[Chinese Journal of Cancer 28:3, 272-276; March 2009]; ©2009 Landes Bioscience
Methods and Technology
Measurements and comparisons for data of small beams of linear accelerators Li Chen,1 Li-Xin Chen,1,* Hong-Qiang Sun,1 Shao-Min Huang,1 Wen-Zhao Sun1, Xing-Wang Gao2 and Xiao-Wu Deng1 1State Key Laboratory of Oncology in South China; Guangzhou, Guangdong P.R. China; and Department of Radiation Oncology; Cancer Center; Sun Yat-sen University; Guangzhou, Guangdong P.R. China; 2Department of Radiation Oncology; No. 458 Hospital of People’s Liberation Army; Guangzhou, Guangdong P.R. China
Key words: ionization chamber, total scatter factor, collimator scatter factor, tissue-maximum ratio, small field Background and Objective: Accurate data acquisition is very important to establish a reliable dose calculation model of the treatment planning system for small radiation fields in intensity modulated radiation therapy (IMRT) and stereotactic radiotherapy (SRT). This study was to analyze and compare small-field measurements using different methods and ionization chambers. Methods: Three types of farmer chambers were used, with active volumes of 0.65 cc, 0.13 cc and 0.01cc respectively. The beam data, including the total scatter factor (Scp), collimator scatter factor (Sc), tissue-maximum ratio (TMR), were acquired in a 30 × 30 × 30 cm3 water phantom under two linear accelerators. Measurements were performed at accelerating potentials of 4, 6, and 8 MV with the beam size ranging from 1 cm × 1cm to 10 cm × 10 cm. The measurements were analyzed and compared. Results: For the beam size of ≥ 3 cm × 3 cm, the differences in Scp and Sc measurements of the 0.65 cc, 0.13 cc and 0.01 cc ion chambers were within 0.8%, while the differences were much greater for the beam size of less than 3 cm × 3 cm (the maximum difference reached 64%). Using 4, 6 and 8 MV x-rays, Sc measured by the 0.13cc chamber with an elongated source-to-surface distance (SSD) ( > 150cm) were 25.4%, 6.9%, 24.6%, and 1.4%, 1.4%, 2.2% greater than those measured by a standard SSD (100 cm) for 1 cm × 1 cm and 2 cm × 2 cm beams respectively; although there was no significant difference in Sc measurements for the beams of ≥ 2 cm × 2 cm using the elongated SSD of the 0.13 cc and the 0.01 cc ion chambers, Sc measured by the 0.13 cc ion chamber were 0.2%, 8.5%, 3.4% less than those measured by the *Correspondence to: Li-Xin Chen; State Key Laboratory of Oncology in South China; Guangzhou, Guangdong 510060 P.R. China; and Department of Radiation Oncology; Cancer Center; Sun Yat-sen University; Guangzhou, Guangdong 510060 P.R. China; Tel.:86.20.87343089; Fax: 86.20.87343394; Email:
[email protected] Submitted: 05/26/08; Revised: 10/08/08; Accepted: 12/25/08 This paper was translated into English from its original publication in Chinese. Translated by: Beijing Xinglin Meditrans Center (http://www.58medtrans.com) and Hua He on 02/20/09. The original Chinese version of this paper is published in: Ai Zheng (Chinese Journal of Cancer), 28(3); http://www.cjcsysu.cn/cn/article.asp?id=14984 Previously published online as a Chinese Journal of Cancer E-publication: http://www.landesbioscience.com/journals/cjc/article/8658
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0.01 cc ion chamber for the 1 cm × 1 cm beam. For the 1 cm × 1 cm beam, the TMR of the depth deeper than 15 cm measured with the 0.01 cc ion chamber was about 4% different compared with that measured with the 0.13 cc ion chamber; for radiation fields of ≥ 2 cm × 2 cm, the differences of TMR between the 0.01 cc and 0.13 cc chambers were within 1%. For the radiation fields of ≥ 3 cm × 3 cm, the measured TMR values had a good consistency with the calculated values obtained from the percentage depth doses (PDDs) at the depth of 0 to 15 cm; but the two values were obviously different at the depths of deeper than 15 cm ( > 2%). Conclusions: For the measurement of small fields, the choice of a suitable detector is important due to the lack of lateral electron equilibrium. Misuse of the detector may affect the accuracy of the measurements for small radiation fields. When the lateral electron equilibrium is not established, the size of the detector used to measure the absorbed dose on the central axis should be considerably smaller than the field size. With the development of the radiotherapy apparatuses and the computer techniques, such as the application of the stereotactic radiotherapy (SRT) and intensity modulated radiation therapy (IMRT),1,2 the measurement and calculation of beam data for small radiation fields become important. In the use of some treatment planning systems (TPSs) and physical validation software, the beam data for small fields or even the zero-point radiation field are required to construct the dose calculation model. These beam data include the percentage depth doses (PDDs) (or tissuemaximum ratio (TMR), total scatter factor (Scp) and collimator scatter factor (Sc). For the step-and-shoot IMRT using multileaf collimator (MLC), the minimum radiation field is 1 cm x 1 cm or even smaller. For the circular radiation field of SRT, the minimum diameter for the radiation field of the collimator of the Elekta -knife (Elekta, Sweden) at the focal plane is 4mm. When the lateral electron equilibrium cannot be established, the size of the detector used to measure the absorbed dose on the central axis should be considerably smaller than the radiation field for the measurement of beam data for small radiation fields ( ≤ 3 cm × 3 cm).3,4 Using the 6 MV x-rays, the results from the Monte Carlo simulation method suggest that the minimum radiation radius of the beam to establish the lateral electron equilibrium is about 1.0 cm.5 When different detectors are used, such as the ionization chamber or the
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Measurements and comparisons for data of small beams of linear accelerators
Table 1
Geometrical parameters of the ionization chambers used in dose measurements
semiconductor detector, to measure the data of small beams, the size and the form of the detectors should suitable for the small radiation fields.6-10 Characteristics such as the directional dependence, stability and energy response should be taken into account as well. This study analyzed and compared small-field measurements using different methods and ionization chambers.
Materials and Methods The ionization chambers with an active volume of 0.6 cc (FC65, IBA company), 0.13cc (CC13, IBA company) and 0.01 cc (CC01, IBA company) and the dosemeter (DOSE1, IBA company) were applied to measure Scp, Sc and TMR of radiation fields ranging from 1 cm × 1 cm–10 cm × 10 cm. All the beam data were acquired using the ELEKTA linear accelerator (ELEKTA Precise Desktop) with accelerating potentials of 4 MV and 8 MV, and the Varian linear accelerator (Varian 600 C) with an accelerating potential of 6 MV. The water-equivalent phantom (RW3 with a density of 1.05 g·cm-3) was used. The parameters of the ionization chambers are listed in Table 1. Measurements of Scp. Scp is defined as the ratio of the dose, at the maximum dose depth (dmax) on the central axis for an interest field size, to the dose measured at the same point in a phantom for a reference of 10 cm × 10 cm field, when both are measured with the same SAD setup using the same number of monitor units (MU). The measurement depth in the water-equivalent phantom was adjusted before the measurement. The measurement depth in the water-equivalent phantom (dm) is related to the measurement depth in the water [dw=dm × pm × (Z/A)m/(Z/A)W] is the physical density, and Z and A are the atomic number and atomic mass number, respectively. Measurement of Sc. Generally, when the ionization chamber with the build-up cap is placed at the mechanical isocenter (SSD = SAD = 100 cm), Sc is calculated using the output dose rate in air for different radiation fields and normalized to the value for the reference radiation field (10 cm × 10 cm). Given the outer radius of the build-up cap (made of copper) is 18 mm using 8 MV rays (16 mm for 6 MV), the small radiation field of 1 cm × 1 cm could not totally cover the build-up cap of the ionization chamber. Therefore, the SSD were elongated to allow the radiation field to cover the ionization chamber and the build-up cap.12,14 Sc values for normal conditions (SSD = 100 cm) and for elongated SSD (SSD ≥ 150 cm) were measured. Measurements of TMR. Two methods were used to measure TMR, the SAD method and PDD method. Using the SAD method, the TMR was measured in RW3 water-equivalent phantom and www.landesbioscience.com
Figure 1. The total scatter factors measured by the three chambers under 4, 6, 8 MV x-rays.
the depths were transformed according to Formula (1). The PDDs were measured in the 3-D water phantom (RFA300, Scanditronix Medical AB Company) for radiation fields ranging from 2 × 2 cm2 to 10 × 10 cm2. The PDDs were transformed to TMRs using the OminiPro Accept software (version 6.1, Scanditronix Medical AB company). TMRs were calculated by the OminiPro Accept using PDDs and the peak scatter factors (PSFs) from British Journal of Radiology (BJR).15 However, TMRs for the minimum beam of the PDDs could not be calculated using PDDs. TMRs of smaller beams could only be directly measured or extrapolated from the measurement data of larger beams.
Results Measurement results of Scp. The measured data of Scp using the three chambers with accelerating potentials of 4, 6, 8 MV are shown in Figure 1. In reference to the data measured by the CC01 ionization chamber, the relative errors between the other two ionization chambers and the CC01 chamber are listed in Table 2. The relative error (%) = (measurement data by the CC13 or FC65 chamber – measurement date by the CC01 chamber)/(measurements by the CC01 chamber) × 100. The results measured by the three ionization chambers were similar for the radiation fields of ≥ 4 cm × 4
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Measurements and comparisons for data of small beams of linear accelerators
Table 2
The relative errors of the total scatter factors measured by the CC13 or FC65 chamber to those measured by the CC01 chamber under 4, 6, 8 MV x-rays
cm. The relative errors using 4, 6, 8 MV rays were within 0.4%, 0.6% and 0.7%, respectively. Greater relative errors were obtained from the three ionization chambers for the radiation fields of ≤ 3 cm × 3 cm, because the lateral electron equilibrium could not be established. The relative error was negatively correlated to the size of the radiation field. The measurement data increased with the decrease of the active volume of the ionization chamber. Measurement results of Sc. Because the volume of the FC65 ionization chamber and its build-up cap were large, Sc was only measured using the CC01 and CC13 chambers with their build-up caps. Using 4, 6 and 8MV rays, Sc measured by the CC13 and CC01 chamber with an elongated source-to-surface distance (SSD) are shown in Figure 2. Using 4, 6 and 8 MV rays, the Sc measured by the CC13 chamber were 0.2%, 8.5% and 3.4% smaller than those measured by the CC01 chamber respectively, for the radiation field of 1 cm × 1 cm. The relative errors of the radiation field of 2 cm × 2 cm or even bigger ones were within 0.8%. When the SSD was elongated, the radiation field of 2 cm × 2 cm could cover the ionization chamber and the build-up cap. The ratios of Sc for the small radiation fields measured with the elongated SSD to that measured with the standard SSD (100 cm) are listed in Table 3. Significant differences in the relative errors were observed between the two methods for the measurement of radiation field of 1 cm × 1 cm or 2 cm × 2 cm, with the maximal relative error of 25%. In contrast, the relative errors for the radiation field of ≥ 3 cm × 3 cm were within 1%. Results of TMR. TMRs for radiation fields of ≥ 3 cm × 3 cm measured by the CC13 and CC01 ionization chambers had no significant difference ( < 1%). TMRs for the radiation fields of 1 cm × 1 cm and 2 cm × 2 cm using 4, 6 and 8 MV rays by the CC01 chamber is listed in Table 4. TMRs were normalized to the value at dmax, the dmax for 4, 6 and 8MV rays was 1.3 cm, 1.5 cm and 2.0 cm respectively. Relative ratios of TMR measured by the two ionization chambers at different depths are shown in Table 5. The measurement data for the radiation field of 2 cm × 2 cm were consistent using the two ionization chambers (with the relative error below 0.8%). However, TMR showed great relative errors for the radiation field of 1 cm × 1 cm at a depth of more than 15 cm (with the maximum value of 4%). PDDs measured in the radiation fields ranging from 2 cm × 2 cm to 10 cm × 10 cm in the RFA300 water phantom were used to calculate the values of TMR 274
Figure 2. The diagrams of collimator scatter factors measured by the CC13 and CC01 ionization chambers.
Table 3
The ratios of collimator scatter factors measured with an elongated source-to-surface distance (SSD) to those measured with the standard SSD by the CC13 or CC01 chamber
The elongated source-chamber distances equal to 150 cm, 176 cm, 181 cm at 4, 6, 8 MV potentials, respectively.
Table 4
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The tissue maximum ratios measured by the CC01 ion chamber
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Measurements and comparisons for data of small beams of linear accelerators
Table 5
The tissue maximum ratios measured by the CC13 chamber to those measured by the CC01 chamber
for the radiation fields ranging from 3 cm × 3 cm to 10 cm × 10 cm by the OminiPro Software. The ratios of TMR measured by the CC13 ionization chamber using 8MV rays to those calculated with PDD curves and BSF values are listed in Table 6. TMRs directly measured by the ionization chamber for the radiation fields ranging from 3 cm × 3 cm to 10 cm × 10 cm were consistent with those calculated with PDDs at the depth of ≤ 15 cm. However, with an increase in the depth, the measured TMR were 2% to 5% higher than those calculated values, which may be due to the different detectors used for measuring PDD and TMR.
Discussion For the radiation fields of ≥ 4 cm × 4 cm, no differences existed in the data measured by the three ionization chambers. However, for small radiation fields, especially for those as small as 1 cm × 1 cm, great differences were observed in Scp and Sc, even though the CC13 and CC01 chambers with small active volumes were used. The standard SSD (SSD = 100 cm) setup is usually used to measure Sc for normal radiation fields, but elongated SSD should be used for small radiation fields to guarantee that the radiation fields can cover the build-up cap, otherwise, great errors would occur. The difference in TMR was relatively small measured by the CC01 and CC13 chambers at a depth of less than 15 cm. However, with the increase of the depth, the difference in measured values by the two chambers was increased. If TMRs were normalized to the values at the maximum depth, the relative errors were within 1%, which might be acceptable in clinical practice. As the measured data for TMR were relative ratios, the relative deviations could be eliminated to some extents, though the lateral electron equilibrium could not be established at some depths. As a result, the relative errors of TMR were much smaller than those of Scp values measured by the two ionization chambers. Positioning is important for the measurement of small radiation fields. Using the ELEKTA 8MV rays, the measurement data of Scp by the CC01 chamber with the SAD method revealed that, a deviation of the measurement setup from the central axis of the radiation fields by 1mm would result in 13.3% errors for the radiation field of 1 cm × 1 cm. For radiation fields of ≥ 2 cm × 2 cm, such a deviation was within 1%, implying that accurate positioning is needed for the accurate measurement of small radiation fields. Pilot studies are needed for more accurate measurement data. www.landesbioscience.com
Table 6
The tissue maximum ratios measured by the CC13 chamber to those calculated by OminiPro software under a 8MV x-ray
Monte Carlo study claimed that the minimum radiation radius for 6 MV rays was from 1.0–1.3 cm when the lateral electron equilibrium could be established. The minimum radiation radius for 10 MV rays was about 1.7 cm.4,5 The radiation fields for the lateral electron equilibrium is increased with an increase in energy. Therefore, radiation to small fields should be carefully planned, with appropriate positioning and detectors.3,9,16-18 Regarding to the choice of detectors, besides directional dependence, stability and energy response, the form and volume of the detector should also be seriously considered. The measurement of small radiation fields needs a lot of work, including the measurement of the off-axis ratio (OAR) and so on. Based on the current study, the measurement and calculation of data for small radiation fields should be further investigated using other detectors and Monte Carlo methods. Acknowledgements
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