the saturation current) measured in the investigated radiation field. The procedure of the determination of Q 4 is illustrated in Fig. 1. 2

3 Ion collection efficiency f R U R Collecting voltage (V) FIG. 1. Principle of determination of Q 4. The figure shows the saturation curves of a recombination chamber for a reference 137 Cs gamma radiation source (open circles) and for 239 Pu-Be neutron source (solid circles). The voltage U R and the value of f R are indicated. Please note, that the voltage-axis is logarithmic up to 100 V and linear for higher voltages. It has been shown [8-10], that the Q 4 approximates well the ICRU-21 quality factor and there is also a method to derive the ICRP-60 quality factor. Dependence of RIQ on local density of ions can be expressed as: µ Q R = (2) 1+ R( µ 1) with R=0.04 for Q 4. Measured value of Q R is obviously averaged over all values of µ. Since radiation quality factor was defined in terms of unrestricted linear energy transfer in water, L, therefore also RIQ is often expressed as a function of L [11]: L / L Q 0 R for L 3.5 kev µm -1 (3) 1+ R( L / L0 1) Q R L / L 0 for L < 3.5 kev µm -1 (4) L 0 = 3.5 kev µm -1. Better approximation can be obtained using restricted LET with energy cut-off of about 500 ev instead of unrestricted one in equation (3 and 4) [9]. 3. Method for determination of H*(10) at medical accelerators The energy of neutrons generated in the vicinity of medical accelerators is limited to several MeV. Moreover, the neutron contribution to the absorbed dose is low. In such fields, the ambient dose equivalent H*(10) is equal to the product of the absorbed dose and recombination index of radiation quality Q 4 [6,8]. H *(10) = D* (10) Q 4, (5) D*(10) absorbed dose in an appropriate phantom, which can be measured by a properly calibrated recombination chamber operating at saturation. 3

4 By definition, Q 4γ = 1 for gamma radiation of 137 Cs. Theory of local recombination of ions predicts that Q 4 for high energy photons and relativistic electrons can be slightly lower than one (eq. 4), because it is a function of restricted LET [9]. Our previous measurements, performed in the beam of 19 F(p,α γ ) 16 O photons at PTB Braunschweig, resulted in the value of Q 4 = 0.95 ± Therefore, any excess of the measured Q 4 over unity in the vicinity of a medical accelerator, would indicate some presence of neutrons. Then, the H*(10) value can be calculated, from the equation (1) and used for radiation protection purposes. Additionally, the neutron dose equivalent, H*(10) n, can be derived using the fact that Q 4 is an additive quantity [8] i.e. D Q4 = Dγ Q4γ + Dn Q4n = ( D Dn ) Q4γ + Dn Q4n (6) D γ D n D Q 4γ Q 4n photon absorbed dose, neutron absorbed dose, total absorbed dose, Q 4 for photons alone Q 4 for neutrons alone. Because Q 4γ = 1, the eq (6) can be transformed to the form: Q 1 * * 4 D (10) n = D (10) (7) Q 1 4n or Q H *(10) = * ( 1) 4n n D (10) Q4 (8) Q 1 4n The value of the Q 4n in the equation (8) is not known, if not determined by an independent method. It can be seen, however, that the uncertainty of H*(10) is not much influenced by an ambiguity of Q 4n. Therefore, an approximate value of Q 4n. it can be assumed, basing on general knowledge of the neutron spectra at medical accelerators. The uncertainty of such estimation is of about 15% and adds about 2% of uncertainty to the value of H*(10) n, calculated from the eq. (8). In case of the neutron absorbed dose calculation from the eq. (7), the uncertainty of Q 4n has larger influence on the final value of D*(10) n, therefore, the use of H*(10) n and eq.(5) is recommended. The accuracy of the measurements is also influenced by volume recombination of ions in the recombination chamber. According the theory of Boag [12], the volume recombination, f V, in a parallel-plate ionization chamber irradiated in a pulse radiation field is given by the following relation: 1 f V = ln(1 + u) (9) u 2 u α qd ai = = e( k k ) U U q density of the generated electrical charge, proportional to the saturation current, i 0 saturation current, U polarizing voltage d distance between electrodes, a a constant for a given chamber. 4

5 Total ion collection efficiency, f, in the chamber can be expressed as a product of ion collection efficiency of the volume recombination f v, and the initial one f i : Therefore: f ( i U ) f ( U ) f ( i U ) f ( i U ) f ( U ) 0, = V 0, i (10) ( U ) U ai f ( U ) U ai = + 0 fi = + 0 i 0, i ln 1 ln 1 (11) ai0 fi ( U ) U ai0 U Parameters a and f i can be obtained from the measurements of f at different dose rates. Then, the eq. (11) can be fitted to the dependence of f on i 0 /U. Unfortunately, such measurements are difficult at the medical accelerators, because the change of the dose rate in isocentre is due to the change of frequency of pulses, while the volume recombination depends on intensity of radiation in a single pulse. Therefore, the measurements can be performed within very limited range of dose rates at different distances from the target. In our measurements, the correction for volume recombination was estimated to be of order of 1% of total ion collection efficiency and at present, its uncertainty considerably influences the accuracy of the measured Q 4. In order to improve the accuracy, a special recombination chamber was designed. The chamber (described in the next section) has a small distance between electrodes (0.5 mm) and this strongly reduces the volume recombination, comparing to the chamber used up to now. Measuring system The measuring system consists of two ionisation chambers connected to a two channel automated electronic circuit, controlled from a PC computer. Each channel of the electronic system includes an electrometer (Keithley 642 connected to the computer via IEEE 4888 bus), a power supply and necessary interfaces. A recombination chamber of F-1 type was used as a main detector for the first measurements at the Varian Clinac 2300C/D accelerator. The F-1 chamber is a 3.8 cm 3 parallel-plate chamber, filled with methane up to a pressure of 0.5 MPa. The chamber has three electrodes, 34 mm in diameter. The wall thickness is 0.6 g/cm 2. The distance between electrodes is equal to 1.75 mm. A high-pressure C-CO 2 ionisation chamber served as a monitor of photon radiation. The values of the ionisation current of the main chamber have to be measured in saturation and at the voltage U R. However, the results are much more precise if the measurements are performed for both polarities of the voltages and appropriately averaged [8]. The sequential application of the positive and negative voltages was controlled by the PC computer. The high-stability voltage supply ZSWN2 (designed at the IAE)was used. The value of U R = 40 V was used for the F-1 chamber. The monitoring chamber was supplied with the constant voltage of 300 V. The chambers were connected to the electronics by electrometric cables, type T3295 BICC (2 mm in diameter, PTFE insulation covered by graphite), which can be up to 50 m long. The F-1 chamber is well sealed and its sensitivity usually does not change more than 0.5% per year. However, according to a good practice, the device should be re-calibrated periodically. The measuring system was calibrated at the Institute of Atomic Energy in reference radiation fields of 137 Cs, in terms of ambient dose equivalent. The calibration involved determination of the recombination voltage U R. The system is under development with the aim to improve its accuracy and reliability. Recently, a new recombination chamber was designed in order to reduce the volume recombination (see Fig. 2) The cylindrical tissue equivalent chamber is 115 mm in length and 18 mm in diameter. The distance between the electrodes is equal to 0.5 mm. The chamber is enclosed in a 0.3 mm thick aluminium container and can be filled with tissue equivalent gas up to a pressure of several MPa. 5

6 FIG. 2. Recombination chamber designed for the measurements of H*(10) in the vicinity of medical accelerators Results and discussion Three series of measurements were performed in the treatment room of the Varian Clinac 2300C/D at the Oncology Centre in Warsaw, with the accelerator producing 6 MV and 15 MV photons. The photon beam was collimated to the area of cm 2 or 4 4 cm 2 at one meter distance from the target. The measurements were made on the treatment couch, 17 cm from the axis of the beam. The chamber was placed on a standard PMMA phantom and sequentially irradiated with the 15 MV and 6 MV beams. The obtained value of the Q 4 for 6 MV beam was equal to Q 4 = 0.99 ± This result means that no neutrons were detected. Such result was obviously expected for this energy of photons and confirmed the proper performance of the measuring method. The values obtained for the 15 MV beam are summarized in the Table 1. The accuracy of the Q 4 values was lower than in the laboratory conditions, mainly due to a necessary correction for volume recombination, which was not well known for the pulsed radiation field in the treatment room. Neutron dose equivalent and neutron dose were calculated from the eq. (7) and (8) assuming that Q 4n =10. Table I. Values of total and neutron doses and dose equivalents obtained for two series of measurements at the accelerator Varian Clinac 2300C/D. Series of the measurements A B Irradiation field at the isocentre cm cm 2 Dose rate at isocentre 6 Gy min -1 2 Gy min -1 Q 4 1,075±0,032 1,144±0,06 H*(10) ± 10 msv min -1 14,2 ± 0,8 msv min -1 H*(10) per total dose in the 35.4 ± 1.5 msv Gy msv Gy-1 isocentre H*(10) n per total dose in the 2.92 ± 1.3 msv Gy -1 2±0,9 msv Gy-1 isocentre D*(10) n / D*(10) 0.9 ± 0.3 % 1.5 ± 0.7% 6

7 Conclusions The main idea of the present study was to create an automated measuring system with a recombination chamber, for direct determination of the total H*(10), or dose equivalent in certain organs, outside the irradiation field in the vicinity of medical accelerators. It was proved that the measurements could be performed in reasonable time of about 20 minutes and with accuracy of better than 10%. Determination of H*(10) is, in principle, sufficient from the point of view of radiation protection but neutron absorbed dose can be also of interest, for comparison with the internationally recommended limits. At present, the uncertainty of the neutron absorbed dose measured by the system constitutes about 0.5% of the total measured value. Therefore, the results can be used for comparison with the recommendations, if the measured value is far enough from the limits. In other cases more precise measurements should be performed. The significant advantage of using the recombination chamber is the direct reading of the result and relatively short time of the measurements. Another advantage is that in future the chamber can be connected to the electrometers that are already in common use in oncology departments. In principle, no special electronics is needed except interfaces for data acquisition and software for online calculations. Acknowledgements The authors are very grateful to Dr W. Bulski, Head of the Medical Physics Department in Maria Curie Oncology Centre in Warsaw, for enabling access to the accelerator and fruitful discussions. The work was partly supported by the Polish State Committee for Scientific Research, under the project number 4T11E References 1. Gudowska, I., Brahme, A., Neutron Irradiation from High-Energy X-ray Medical Accelerators. Nukleonika. 41(2): , (1996). 2. Kase, K. R., Mao, X. S., Nelson, W. R., Liu, J. C., Kleck, J. H., Elsalim, M., Neutron Fluence and Energy Spectra around the Varian Clinac 2100C/2300C Medical Accelerator. Health Physics, 74(1): 38-44, (1998). 3. Thomas, D.J., Bardell, A.G., Macaulay, E.H., Characterisation of a gold foil-based Bonner sphere set and measurements of neutron spectra at medical accelerator. Nucl. Instr. and Meth. in Phys. Res. A., 476: 31-35, (2002). 4. Bourgois, L., Delacroix D., Ostrowsky A., Use of Bubble Detectors to Measure Neutron Contamination of a Medical Accelerator Photon Beam. Radiat. Prot. Dosim., 74(4): , (1997). 5. International Standard IEC (1998). 6. Golnik, N. Review of Recent Achievements of Recombination Methods. Radiat. Prot. Dosim., 70(1-4): , (1997). 7. Golnik, N., Kaminski, P., Zielczynski, M., A measuring system with recombination chamber for neutron dosimetry around medical accelerators. Radiat. Prot. Dosim., submitted. 8. Zielczyński, M., Golnik, N., Recombination Index of Radiation Quality - Measuring and Applications. Radiat. Prot. Dosim., 52: , (1994). 9. Golnik, N. Recombination Methods in the Dosimetry of Mixed Radiation. Report of the Institute of Atomic Energy, Swierk, Poland, IAE -20/A, ISSN (1996). 10. Golnik, N., Brede H. J. and Guldbakke, S. Response of REM-2 Recombination Chamber to H*(10) of Monoenergetic Neutrons. Radiat. Prot. Dosim. 74(3), (1997). 11. Golnik, N. and Zielczyński, M. The Concept of RIQ and Its Adaptation to Recent Recommendations of ICRP for External Neutron Fields. Nukleonika, 41(2), (1996). 12. Boag, J. W. Ionization chambers. in Radiation Dosimetry, edited by F.M. Attix and W.C. Roesch, (eds.), Acad. Press., Vol II, (1968). 7