CALIBRATION OF A GAMMAMED 12i 192 Ir HIGH DOSE RATE SOURCE

Transcription

1 VI. simpozij HDZZ, Stubičke Toplice HR CALIBRATION OF A GAMMAMED 12i 192 Ir HIGH DOSE RATE SOURCE Tomislav Bokulić, Mirjana Budanec, Iva Mrčela, Ana Frobe and Zvonko Kusić Department of Oncology and Nuclear Medicine, University Hospital "Sestre milosrdnice", Vinogradska c. 29, HR Zagreb, Croatia tbokulic@public.srce.hr INTRODUCTION High dose rate brachytherapy (HDRBT) with remote afterloading Gammamed 12i unit has been used in Department of Oncology and Nuclear Medicine for about three years. Current clinical applications involve intracavitary treatment of the following localisations: cervix, endometrium, bronchus, oesophagus and nasopharinx. The 192 Ir miniature source in the form of a small pellet (1.1 mm outer diameter and 5.6 mm length) is laser welded to a wire that pushes and pulls it through the plastic catheter to the desired location in an applicator. One of essential steps in HDRBT quality assurance and safety programme is calibration of the 192 Ir source. The calibration is usually made with a well-type ionisation chamber or by a thimble type (Farmer) chamber free in air. The 192 Ir calibration coefficient for Farmer type chambers can be obtained by interpolative technique with two beam qualities (250 kvp X rays and 60 Co), recommended by the IAEA [1]. Alternatively, 250 kvp X rays and l37 Cs beam qualities, like in USA laboratories [2], or even 11 beam qualities (nine X rays, 60 Co and l37 Cs) [3] can be utilised in a calibration technique. In this work we present the results of calibration of 11 sources performed with Standard Imaging HDR1000+ well chamber. We determined the axial most sensitive measurement point in the chamber and investigated the effect of the scatter environment on the chamber's output- ionisation current (charge). The calibration results were compared to the measurements in air done by the 0.6 cm 3 Farmer type chamber. However, we used the only available 60 Co calibration coefficient. The conventional electrometer/chamber corrections and nonuniformity (exposure gradient) corrections due to finite dimensions of the chamber were utilised. The influence of room scatter was also estimated. MATERIAL AND METHODS Since the introduction of the HDRBT procedures in department, the calibration of 192 Ir source has been performed by the large volume (V=245 cm 3 ) reentrant well chamber HDR attached to the Excalibur CDX 2000A (Standard 134

2 Imaging) electrometer. The voltage (+300 V) is applied through the triax cable to the collecting electrode, a thin-walled aluminium tube. The moving HDR source in a plastic needle can be positioned in the chamber volume either by means of a plastic spacer of desired length or by programming the correct stopping position in combination with the origin displacement. The well type chamber calibration has always been performed the day after the source change and before any clinical application. Together with the chamber constancy check and calculation in convenient spreadsheet forms, the calibration takes about one hour. The constancy check is done with a calibrated 137 Cs (3.7 GBq) source in an appropriate source holder. Air kerma strength (AKS), or numerically equal reference air kerma rate (K R ), of the source is determined from the following expression: K R =N K -(M u /t)-k T,pk recomb -N elec (1) where iv K is the reference air kerma rate calibration factor of the well type chamber supplied by the accredited laboratory, M u is the scale unit reading (charge collected during time t) and & T, P, &recomb and N e \ec are corrections for the temperature and pressure, recombination losses and the electrometer calibration factor, respectively. The latter coefficient applies if the electrometer was calibrated separately. The calibration point inside the chamber volume corresponds to the source at the position of maximum response. At this point, the uncertainty in the reference air kerma rate determination, due to positional uncertainty, is minimised. The calibration point measurements were performed at different positions of the source along the axis of the chamber by programming the stopping positions with different origin definition. The chamber was also repeatedly positioned at various distances from the wall and from the floor to determine relative increase in ionisation current due to scatter. In free in air measurements, the AKS was determined for a number of sources with 0.6 cm 3 Fanner type chambers (30001, PTW-Freiburg) and UNIDOS electrometer. The catheter and straight metal applicator were attached to a special lightweight plastic holder (Figure 2). The holder has a calibrated ruler that enables precise positioning of the chamber at the desired distance from the source. The K R in this measurement can be calculated at different source to chamber distances from: K R =N K -(M u /t)-k air -k scal <kn-k T, p (d/drej)2 (2) N K is the air kerma calibration factor of the ionisation chamber at the actual photon energy, M a is the measured charge collected during the time t, k air is the correction for attenuation of the primary photons by the air between the source and the chamber, k scan is the correction for scattered radiation from the walls, floor, measurement set-up, air, etc. k n is the non-uniformity correction factor, accounting for the non-uniform electron fluence within the air cavity, k TyP is the correction for 135

3 ambient temperature and pressure. Transit effects during source transfer are excluded with the application of an interval measurement starting when the source is already exposed at the measuring position. The measurement distance d is the distance between the centre of the source and the centre of the ionisation chamber; a^ef is the reference distance of 1 m. RESULTS Figure 1 shows the results of the well chamber calibration of 11 sources, each received at approximately three monthly intervals. The relative deviation of the measured AKS from the manufacturer's certificate specification has always been less than 3.4 %. After the introduction of more strict protocol (chamber position and holder, longer thermal equilibration, own pressure measurement) these deviations were kept below 2%. The position of maximum ionisation current in the chamber is at 56 mm from the chamber's bottom. It is slightly higher compared to the manufacturer's specification (48 mm). The lower row in Figure 1 indicates the necessity to perform measurements at distances of more than 25 cm from the floor or nearest wall in order to have only a negligible increase of collected ionisation charge (current). The well chamber charge measurements were highly reproducible (< 0.1%). The overall measurement uncertainty 2.67% (1SD) is conservatively estimated from manufacturer's specification for the chamber and estimates for correction coefficients. For determination of the AKS from the measured air kerma at the distance d, it is necessary to correct for the attenuation of the primary photons between the source and the ionisation chamber. Factors kair at different distances between the source and the ionisation chamber were interpolated from tables given in [1]. For distances smaller than 40 cm, this correction is less than 0.5%. The room scatter was determined under the assumption that it is a constant contributor at all measurement distances, independent of the source to chamber distance. Equal time measurements were done at distances of 10, 15, 20, 25 and 30 cm. Regression analysis of measured primary and room scatter versus {do/df gave constant room scatter and primary exposure at d 0. The room scatter fraction was approximately 0.58% at 20 cm source to chamber distance. We also applied Kondo and Randolph [4] nonuniformity kn corrections. Similar results can be obtained with Dove results [5] (Table 1). Large discrepancies between measured and specified AKS, seen on the graph (Figure 2, right), indicate the problem of nonuniformity correction and positional uncertainty at near source position, and weak signal as well as relatively larger scatter influence at farther source positions. 136

4 Figure 1. Top row: Relative deviation of the measured AKS and the manufacturer's certificate for 11 sources on departmental calibration (left). Axial response of HDR1000+ well chamber measured from the applicator's end (right). Bottom row: Relative response (ionisation current) of the well chamber versus the distance of the chamber from the wall (Limits of 0.1% differences are indicated) (left). Relative response for different heights (right). (%dev=100*(aks(exp)- AKS(m))/AKS(m); exp=measured; m=manufacturer spec.) 137

5 D 10 i. 20. source Ir-192 ionisation chamber Figure 2. A sketch of geometry in Farmer type chamber measurement (left). An example of results obtained at various distances after the application of correction factors according to expression (2) (right). Table 1. Nonuniformity correction factors. d(m) Kondo[41 Dovef The estimated uncertainty in a calibration of a T Ir using the simultaneous measurement at seven distances is according to literature [1], typically 1.5% (1SD). Our estimate obtained by repeated measurement and estimated uncertainties for correction coefficients gave 1.48 % (1SD) at 15 cm source to chamber distance. CONCLUSION 192, Due to the ease of implementation and speed, a routine iyz Ir source calibration should be preferably done with a suitable well type chamber similar to the one described in this article. The largest discrepancy of the measured AKS and manufacturer's specification was 3.4%, becoming less than 2% with more stringent measurement protocol. The calibration with a Farmer type chamber can be properly implemented only with appropriate calibration coefficients at two beam qualities ( 60 Co or ul Cs, and 250 kvp X rays). Otherwise, it could be considered only as a redundant check, in addition to the proper well chamber calibration. Regular and adequate chamber calibration should be provided by accredited laboratory to all institutions performing HDRBT. 138

6 REFERENCES [1] International Atomic Energy Agency (IAEA). Calibration of photon and beta ray sources used in brachytherapy, IAEA-TECDOC-1274, Vienna: IAEA; [2] Goetsch SJ, Attix FH, Pearson DW, Thomadsen BR. Calibration of l92 Ir high-doserate afterloading systems, Med Phys 1991; 18:462^167. [3] Buermann L, Kramer HM, Schrader H, Selbach HJ. Activity determination of 192 Ir solid sources by ionisation chamber measurements using calculated corrections for self-absorption. Nucl Instr Meth Phys Res A 1994;339: [4] Kondo VS, Randolph ML. Effect of finite size of ionisation chambers on measurements of small photon sources. Rad Res 1960;13: [5] Dove DB. Effect of dosemeter size on measurements close to a radioactive source, BrJRadiol 1959;62: ABSTRACT High-dose-rate (HDR) brachytherapy has been used in our department for cancer treatment for about three years. The HDR 192 Ir source is usually calibrated using a well-type ionisation chamber or a thimble chamber free in air. This paper presents the calibration of 11 sources using Standard Imaging HDR1000 well chamber. We investigated the effect of location of the chamber in the room and scatter environment on output ionisation current and determined the most sensitive measurement point in the chamber. The largest discrepancy between the measured air kerma rate (AKR) and manufacturer's specification was 3.4%.These results were compared with calibration in the air done using the 0.6 ccm Farmer-type chamber, attached to a special lightweight holder. Instead of interpolative techniques recommended by either the IAEA or authorised laboratories, we used a calibration coefficient for 60 Co quality only. Beside the conventional electrometer/chamber corrections, the exposure gradient corrections due to the finite dimensions of the chamber were employed. The influence of room scatter was also estimated. Preliminary results of calibration in air indicated that well chamber calibration and Farmer chamber calibration with 60 Co calibration factor were within 2%. 139