Reference dosimetry in a scanned pulsed proton beam using ionisation chambers and a Faraday cup

Lorin, Stefan; Grusell, Erik; Tilly, Nina; Medin, Joakim; Kimstrand, Peter; Glimelius, Bengt

doi:10.1088/0031-9155/53/13/008

Cited by 28 publications

(49 citation statements)

References 19 publications

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“…For the beam to be pulsed or pulsed‐scanned, t beam must be short compared to t IC and the time between pulses must be long compared to t IC . Other possible intermediate conditions have been described by Lorin et al . The Proteus‐235 beam pulses consist of 0.8 ns micro‐pulses separated by 10 ns intervals.…”

Section: Methodsmentioning

confidence: 87%

“…concluded that for proton beams used for treating ocular targets (usually operating at high dose rates), the recombination correction factor can be overestimated by up to 2% if the recommendation from TRS‐398 is followed without consideration. Lorin et al . showed that the signal from a thimble chamber should be corrected when evaluating a scanning proton beam.…”

Section: Introductionmentioning

confidence: 99%

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Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

et al. 2017

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Purpose: To evaluate the effect on charge collection in the ionization chamber (IC) in proton pencil beam scanning (PBS), where the local dose rate may exceed the dose rates encountered in conventional MV therapy by up to three orders of magnitude. Methods:We measured values of the ion recombination (k s ) and polarity (k pol ) correction factors in water, for a plane-parallel Markus TM23343 IC, using the cyclotron-based Proteus-235 therapy system with an active proton PBS of energies 30-230 MeV. Values of k s were determined from extrapolation of the saturation curve and the Two-Voltage Method (TVM), for planar fields. We compared our experimental results with those obtained from theoretical calculations. The PBS dose rates were estimated by combining direct IC measurements with results of simulations performed using the FLUKA MC code. Values of k s were also determined by the TVM for uniformly irradiated volumes over different ranges and modulation depths of the proton PBS, with or without range shifter. Results: By measuring charge collection efficiency versus applied IC voltage, we confirmed that, with respect to ion recombination, our proton PBS represents a continuous beam. For a given chamber parameter, e.g., nominal voltage, the value of k s depends on the energy and the dose rate of the proton PBS, reaching c. 0.5% for the TVM, at the dose rate of 13.4 Gy/s. For uniformly irradiated regular volumes, the k s value was significantly smaller, within 0.2% or 0.3% for irradiations with or without range shifter, respectively. Within measurement uncertainty, the average value of k pol , for the Markus TM23343 IC, was close to unity over the whole investigated range of clinical proton beam energies. Conclusion: While no polarity effect was observed for the Markus TM23343 IC in our pencil scanning proton beam system, the effect of volume recombination cannot be ignored.

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Section: Methodsmentioning

confidence: 87%

Section: Introductionmentioning

confidence: 99%

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

et al. 2017

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“…Proton therapy and ion therapy centers might have, in the future, access to clinical dedicated calorimeters to determine the absorbed dose‐to‐water to avoid additional uncertainty on the ion chamber charge measurement (e.g., due to large ion recombination effects). Dedicated calorimeters would also avoid the need to determine beam quality correction factors.…”

Section: Resultsmentioning

confidence: 99%

Consistency in quality correction factors for ionization chamber dosimetry in scanned proton beam therapy

et al. 2017

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The Alfonso formalism was applied to scanned proton beams. In our MC simulations, neither the difference in the beam profiles (scanned beam vs hypothetical beam) nor the different incident beam energies influenced significantly the beam correction factors. This suggests that ionization chamber quality correction factors in scanned or broad proton beams are indistinguishable within the calculation uncertainties provided dose distributions achieved by both modalities are similar and compliant with the TRS-398 reference conditions.

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“…This approach, based on absolute dose per MU, is equivalent to the approach based on absolute dose per particle used at other institutions. 4,23 …”

Section: Iic Conversion Of Mc-generated Iddsmentioning

confidence: 99%

Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

et al. 2013

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Purpose: To present our method and experience in commissioning dose models in water for spot scanning proton therapy in a commercial treatment planning system (TPS). Methods: The input data required by the TPS included in-air transverse profiles and integral depth doses (IDDs). All input data were obtained from Monte Carlo (MC) simulations that had been validated by measurements. MC-generated IDDs were converted to units of Gy mm 2 /MU using the measured IDDs at a depth of 2 cm employing the largest commercially available parallel-plate ionization chamber. The sensitive area of the chamber was insufficient to fully encompass the entire lateral dose deposited at depth by a pencil beam (spot). To correct for the detector size, correction factors as a function of proton energy were defined and determined using MC. The fluence of individual spots was initially modeled as a single Gaussian (SG) function and later as a double Gaussian (DG) function. The DG fluence model was introduced to account for the spot fluence due to contributions of large angle scattering from the devices within the scanning nozzle, especially from the spot profile monitor. To validate the DG fluence model, we compared calculations and measurements, including doses at the center of spread out Bragg peaks (SOBPs) as a function of nominal field size, range, and SOBP width, lateral dose profiles, and depth doses for different widths of SOBP. Dose models were validated extensively with patient treatment field-specific measurements. Results: We demonstrated that the DG fluence model is necessary for predicting the field size dependence of dose distributions. With this model, the calculated doses at the center of SOBPs as a function of nominal field size, range, and SOBP width, lateral dose profiles and depth doses for rectangular target volumes agreed well with respective measured values. With the DG fluence model for our scanning proton beam line, we successfully treated more than 500 patients from March 2010 through June 2012 with acceptable agreement between TPS calculated and measured dose distributions. However, the current dose model still has limitations in predicting field size dependence of doses at some intermediate depths of proton beams with high energies. Conclusions: We have commissioned a DG fluence model for clinical use. It is demonstrated that the DG fluence model is significantly more accurate than the SG fluence model. However, some deficiencies in modeling the low-dose envelope in the current dose algorithm still exist. Further improvements to the current dose algorithm are needed. The method presented here should be useful for commissioning pencil beam dose algorithms in new versions of TPS in the future.

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Reference dosimetry in a scanned pulsed proton beam using ionisation chambers and a Faraday cup

Cited by 28 publications

References 19 publications

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

Ion recombination and polarity correction factors for a plane–parallel ionization chamber in a proton scanning beam

Consistency in quality correction factors for ionization chamber dosimetry in scanned proton beam therapy

Commissioning dose computation models for spot scanning proton beams in water for a commercially available treatment planning system

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