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Feasibility study on the verification of actual beam delivery in a treatment room using EPID transit dosimetry.

Baek TS, Chung EJ, Son J, Yoon M - Radiat Oncol (2014)

Bottom Line: The proposed method was evaluated by comparing the calculated dose map from TPS and EPID measurement on the same plane using a gamma index method with a 3% dose and 3 mm distance-to-dose agreement tolerance limit.The passing rate of the transit dose for 24 IMRT fields was lower with the anthropomorphic phantom, averaging 86.8% ± 3.8%, a reduction partially due to the inaccuracy of TPS calculations for inhomogeneity.The simulation study indicated that the passing rate of the gamma index was significantly reduced, to less than 40%, when a wrong field was erroneously irradiated to patient in the treatment room.

View Article: PubMed Central - PubMed

Affiliation: Department of Bio-convergence Engineering, Korea University, Jeongneungro 161, Seongbuk-gu, Seoul, 136-703, Korea. taesb@nhimc.or.kr.

ABSTRACT

Purpose: The aim of this study is to evaluate the ability of transit dosimetry using commercial treatment planning system (TPS) and an electronic portal imaging device (EPID) with simple calibration method to verify the beam delivery based on detection of large errors in treatment room.

Methods and materials: Twenty four fields of intensity modulated radiotherapy (IMRT) plans were selected from four lung cancer patients and used in the irradiation of an anthropomorphic phantom. The proposed method was evaluated by comparing the calculated dose map from TPS and EPID measurement on the same plane using a gamma index method with a 3% dose and 3 mm distance-to-dose agreement tolerance limit.

Results: In a simulation using a homogeneous plastic water phantom, performed to verify the effectiveness of the proposed method, the average passing rate of the transit dose based on gamma index was high enough, averaging 94.2% when there was no error during beam delivery. The passing rate of the transit dose for 24 IMRT fields was lower with the anthropomorphic phantom, averaging 86.8% ± 3.8%, a reduction partially due to the inaccuracy of TPS calculations for inhomogeneity. Compared with the TPS, the absolute value of the transit dose at the beam center differed by -0.38% ± 2.1%. The simulation study indicated that the passing rate of the gamma index was significantly reduced, to less than 40%, when a wrong field was erroneously irradiated to patient in the treatment room.

Conclusions: This feasibility study suggested that transit dosimetry based on the calculation with commercial TPS and EPID measurement with simple calibration can provide information about large errors for treatment beam delivery.

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Related in: MedlinePlus

Experimental setup for EPID and dose calibration. (a) EPID calibration with ion chamber measurement, (b) radiation dose as a function of EPID in calibrated units and (c) dose conversion factor for EPID signals and CU.
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Fig2: Experimental setup for EPID and dose calibration. (a) EPID calibration with ion chamber measurement, (b) radiation dose as a function of EPID in calibrated units and (c) dose conversion factor for EPID signals and CU.

Mentions: For image acquisition and transit dose analysis, the EPID was first calibrated according to the vendor’s guidelines with dark field, flood field and the diagonal profile correction which was measured at dmax in water for a 40 × 40 cm2 open field. Then, EPID response was scaled such that 1 Calibrated Unit (CU) corresponds to 100 MU delivered by a 10 × 10 cm2 open field at 100 cm source-to-detector distance (SDD). It has been reported that the mechanical parts of the EPID produces a non-uniform back scattering in Varian EPID [16]. To remove the non-uniform backscattering pattern in calibration process, the impact of the backscatter that was present during the flood-field calibration was deduced and, then, the estimated non-uniform back-scatter pattern is corrected [17]. The absolute dose of the transit beam was first measured with EPID in calibrated units (CU) and converted to the absorbed dose. Figure 2(a) shows the experimental setup for EPID calibration with ion chamber measurement where we used 10 × 10 cm2 field size and 20 cm-thick homogeneous phantom as a simple approximation of field size and patient thickness, respectively. Both Eclipse calculation and measurement were done in 8 mm depth [8]. To convert the CU to dose, the calibration curve was acquired by comparison between dose measured by ion chamber and CU measured by EPID signal. Figure 2(b) shows that CU vs. dose on the beam axis as a function of CU (i.e., beam-on time). Although it seems like there is a linear relationship between dose and CU, the relationship is non-linear at low CU values. Figure 2(c) shows that the absorbed dose per CU decreases as the CU values increases and then it becomes relatively constant. Using this conversion factor, the measured EPID signal was converted to the absorbed dose. The data produced by the TPS and EPID were compared using the RIT 113 software module (Radiological Imaging Technology, Ver. 5.2, Colorado Springs, CO, USA) which usually takes less than 3 seconds for each field.Figure 2


Feasibility study on the verification of actual beam delivery in a treatment room using EPID transit dosimetry.

Baek TS, Chung EJ, Son J, Yoon M - Radiat Oncol (2014)

Experimental setup for EPID and dose calibration. (a) EPID calibration with ion chamber measurement, (b) radiation dose as a function of EPID in calibrated units and (c) dose conversion factor for EPID signals and CU.
© Copyright Policy - open-access
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4262986&req=5

Fig2: Experimental setup for EPID and dose calibration. (a) EPID calibration with ion chamber measurement, (b) radiation dose as a function of EPID in calibrated units and (c) dose conversion factor for EPID signals and CU.
Mentions: For image acquisition and transit dose analysis, the EPID was first calibrated according to the vendor’s guidelines with dark field, flood field and the diagonal profile correction which was measured at dmax in water for a 40 × 40 cm2 open field. Then, EPID response was scaled such that 1 Calibrated Unit (CU) corresponds to 100 MU delivered by a 10 × 10 cm2 open field at 100 cm source-to-detector distance (SDD). It has been reported that the mechanical parts of the EPID produces a non-uniform back scattering in Varian EPID [16]. To remove the non-uniform backscattering pattern in calibration process, the impact of the backscatter that was present during the flood-field calibration was deduced and, then, the estimated non-uniform back-scatter pattern is corrected [17]. The absolute dose of the transit beam was first measured with EPID in calibrated units (CU) and converted to the absorbed dose. Figure 2(a) shows the experimental setup for EPID calibration with ion chamber measurement where we used 10 × 10 cm2 field size and 20 cm-thick homogeneous phantom as a simple approximation of field size and patient thickness, respectively. Both Eclipse calculation and measurement were done in 8 mm depth [8]. To convert the CU to dose, the calibration curve was acquired by comparison between dose measured by ion chamber and CU measured by EPID signal. Figure 2(b) shows that CU vs. dose on the beam axis as a function of CU (i.e., beam-on time). Although it seems like there is a linear relationship between dose and CU, the relationship is non-linear at low CU values. Figure 2(c) shows that the absorbed dose per CU decreases as the CU values increases and then it becomes relatively constant. Using this conversion factor, the measured EPID signal was converted to the absorbed dose. The data produced by the TPS and EPID were compared using the RIT 113 software module (Radiological Imaging Technology, Ver. 5.2, Colorado Springs, CO, USA) which usually takes less than 3 seconds for each field.Figure 2

Bottom Line: The proposed method was evaluated by comparing the calculated dose map from TPS and EPID measurement on the same plane using a gamma index method with a 3% dose and 3 mm distance-to-dose agreement tolerance limit.The passing rate of the transit dose for 24 IMRT fields was lower with the anthropomorphic phantom, averaging 86.8% ± 3.8%, a reduction partially due to the inaccuracy of TPS calculations for inhomogeneity.The simulation study indicated that the passing rate of the gamma index was significantly reduced, to less than 40%, when a wrong field was erroneously irradiated to patient in the treatment room.

View Article: PubMed Central - PubMed

Affiliation: Department of Bio-convergence Engineering, Korea University, Jeongneungro 161, Seongbuk-gu, Seoul, 136-703, Korea. taesb@nhimc.or.kr.

ABSTRACT

Purpose: The aim of this study is to evaluate the ability of transit dosimetry using commercial treatment planning system (TPS) and an electronic portal imaging device (EPID) with simple calibration method to verify the beam delivery based on detection of large errors in treatment room.

Methods and materials: Twenty four fields of intensity modulated radiotherapy (IMRT) plans were selected from four lung cancer patients and used in the irradiation of an anthropomorphic phantom. The proposed method was evaluated by comparing the calculated dose map from TPS and EPID measurement on the same plane using a gamma index method with a 3% dose and 3 mm distance-to-dose agreement tolerance limit.

Results: In a simulation using a homogeneous plastic water phantom, performed to verify the effectiveness of the proposed method, the average passing rate of the transit dose based on gamma index was high enough, averaging 94.2% when there was no error during beam delivery. The passing rate of the transit dose for 24 IMRT fields was lower with the anthropomorphic phantom, averaging 86.8% ± 3.8%, a reduction partially due to the inaccuracy of TPS calculations for inhomogeneity. Compared with the TPS, the absolute value of the transit dose at the beam center differed by -0.38% ± 2.1%. The simulation study indicated that the passing rate of the gamma index was significantly reduced, to less than 40%, when a wrong field was erroneously irradiated to patient in the treatment room.

Conclusions: This feasibility study suggested that transit dosimetry based on the calculation with commercial TPS and EPID measurement with simple calibration can provide information about large errors for treatment beam delivery.

Show MeSH
Related in: MedlinePlus