an overall stem effect, including stem leakage and stem scatter, for a tm30013 farmer-type chamber

Journal of the Korean Physical Society, Vol. 58, No. 6, June 2011, pp. 1688∼1696

An Overall Stem Effect, including Stem Leakage and Stem Scatter, for aTM30013 Farmer-type Chamber

Dae Cheol Kweon

Department of Radiologic Science, Shin Heung College University, Uijeongbu 480-701, Korea

Jae-Seung Lee∗

Department of Radiation Oncology, Good Samaritan Hospital, Pohang 791-704, Korea andDepartment of Physics, Soonchunhyang University, Asan 336-745, Korea

Eun-Hoe Goo

Department of Radiology, Seoul National University Hospital, Seoul 110-744, Korea andDepartment of Physics, Soonchunhyang University, Asan 336-745, Korea

Moon-Jib Kim

Department of Physics, Soonchunhyang University, Asan 336-745, Korea

Jae-Eun Jung

Department of Biomedical Engineering, Sahm Yook Seoul Medical Center, Seoul 130-711, Korea

Kyung-Rae Dong

Department of Radiological Technology, Gwangju Health College University, Gwangju 501-701, Korea andDepartment of Nuclear Engineering, Chosun University, Gwangju 501-759, Korea

Woon-Kwan Chung

Department of Nuclear Engineering, Chosun University, Gwangju 501-759, Korea

In-Chul Im and Yun-Sik Yu

Department of Radiological Science, Dongeui University, Busan 614-714, Korea

(Received 21 March 2011, in final form 9 May 2011)

The stem effect is a leakage current generated when the chamber stem is included in the radiationfield size. Such an effect can be divided into stem leakage and stem scatter. When a chamber iscalibrated in air, the chamber response is likely to be affected by the photons scattered from thechamber stem. These interactions contribute to the apparent measured exposure. We calculated theoverall stem effect correction factor that was caused by the metal stem of the ionization chamber.We measured the stem effect of a Farmer-type ionization chamber that had recently been in use forexposure dose measurements. In addition, we calculated and compared the ratios of stem leakageand stem scatter to the overall stem effect. We measured an overall stem effect, including thestem leakage and the scatter of PTW model TM 30013 (vented to air, sensitive volume 0.6 cm3)Farmer chamber, in the exposure measurement. We measured the dependences of the stem scatter(ksem.scatter) and the stem leakage (ksem.leak) on the length of chamber stem exposure when measuringthe exposure dose of high-energy X-rays generated by a linear accelerator (LINAC). Electronsejected from the metal stem were collected by the central electrode, increased to a maximum andthen decreased. Most of the overall stem effect was caused by stem scatter and was determined towithin 4% according to the length of the stem exposed in repeated measurements of with variousradiation fields.

PACS numbers: 87.53.-j, 87.58.Sp, 87.59.BhKeywords: Stem leakage, Stem scatter, Stem effect, Exposure dose, Ionization chamber, Exposure measure-mentDOI: 10.3938/jkps.58.1688

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An Overall Stem Effect, including Stem Leakage and Stem Scatter · · · – Dae Cheol Kweon et al. -1689-

Table 1. Physical characteristics of the Farmer-type ionization chamber used in this study.

Dimension of sensitive volume Wall thicknessStem materials

Chamber (Purity of Al, %) Length of stem

modela) Radius Length Volumne PMMA Graphite Chamber Dummy (mm)b)

(mm) (mm) (cm3) (g/cm3) (g/cm3) stem stem

TM30013 3.05 26.0 0.600 1.19 1.85 99.98 99.28 107.7

a)Vented to air, waterproof, fully-guarded chamber.b)This, except for the external diameter of the sensitive volume, is defined as the total ionization chamber length.

I. INTRODUCTION

The ionization chamber was commercialized for thepurpose of measuring high-energy radiation doses in ra-diation therapy. Such ionization chambers have recentlybeen designed to improve spatial resolution by reduc-ing the energy dependency and the sensitive volumeand to ensure both convenience in the structure andthe handling of the chamber and stability in the electri-cal characteristics [1,2]. The International Commissionon Radiation Units and Measurements (ICRU) has rec-ommended an overall accuracy in tumor dose deliveryof around ±5% [3,4], and the American Association ofPhysicists in Medicine (AAPM) has reported that theoverall uncertainty of the prescription dose delivered toa certain reference point is approximately 5.6% [5]. Theanalysis shows that an ionization chamber suitable forcalibrating radiation therapy beams and provided witha 60Co exposure calibration factor from the AccreditedDose Calibration Laboratory (ADCL) has a cumulativeuncertainty of approximately 1.6% [5,6]. The cumula-tive uncertainty of an ionization chamber includes theion recombination loss, the spatial resolution, the polar-ity effect, and the stem effect [7–10].

The generally referred to stem effect can cause elec-tron scattering in the chamber stem, which results in adifferent type of leakage current [11,12]. The stem effectcan be divided into stem leakage and stem scatter. Stemleakage arises as a consequence of direct irradiation ofthe chamber volume, as well as the insulators and thecables in the chamber. Stem scatter arises from the ef-fect of scattered radiation in the stem that reaches thesensitive volume [11]. These interactions contribute tothe apparent measured exposure [13].

Typically, the calibration of the ionization chamber isperformed with a fixed field that may cover the entirestem or only a small portion of the stem in a geomet-rically defined field border. However, the clinical mea-surement is conducted with the chamber stem includedin a range that is different from the range for calibrationand with a field that is different from the one used for

∗E-mail: [email protected]; Fax: +82-54-245-6529

the calibration [14]. In this case, due to radiation inter-action, electrons ejected from air near the chamber end,from the dielectric in the metal stem or from the cablecan reach the central electrode, generating the stem ef-fect, which reduces charges [14–16]. Such a stem effectdepends on the length of the chamber stem in the fieldborder. Thus, a correction will be necessary wheneverthe length of the exposed stem differs from that at thetime of the chamber calibration [15].

Therefore, this study intends to calculate the stem ef-fect correction factor (Psem). This factor is the result,which is shown in different forms, of a leakage currentthat is frequently inevitable in the measurement of ex-posure doses of high-energy photon beams. We mea-sured the dependences of stem leakage (ksem.leak) andthe stem scatter (ksem.scatter) on the length of the cham-ber stem exposure when measuring the exposure dose ofhigh-energy X-rays generated by using a linear acceler-ator (LINAC), and we calculated the overall stem effectcorrection factor that was caused by the metal stem ofthe ionization chamber. In addition, we calculated andcompared the ratios of the stem leakage and the stemscatter to the overall stem effect.

II. MATERIALS AND METHODS

1. Measuring Equipment

In order to measure the stem effect, we utilized aFarmer-type ionization chamber (TM 30013, PTW, Ger-many) that had a sensitive volume of 0.6 cm3, which iscommonly used for the measurement of exposure dose.Table 1 shows the detailed specifications of the ionizationchamber used for the measurement. Acrylic build-upcaps are used with ionization chambers for in air mea-surements in high-energy photon beams when chargedparticle equilibrium (CPE) is desired. In this case, theused build-up caps were 1.5 and 2.5 cm for the 6- andthe 10-MV X-ray beams, respectively.

The electrometer (UNIDOS, PTW, Germany) has anoperative voltage of 400 V with a calibration factor(Pelec) of 1.00 ± 0.5%. According to the international

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Fig. 1. (Color online) PTW model TM 30013 (ventedsensitive volume of 0.6 cm3) Farmer-type ionization cham-ber used in the measurement, dummy stem manufactured tomeasure the stem scatter, and structural drawing. The ma-terials for the chamber stem and the dummy stem are 99.98percent and 99.28 percent pure aluminums. The dimensionsare given in mm.

calibration protocol [17,18], the ionization chambers, theelectrometer, and the cables were calibrated to have un-certainties of less than ±1% by using second standardcalibration laboratories.

Figure 1 shows the PTW model TM 30013 ionizationchamber (vented sensitive volume of 0.6 cm3) and thedummy stem for measurement of stem scatter in thisstudy. A dummy stem was used for the computer nu-merical control (CNC) automatic lathe machine (MS26,CHRONOMICS, Switzerland) to manufacture an alu-minum stick with a diameter of 20 mm by precision ma-chining. The allowable error in the manufacturing pro-cess was less than ±0.1 mm while the material for thedummy stem was aluminum with a purity of 99.28%.

2. Measurement Methods

Figure 2 shows the geometric arrangement used formeasuring the exposure dose with an ionization cham-ber. The distance from the radiation source to the centeraxis of the sensitive volume was set at 100 cm. High-density styrofoam was placed and kept at a distance of20 cm from the couch to make sure that no other ma-terials that scattered radiation, excluding air, existed inthe vicinity of the ionization chamber. To ensure theaccuracy of measurement, we repeated the measurementof the exposure dose in 100 MU (monitor unit), which isthe mechanical dose unit of linear accelerator, 10 timesfor each field before calculating the average, standarddeviation and uncertainty type A of the repeated mea-surements.

3. Parameters Measured in this Study

A. Dependency of chamber orientation

The calibration of the stem leakage was done by turn-ing around the ionization chamber or the collimator. In

Fig. 2. Geometric arrangement used for measuring theexposure dose with an ionization chamber.

the calibration after the measurement of the radiationdose by using the calibrated ionization chamber, if themeasurement had been conducted in a different direc-tion from the exposure direction, the sensitivity of theionization chamber differed according to the direction ofthe radiation. In this case, the effect of the orientationof the ionization chamber should be measured [19,20].The direction dependency of the ionization chamber isdefined as the ratio of the response function of the ion-ization chamber direction, which is rotated by 90(, to thecalibrated direction of the ionization chamber when thefield size is 10 × 10 cm2.

B. Dependency of chamber materials

The calibration of the stem scatter is done by puttingthe dummy stem, which had the same physical charac-teristics as those of the chamber stem, in the oppositiondirection to the sensitive volume. In this study, we usedaluminum with a purity of 99.98% as the chamber stemand with a purity of 99.28% as the dummy stem (Ta-ble 1). Therefore, in order to measure the effect of thestem material, we manufactured aluminum plates thathad aluminum purities of 99.98% and 99.28% and size of150 × 150 × 1 mm3. We propped up the manufacturedaluminum plates with high-density Styrofoam to placethem on the ionization chamber and conducted the mea-surement at a field size of 10 × 10 cm2. The dependencyon the stem material was defined as the ratio of the re-sponse function of the aluminum plate with a purity of99.28% to that of the aluminum plate with a purity of99.98%.

4. Measurement of the Stem Leakage and Scat-ter

A. Measurement of the stem leakage

Figure 3(a) shows the stem leakage measurementmethod used in the experiment. Since dose decreases


rapidly at the edge of beam, it is recommended to includeapproximately 10% of the stem for measurement of thehigh-energy X-rays emitted by a linear accelerator [14].Therefore, we set the length of the Y direction (Yjaw) ataround 5 cm, and we increased the size of the rectangu-lar field (Xjaw × Yjaw) by 2 cm from 5 × 5 cm2 to 33 ×5 cm2 to ensure that the stem exposure increased by 1cm. The first measurement measured ionization currentsthat did not include the chamber stem in position (1) ofFig. 3(a). The values from the first measurement pro-vided fundamental responses to the sensitive volume ofthe ionization chamber. The second measurement mea-sured the dependence of the stem effect on the length ofthe stem exposed as the field of the X direction (Xjaw)was increased at a constant interval at position (2) ofFig. 3(a) and the ionization chamber was rotated by 90degrees. The values from the second measurement pro-vided the contributions to the stem effect for various thelengths of the stem exposure.

B. Measurement of the stem scatter

Figure 3(b) shows the stem scatter measurementmethod used in the experiment. Ma and Nahum [21] con-ducted a study on the stem effect correction factor fora medium-energy X-ray beam by using a Monte Carlocalculation. They reported that the stem scatter wasthe highest when the dummy stem was placed in a direc-tion opposite to the sensitive volume [21,22]. In order tomeasure the stem scatter of the ionization chamber, weincreased the size of the square field (Xjaw×Yjaw) by 2 ×2 cm2 from 5 × 5 cm2 to 33 × 33 cm2 to make sure thatthe stem exposure increased by 1 cm. We conducted thefirst measurement after placing the dummy stem in a di-rection opposite to the sensitive volume of the ionizationchamber and the second measurement after removing thedummy stem.

5. Analysis of the Overall Stem Effect Correc-tion Factor

A. Analysis of the stem leakage

In the stem leakage correction factor (kstem.leak), if thevalue measured by the electrometer in position (1) ofFig. 3(a) is the ionization current Istemless that does notcontain the chamber stem and if the values measuredby the electrometer in position (2) of Fig. 3(a) are theionization currents Istem for various the lengths of thestem exposed, the following can be defined:

Istem − Istemless: Ionization currents due to electronsejected from the chamber stem, Istem−(Istem−Istemless):Ionization currents due to collective electrons from thesensitive volume where the chamber stem is excluded.

Fig. 3. (Color online) Geometry of the stem leakage andthe stem scatter determination. (a) The measurements of thestem leakage were made with the chamber oriented in eachof two positions as shown in the figure. (b) The influence ofthe stem was determined experimentally by using a dummystem placed on top of the ionization chamber in a positionopposite the original stem.

Therefore, the stem leakage correction factor (kstem.leak)can be determined as follows:

kstem.leak =Istem − (Istem − Istemless)

Istem=

Istemless

Istem. (1)

B. Analysis of the stem scatter

It is ideal to collect only the electric charges that aregenerated in the sensitive volume of the ionization cham-ber. However, when photons are beamed to the ioniza-tion chamber, the stem linked to the sensitive volumeis exposed while the ionization current increases due tophotons scattered in the stem. When the chamber stemcontributes to scattered rays, it is called stem scatter.Therefore, when Ids is the ionization current measuredafter arranging the dummy stem in a direction oppositeto the ionization chamber and I is the ionization currentmeasured after removing the dummy stem, the ionizationcurrents can be defined as follows:

Ids−I: ionization current caused by the chamber stem,and I−(Ids−I): ionization current generated in the sen-sitive volume, excluding the chamber stem. Therefore,


the stem scatter correction factor (kstem.scatter) can bedetermined as follows:

kstem.scatter =I − (Ids − I)

I=

2I − IdsI

. (2)

C. Analysis of the stem effect correction factors (Pstem)

The stem effect is the leakage current generated whenthe chamber stem is included in the radiation field size.Such an effect can be divided into stem leakage and stemscatter [11]. Therefore, the overall stem effect in the mea-surement of a high-energy photon beam by using an ion-ization chamber is equal to the stem leakage multipliedby the stem scatter. When the stem leakage was andkstem.leak the stem scatter was kstem.scatter in the mea-surement above, the overall stem effect correction factor(Pstem) was determined as below:

Pstem = kstem.leak × kstem.scatter. (3)

In addition, when the ratios of the stem leakage and thestem scatter to the overall stem effect were kstem.leak andkstem.scatter, respectively, such ratios were determined asfollows:

Rstem.leak =kstem.leak

Pstem× 100(%) (4)

kstem.scatter =kstem.scatter

Pstem× 100(%). (5)

III. RESULTS

We used the PTW model TM 30013 (vented sensitivevolume of 0.6 cm3) Farmer-type chamber for high-energyX-rays emitted from a LINAC with a view to calculatingthe dependence of the overall stem effect correction fac-tor of the ionization chamber on the length of the stemexposed in the exposure measurement. The results areas follows:

1. Effect of Chamber Orientation and Stem Ma-terials

Table 2 shows the dependency that the ionizationchamber has on measurement direction in the stem leak-age measurement and the dependency that the chamberstem and the dummy stem have on material in the stemscatter measurement. When the field size was 10 × 10cm2, we conducted the measurement of a 100-MU expo-sure dose, repeating it ten times, in the calibrated direc-tion of the ionization chamber and in the exposure di-rection with the ionization chamber was rotated by 90◦.The measurement result was less than 0.02%, which was

Table 2. Chamber orientation and stem material depen-dencies, with their estimated relative uncertainties, for theTM 30013 ionization chamber measured for 6- and 10-MVX-rays.

ValuesTM30013

6 MV 10 MV

Orientation dependency (%) 0.0101 0.0179

100× relative uncertaintya) 0.0244 0.0207

Stem material dependency (%) 0.0271 0.0397

100× relative uncertainty 0.0248 0.0298

a) Represents the relative standard uncertainty estimated byusing statistical methods, type A.

very small enough to state that the direction dependencyof the ionization chamber could be ignored in the stemleakage measurement. In addition, when we used alu-minum plates with purities of 99.98% and 99.28% andsizes of 150 × 150 × 1 mm3, the dependency that thechamber stem and the dummy stem had on material wasless than 0.04% under all types of experiment conditions.This means that the dependency on the quality of thestem material was small enough to be disregarded in thestem scatter measurement.

2. Stem Leakage of Ionization Chamber

Table 3 shows the stem leakage correction factors(kstem.leak) versus the length of stem exposed to 6-MVand 10-MV X-rays for a PTW model TM 30013 Farmer-type chamber. We calculated the uncertainty for thestem leakage correction factor (kstem.leak) and the re-peated measurement based on the ionization currentsthat were measured in position (1) and position (2) ofFig. 3(a) by using the method of the recommended stemleakage measurement and Eq. (1). The exposure dose inthe small field (the length of the stem exposure was 2 cmor less) that included only the sensitive volume of the ion-ization chamber was the result of the primary radiationbeam that penetrated through the air, in effect, withoutany interaction. In this case, the stem leakage correc-tion factor (kstem.leak) was less than 0.04%, which wassmall enough to be ignored. However, with increasing inthe length of the stem exposure, the stem leakage correc-tion factor (kstem.leak) increased gradually and reached amaximum value in the middle part of the chamber stem(the length of the stem exposure was 5 cm or less) be-fore decreasing on an irregular basis. The measurementresults showed that when 6-MV X-rays were used, 0.04∼ 0.33% of the electrons emitted from the metal stemreached the center electrode, making a contribution toexposure dose. When 10-MV X-rays were used, 0.03 ∼0.32% of such electrons reached the center electrode. The


Table 3. Stem leakage correction factors (kstem.leak) de-termined for various lengths of the stem exposed to 6- and10-MV X-ray in the PTW model TM30013 (vented sensitivevolume of 0.6 cm3) Farmer-type ionization chamber.

Length of stem6 MV 10 MV

exposed (cm) kstem.leaka) Relativeb)

kstem.leakRelative

uncertainty uncertainty

1 1.0004 0.0335 1.0003 0.0269

2 0.9998 0.0306 0.9997 0.0371

3 0.9979 0.0300 0.9982 0.0291

4 0.9965 0.0221 0.9964 0.0267

5 0.9967 0.0327 0.9968 0.0214

6 0.9980 0.0314 0.9980 0.0482

7 0.9991 0.0291 0.9989 0.0277

8 0.9995 0.0335 0.9992 0.0249

9 0.9989 0.0258 0.9989 0.0277

10 0.9995 0.0298 0.9992 0.0258

11 0.9991 0.0277 0.9989 0.0359

12 0.9993 0.0249 0.9995 0.0249

13 0.9997 0.0233 0.9995 0.0291

14 0.9993 0.0180 0.9992 0.0348

15 0.9994 0.0340 0.9993 0.0298

a)The stem leakage correction factors.b)Expressed as 100× relative uncertainty. Represents therelative standard uncertainty estimated by using statisticalmethods, type A.

average uncertainty was 2.55 × 10−2% for the measure-ment repeated for ten times with the same measurementmethod.

3. Stem Scatter of Ionization Chamber

In a square field where the length of the stem expo-sure was increased by 1 cm, we measured the ionizationcurrent after placing the dummy stem in a direction op-posite to the sensitive volume of the ionization chamberand to the ionization current after removing the dummystem. Based on measured values of the ionization currentand Eq. (2), we calculated the uncertainty for the stemscatter correction factor (kstem.scatter) and the repeatedmeasurements. Table 4 show the relation of the stemscatter correction factor (kstem.scatter) to the length ofstem exposure when 6-MV and 10-MV X-rays were usedfor the Farmer-type chamber of TM 30013 in a PTWmodel. Unlike the case with stem leakage, as soon as thebeam reached the chamber stem, 2.48% and 2.88% ofthe stem scatter took place in the metal stem for 6- and10-MV X-rays, respectively. The stem scatter increasedgradually with increasing length of stem exposure beforereaching maximum values of 3.36% for 6-MV X-rays and

Table 4. Stem scatter correction factors (kstem.scatter) de-termined for various lengths of the stem exposed to 6- and10-MV X-ray in the PTW model TM30013 (vented sensitivevolume of 0.6 cm3) Farmer-type ionization chamber.

Length of6 MV 10 MV

stem

exposedkstem.scatter

a) Relativeb)

kstem.scatterRelative

(cm) uncertainty uncertainty

1 0.9752 0.0260 0.9712 0.0340

2 0.9734 0.0394 0.9689 0.0300

3 0.9724 0.0298 0.9684 0.0267

4 0.9695 0.0267 0.9660 0.0394

5 0.9684 0.0327 0.9655 0.0260

6 0.9664 0.0327 0.9630 0.0260

7 0.9680 0.0298 0.9637 0.0340

8 0.9688 0.0340 0.9662 0.0416

9 0.9690 0.0307 0.9677 0.0333

10 0.9689 0.0153 0.9684 0.0277

11 0.9696 0.0300 0.9690 0.0267

12 0.9723 0.0300 0.9690 0.0267

13 0.9723 0.0300 0.9719 0.0249

14 0.9739 0.0300 0.9737 0.0277

15 0.9742 0.0269 0.9736 0.0359

a)The stem scatters correction factors.b)Expressed as 100× relative uncertainty. Represents therelative standard uncertainty estimated by using statisticalmethods, type A.

3.70% for the 10-MV X-rays in the middle part of thechamber stem (the length of stem exposure was 6 cmor less). The results showed a significant increase when10-MV X-rays were used. The stem scatter decreasedirregularly after the middle part of the metal stem ofthe ionization chamber (the length of the stem exposurewas 7 cm or higher). After the end part of the stem,the ratios, at which the stem scatter contributed to theexposure dose, were almost the same for 6-MV and 10-MV X-rays. The average uncertainty was 3.24 × 10−2%for the measurement repeated ten times with the samemeasurement method.

4. Overall Stem Effect of Ionization Chamber

In order to investigate the stem effect of the ionizationchamber (the sensitive volume is 0.6 cm3) that is fre-quently used in the measurement of the exposure dose,we measured the stem leakage and the stem scatter andused Eq. (3) to calculate the overall stem effect correctionfactor (Pstem). In addition, we used Eqs. (4) and (5) tocalculate the ratios at which the stem leakage (Rstem.leak)and the stem scatter (Rstem.scatter) contributed to the


Fig. 4. (Color online) Correction factors of the overall stemeffect and the stem scatter versus the lengths of the exposedstem compare for 6- and 10-MV X-rays in the PTW modelTM 30013 chamber.

Fig. 5. (Color online) Ratios of the stem leakage and thestem scatter to the overall stem effect. The stem scattercontributed around 95% to the overall stem effect in the partsthat excluded the middle of the chamber stem.

overall stem effect. Figure 4 shows the overall stem ef-fect correction factor (Pstem), which includes both thestem leakage and the stem scatter that may take placein the metal stem of the ionization chamber. The re-sult shows that the stem effect contributed 2.44 ∼ 3.56%and 2.85 ∼ 3.89% to exposure dose in all regions of themetal stem for 6- and 10-MV X-rays, respectively. Inparticular, the stem effect contributed to the exposuredose at the maximum field size (which included the cal-ibrated field size of the ionization chamber at 10 × 10cm2) mostly used in clinical trials. Figure 5 shows the ra-tios at which the stem leakage (Rstem.leak) and the stemscatter (Rstem.scatter) contributed to the overall stem ef-fect. Excluding the middle part of the chamber stem (thelength of the stem exposure is 4 ∼ 6 cm), the stem effectcaused by the stem leakage was approximately less than5%, which is very small, and around 95% of the stemeffect took place in stem scatter. Therefore, as shown in

Fig. 5, the stem effect has a correction factor value thatis similar to that of stem scatter.

IV. DISCUSSION

In radiation therapy where high-energy photons areused, precise measurement of the dose from a radiationsource and a evaluation of the measurement error arecritical to the delivery of a precise prescription dose tothe tumor, for an evaluation of the tolerance dose fornormal tissue and for quality assurance (QA) of equip-ment [23,24]. In general, the ionization chamber has beenknown to show the best characteristics for measuring ra-diation dose [25]. However, since the ionization chamberis complicated and is influenced by the measurement en-vironment, it has diverse correction factors [7–10]. Whenmeasurements are conducted in a geometric field that isdifferent from the one used for ionization chamber cali-bration, stem calibration is recommended because of thestem effect, in which electrons that are emitted fromthe metal stem and insulator, depending on the lengthof the ionization chamber stem, reach the central elec-trode to reduce the electric charge [14,15]. Nevertheless,it is impossible to get a calibration for each measure-ment and the Accredited Dose Calibration Laboratory(ADCL) does not implement the calibration separately.As research on the stem effect, most studies that havebeen conducted thus far have focused on stem leakage forphotons in the low-energy region (1 MeV or less). Therehas not been any study on stem scatter nor on the overallstem effect.

Consequently, we sought to calculate the overall stemeffect correction factor (Pstem) that included the stemleakage and the stem scatter that might take place inthe chamber stem when measuring the exposure dose forhigh-energy photons, which are used in radiation ther-apy. We measured the ionization chamber’s dependencyon measurement direction and material of the dummystem, both of which could play a role as other parame-ters in measuring the overall stem effect. The values ofthe dependency were less than 0.04%, which was smallenough to be disregarded. Therefore, it was not neces-sary to consider such influential factors in the measure-ment results.

According to the measurement results for the stemleakage, the exposure dose for a field size that includedthe sensitive volume of the ionization chamber and a partof the metal stem was the result of the primary beam,which penetrated through the air without any interac-tion. The contribution of the stem leakage to the ex-posure dose was less than 0.04%, which was very small.However, as the length of the stem exposure increased,the stem leakage increased constantly and reached amaximum value of 0.34% in the middle part of the cham-ber stem (the length of the stem exposure is 4 ∼ 5 cm)before gradually decreasing on an irregular basis after-


ward. Such stem leakage showed similar correction fac-tors for 6- and 10-MV X-rays without any dependencyon the photon energy (Table 3).

Furthermore, the measurement result for stem scattershowed that unlike the case with the stem leakage, assoon as the beam reached the chamber stem, the scat-tered photons generated in the metal stem made a con-tribution to the exposure dose. As the length of the stemexposure increased, the stem scatter increased graduallyand reached maximum values of 3.36% for 6 MV X-raysand 3.70% for 10 MV X-rays in the middle part of thechamber stem (the length of the stem exposure is 6 cm)before decreasing on an irregular basis afterward. Suchstem scatter increased with increasing photon energy inthe chamber stem and showed similar correction factorsregardless of the photon energy after the end part of thestem (Table 4).

Based on the results mentioned above, it can be saidthat the stem effect, including stem leakage and the stemscatter, was attributable to (1) the ions within the stemthat could be measured and (2) the ions that were gener-ated between the end part of the stem and the cable. Theelectrons emitted from the air in the space from the mid-dle part of the stem to the end part of the stem have anincreasing probability of ion recombination as they movefarther away from the collection electric field, which hin-ders a normal measurement. Therefore, the stem effectdecreased gradually after the middle part of the cham-ber stem. However, as the beam reached the metal stem,the electrons emitted from the metal stem arrived at thecenter electrode, making a clear contribution to the ex-posure dose. Such a stem effect was found to increaselinearly up to the middle part of the metal stem. Inconclusion, the Farmer-type ionization chamber that isfrequently used to measure the exposure dose, with asensitive volume of 0.6 cm3, showed a stem effect of lessthan around 4% that increased linearly up to the middlepart of the metal stem, but afterward decreased linearlybecause the distance from the collection electric field be-came large, even though the length of the stem exposureincreased, which eventually increased the recombinationof ions. In such a region of ion recombination, the stemeffect showed similar correction factors regardless of thephoton energy because the effect had nothing to do withthe number of the electrons emitted from the metal stem.

In this study on the stem effect, we suggest two points.First, when the ionization chamber has a small sensitivevolume and the a metal stem with a operating voltage of400 V (for example, the ionization chamber has a sensi-tive volume of 0.125 cm3 with a stem length of 4.25 cm),the ion recombination region will decrease, and the num-ber of the electrons collected in the center electrode willincrease because the distance between the center elec-trode and the chamber stem decreases. Therefore, thestem effect will increase linearly throughout the metalstem, which requires consideration of more correctionfactors to the stem effect. Second, it should be notedthat the stem effect makes it, maximum contribution to

exposure dose when such an effect is generated from themetal stem of the ionization chamber at the field size (in-cluding the calibrated field size of the ionization chamberat 10 × 10 cm2) that is mostly used in the clinical trial.Figure 5 shows the ratios of the stem leakage and thestem scatter to the overall stem effect. In all regions ofthe chamber stem, excluding the middle part the ratio ofthe stem leakage to the overall stem effect was found tobe less than around 5%, which is very small, and most ofthe stem effect was attributable to stem scatter (Table5). As a result, we expect to find in this study that thestem scatter, when a dummy stem is used, will be thestem effect of the ionization chamber because the stemleakage emitted from the chamber stem contributes verylittle to the overall stem effect of the Farmer-type ion-ization chamber that is used for exposure dose measure-ments with a sensitive volume of 0.6 cm3 and becausemost of the stem effect is attributable to stem scatter.

V. CONCLUSION

We measured the stem effect of a Farmer-type ioniza-tion chamber that had recently been in use for exposuredose measurements and present some conclusions basedon the measurement results. We repeated the measure-ments for five weeks in the same method to observe thestem effect for various changes in the measurement envi-ronment. The measurement results showed that the stemeffect increased slightly with decreasing temperature orincreasing pressure. Since the stem effect correction fac-tor showed a slight change, not absolutely but relatively,according to the given measurement environment, an in-dividual measurement of the stem effect of ionizationchambers to be applied in clinical trials in the division ofradiation oncology, are required. Furthermore, radiationtherapy focuses on verification of prescription dose ata certain depth of tissue rather than on measurement ofthe exposure dose, so that the absorbed dose is measuredin most of the cases. Also there exist various absorberswith the different depths for measurement. For thesereasons, we believe that a study should be conducted ona new stem correction method for the absorbed dose.

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an overall stem effect, including stem leakage and stem scatter, for a tm30013 farmer-type chamber

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