A simplified approach to characterizing a kilovoltage source spectrum for accurate dose computation

Poirier, Yannick; Kouznetsov, A.; Tambasco, Mauro

doi:10.1118/1.4711750

Cited by 18 publications

(43 citation statements)

References 20 publications

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“…However, the absorbed dose in a patient can only be accurately computed if the input parameters are accurate. We have previously validated both our X‐ray source/beam characterization method and kVDoseCalc experimentally for a stationary beam incident on homogeneous and heterogeneous phantoms 10 , 12 . The homogeneous cylindrical phantom measurements in this work show that kVDoseCalc can accurately compute the dose deposited by a rotating X‐ray source defining a rotating beam incident on curved surfaces.…”

Section: Discussionmentioning

confidence: 68%

“…This method was validated computationally using the MCNP (6) and EGSnrc (7) MC simulation techniques (5) . It was also validated experimentally for therapeutic kV x‐ray beams, 8 , 9 radiographic imaging procedures using open beams, (10) diagnostic CT scanners, (11) and radiographic beams produced with added bowtie filters (12) . A machine‐specific virtual point source model was developed by Poirier et al 10 , 12 to simulate the X‐ray beam input.…”

Section: Introductionmentioning

confidence: 99%

“…It was also validated experimentally for therapeutic kV x‐ray beams, 8 , 9 radiographic imaging procedures using open beams, (10) diagnostic CT scanners, (11) and radiographic beams produced with added bowtie filters (12) . A machine‐specific virtual point source model was developed by Poirier et al 10 , 12 to simulate the X‐ray beam input. This source model is characterized from simple in‐air ionization chamber measurements without the need for specialized equipment or knowledge of MC modeling techniques.…”

Section: Introductionmentioning

confidence: 99%

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Experimental validation of a kV source model and dose computation method for CBCT imaging in an anthropomorphic phantom

Poirier¹,

Tambasco

2016

J Applied Clin Med Phys

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We present an experimental validation of a kilovoltage (kV) X‐ray source characterization model in an anthropomorphic phantom to estimate patient‐specific absorbed dose from kV cone‐beam computed tomography (CBCT) imaging procedures and compare these doses to nominal weighted CT‐dose index (CTDIw) dose estimates. We simulated the default Varian on‐board imager 1.4 (OBI) default CBCT imaging protocols (i.e., standard‐dose head, low‐dose thorax, pelvis, and pelvis spotlight) using our previously developed and easy to implement X‐ray point‐source model and source characterization approach. We used this characterized source model to compute absorbed dose in homogeneous and anthropomorphic phantoms using our previously validated in‐house kV dose computation software (kVDoseCalc). We compared these computed absorbed doses to doses derived from ionization chamber measurements acquired at several points in a homogeneous cylindrical phantom and from thermoluminescent detectors (TLDs) placed in the anthropomorphic phantom. In the homogeneous cylindrical phantom, computed values of absorbed dose relative to the center of the phantom agreed with measured values within ≤2% of local dose, except in regions of high‐dose gradient where the distance to agreement (DTA) was 2 mm. The computed absorbed dose in the anthropomorphic phantom generally agreed with TLD measurements, with an average percent dose difference ranging from 2.4%±6.0% to 5.7%±10.3%, depending on the characterized CBCT imaging protocol. The low‐dose thorax and the standard dose scans showed the best and worst agreement, respectively. Our results also broadly agree with published values, which are approximately twice as high as the nominal CTDIw would suggest. The results demonstrate that our previously developed method for modeling and characterizing a kV X‐ray source could be used to accurately assess patient‐specific absorbed dose from kV CBCT procedures within reasonable accuracy, and serve as further evidence that existing CTDIw assessments underestimate absorbed dose delivered to patients.PACS number(s): 87.57.Q‐, 87.57.uq, 87.10.Rt

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Section: Discussionmentioning

confidence: 68%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Experimental validation of a kV source model and dose computation method for CBCT imaging in an anthropomorphic phantom

Poirier¹,

Tambasco

2016

J Applied Clin Med Phys

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“…The approach of using a combination of two beam quality indicators has been validated by other groups who have used it successfully to characterize the spectrum of an x‐ray beam for Monte Carlo dose computations using HVL 1 and kVp in one instance and HVL 1 and HVL 2 in another …”

Section: Methodsmentioning

confidence: 99%

Characterization of nanoDot optically stimulated luminescence detectors and high‐sensitivity MCP‐N thermoluminescent detectors in the 40–300 kVp energy range

2017

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Purpose: To investigate empirically the energy dependence of the detector response of two in vivo luminescence detectors, LiF:Mg,Cu,P (MCP-N) high-sensitivity TLDs and Al 2 O 3 :C OSLDs, in the 40-300-kVp energy range in the context of in vivo surface dose measurement. As these detectors become more prevalent in clinical and preclinical in vivo measurements, knowledge of the variation in the empirical dependence of the measured response of these detectors across a wide spectrum of beam qualities is important. Method: We characterized a large range of beam qualities of three different kilovoltage x-ray units: an Xstrahl 300 Orthovoltage unit, a Precision x-Ray X-RAD 320ix biological irradiator, and a Varian On-Board Imaging x-ray unit. The dose to water was measured in air according to the AAPM's Task Group 61 protocol. The OSLDs and TLDs were irradiated under reference conditions on the surface of a water phantom to provide full backscatter conditions.To assess the change in sensitivity in the long term, we separated the in vivo dosimeters of each type into an experimental and a reference group. The experimental dosimeters were irradiated using the kilovoltage x-ray units at each beam quality used in this investigation, while the reference group received a constant 10 cGy irradiation at 6 MV from a Varian clinical linear accelerator. The individual calibration of each detector was verified in cycles where both groups received a 10 cGy irradiation at 6 MV. Results: The nanoDot OSLDs were highly reproducible, with AE1.5% variation in response following >40 measurement cycles. The TLDs lost~20% of their signal sensitivity over the course of the study.The relative light output per unit dose to water of the MCP-N TLDs did not vary with beam quality for beam qualities with effective energies <50 keV (~150 kVp/6 mm Al). At higher energies, they showed a reduced (~75-85%) light output per unit dose relative to 6 MV x rays.The nanoDot OSLDs exhibited a very strong (120-408%) dependency of the light output relative to 6 MV x rays. Variations up to 15% between different x-ray units with equivalent effective energies were also observed. Conclusions: While convenient for clinical use, nanoDot OSLDs exhibit a strong variation in their measured light output per unit dose relative to 6 MV in the 40-300 kV x-ray range. This variability differs unit-to-unit, limiting their effective use for in vivo dosimetry applications in the kilovoltage x-ray energy range. MCP-N TLDs offer a much more stable response, but suffer from variations in sensitivity over time dependent on radiation history, which requires careful experimental handling.

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“…The lower extremity of the end plate was placed at the nominal FSD; the atomic densities (

atom / {cm}^{3}

) were calculated using the nominal PMMA polymer composition

false({normalC}_{5} {normalO}_{2} {normalH}_{8} false) n

and density (

1.18 g / {cm}^{3}

). The spectrum of the 100 kVp beam was characterized by matching the measured (6.26 mm Al) HVL and nominal kVp with spectra generated by the third‐party freeware Spektr (7) using the method described by Poirier et al (5) The highest kVp spectra that can be generated by Spektr is 140 kVp; for this reason the 150 and 200 kVp spectra were generated by inputting the nominal inherent 4 mm Be and added filtrations (provided in Table 1) into SpekCalc (8) . The spectra generated by SpekCalc matched the measured HVL within 0.01 mm Al.…”

Section: Methodsmentioning

confidence: 99%

Assessing the deviation from the inverse square law for orthovoltage beams with closed‐ended applicators

Gräfe

Poirier

Jacso³

et al. 2014

J Applied Clin Med Phys

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In this report, we quantify the divergence from the inverse square law (ISL) of the beam output as a function of distance (standoff) from closed‐ended applicators for a modern clinical orthovoltage unit. The divergence is clinically significant exceeding 3% at a 1.2 cm distance for 4 × 4 and 10.15em×.15em10cm2 closed‐ended applicators. For all investigated cases, the measured dose falloff is more rapid than that predicted by the ISL and, therefore, causes a systematic underdose when using the ISL for dose calculations at extended SSD. The observed divergence from the ISL in closed‐ended applicators can be explained by the end‐plate scattering contribution not accounted for in the ISL calculation. The standoff measurements were also compared to the predictions from a home‐built kV dose computation algorithm, kVDoseCalc. The kVDoseCalc computation predicted a more rapid falloff with distance than observed experimentally. The computation and measurements agree to within 1.1% for standoff distances of 3 cm or less for 4.15em×.15em4cm2 and 10.15em×.15em10cm2 field sizes. The overall agreement is within 2.3% for all field sizes and standoff distances measured. No significant deviation from the ISL was observed for open‐ended applicators for standoff distances up to 10 cm.PACS numbers: 87.55.‐x, 87.55.kh

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A simplified approach to characterizing a kilovoltage source spectrum for accurate dose computation

Cited by 18 publications

References 20 publications

Experimental validation of a kV source model and dose computation method for CBCT imaging in an anthropomorphic phantom

Experimental validation of a kV source model and dose computation method for CBCT imaging in an anthropomorphic phantom

Characterization of nanoDot optically stimulated luminescence detectors and high‐sensitivity MCP‐N thermoluminescent detectors in the 40–300 kVp energy range

Assessing the deviation from the inverse square law for orthovoltage beams with closed‐ended applicators

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