Systematic evaluation of spatial resolution and gamma criteria for quality assurance with detector arrays in stereotactic radiosurgery.

PURPOSE: To characterize detector array spacing and gamma index for quality assurance (QA) of stereotactic radiosurgery (SRS) deliveries. Use the Nyquist theorem to determine the required detector spacing in SRS fields, and find optimal gamma indices to detect MLC errors using the SRS MapCHECK, ArcCHECK, and a portal imaging device (EPID). METHODS: The required detector spacing was determined via Fourier analysis of small radiation fields and profiles of typical SRS treatment plans. The clinical impact of MLC errors of 0.5, 1, and 2 mm was evaluated. Global gamma (low-dose threshold 10%) was evaluated for the three detector systems using various combinations of the distance to agreement and the dose difference. RESULTS: While MLC errors only slightly affected mean dose to PTV and a 2 mm thick surrounding structure (PTV_2 mm), significant PTV underdose incurred with increase in maximum dose to PTV_2 mm. Gamma indices with highest sensitivity to the introduced errors at 95% tolerance level for plans on target volumes of 3.2 cm3 (plan 3 cc) and 35.02 cm3 (plan 35 cc) were 2%/1 mm for the SRS MapCHECK and 2%/3 mm for the ArcCHECK, with 3%/1 mm (plan 3cc) and 2%/1 mm (plan 35cc) for the EPID. Drops in passing rates for a 2 mm MLC error were (46.2%, 41.6%) for the SRS MapCHECK and (12.2%, 4.2%) for the ArcCHECK for plan 3cc and plan 35cc, respectively. For Portal Dose, values were 4.5% (plan 3cc) and 7% (plan 35cc). The Nyquist frequency of two SRS dose distributions lie between 0.26  and 0.1 mm-1 , corresponding to detector spacings of 1.9 and 5 mm. Evaluation of SRS MapCHECK data with doubled detector density indicates that increased detector density may reduce the system's sensitivity to errors, necessitating a tighter gamma index. CONCLUSIONS: The present results give insight on the performance of detector arrays and gamma indices for the investigated detectors during SRS QA.

Charge collection efficiency of commercially available parallel-plate ionisation chambers in ultra-high dose-per-pulse electron beams.

Objective. This investigation aims to experimentally determine the charge collection efficiency (CCE) of six commercially available parallel-plate ionisation chamber (PPIC) models in ultra-high dose-per-pulse (UHDPP) electron beams.Approach. The CCE of 22 PPICs has been measured in UHDPP electron beams at the National Metrology Institution of Germany (PTB). The CCE was determined for a dose per pulse (DPP) range between 0.1 and 6.4 Gy (pulse duration of 2.5µs). The results obtained with the different PPICs were compared to evaluate the reproducibility, intra- and inter-model variation, and the performance of a CCE empirical model.Main results. The intra-model variation was, on average, 4.0%, which is more than three times the total combined relative standard uncertainty and was found to be greater at higher DPP (up to 20%). The inter-model variation for the PPIC with 2 mm electrode spacing, which was found to be, on average, 10%, was also significant compared to the relative uncertainty and the intra-model variation. The observed CCE variation could not be explained only by the expected deviation of the electrode spacing from the nominal value within the manufacturing tolerance. It should also be noted that a substantial polarity effect, between 0.914(5) and 1.201(3), was observed, and significant intra- and inter-model variation was observed on this effect.Significance. For research and pre-clinical study, the commercially available PPIC with a well-known CCE (directly measured for the specific chamber) and with a small electrode spacing could be used for relative and absolute dosimetry with a lower-limit uncertainty of 1.6% (k= 1) in the best case. However, to use a PPIC as a secondary standard in UHDPP electron beams for clinical purposes would require new model development to reduce the ion recombination, the polarity effect, and the total standard uncertainty on the dose measurement.

The probe-format graphite calorimeter, Aerrow, for absolute dosimetry in ultrahigh pulse dose rate electron beams.

PURPOSE: The purpose of this investigation is to evaluate the use of a probe-format graphite calorimeter, Aerrow, as an absolute and relative dosimeter of high-energy pulse dose rate (UHPDR) electron beams for in-water reference and depth-dose-type measurements, respectively. METHODS: In this paper, the calorimeter system is used to investigate the potential influence of dose per pulses delivered up to 5.6 Gy, the number of pulses delivered per measurement, and its potential for relative measurement (depth-dose curve measurement). The calorimeter system is directly compared against an Advanced Markus ion chamber. The finite element method was used to calculate heat transfer corrections along the percentage depth dose of a 20-MeV electron beam. Monte Carlo-calculated dose conversion factors necessary to calculate absorbed dose-to-water at a point from the measured dose-to-graphite are also presented. RESULTS: The comparison of Aerrow against a fully calibrated Advanced Markus chamber, corrected for the saturation effect, has shown consistent results in terms of dose-to-water determination. The measured reference depth is within 0.5 mm from the expected value from Monte Carlo simulation. The relative standard uncertainty estimated for Aerrow was 1.06%, which is larger compared to alanine dosimetry (McEwen et al. https://doi.org/10.1088/0026-1394/52/2/272) but has the advantage of being a real-time detector. CONCLUSION: In this investigation, it was demonstrated that the Aerrow probe-type graphite calorimeter can be used for relative and absolute dosimetries in water in an UHPDR electron beam. To the author's knowledge, this is the first reported use of an absorbed dose calorimeter for an in-water percentage depth-dose curve measurement. The use of the Aerrow in quasi-adiabatic mode has greatly simplified the signal readout, compared to isothermal mode, as the resistance was directly measured with a high-stability digital multimeter.

Experimental determination of the recombination correction factor kS for SNC 125c, SNC 350p and SNC 600c ionization chambers in pulsed photon beams.

Accurate ionization chamber measurements of the absorbed dose to water require the correction of incomplete collection of charges created within the chamber volume. According to current dosimetry protocols such as the TRS-398 or the DIN 6800-2, incomplete charge collection is accounted for by the correction factor ks, which can be determined numerically or experimentally. The method proposed by Burns & McEwen (Phys. Med. Biol., 1998) was used in this study to determine the coefficients γ and δ used for the calculation of the correction factor ks of three ionization chambers, the SNC 125c, the SNC 600c and the SNC 350p (all Sun Nuclear Corp., Melbourne, Florida) for an absorbed dose to water range of 0.2mGy to 1.6mGy per pulse in pulsed photon beams. The shift of the effective point of measurement from the reference point Δz and the correction factor kr were determined for the SNC 350p according to the draft DIN 6800-2:2019-07.

The practical application of scintillation dosimetry in small-field photon-beam radiotherapy.

Plastic scintillation detectors are a new instrument of stereotactic photon-beam dosimetry. The clinical application of the plastic scintillation detector Exradin W1 at the Siemens Artiste and Elekta Synergy accelerators is a matter of current interest. In order to reduce the measurement uncertainty, precautions have to be taken with regard to the geometrical arrangement of the scintillator, the light-guide fiber and the photodiode in the radiation field. To determine the "Cerenkov light ratio" CLR with a type A uncertainty below 1%, the Cerenkov calibration procedure for small-field measurements based on the two-channel spectral method was used. Output factors were correctly measured with the W1 for field sizes down to 0.5×0.5cm2 with a type A uncertainty of 1.8%. Measurements of small field dose profiles and percentage depth dose curves were carried out with the W1 using automated water phantom profile scans, and a type A uncertainty for dose maxima of 1.4% was achieved. The agreement with a synthetic diamond detector (microDiamond, PTW Freiburg) and a plane parallel ionization chamber (Roos chamber, PTW Freiburg) in relative dose measurements was excellent. In oversight of all results, the suitability of the plastic scintillation detector Exradin W1 for clinical dosimetry under stereotactic conditions, in particular the tried and tested procedures for CLR determination, output factor measurement and automated dose profile scans in water phantoms, have been confirmed.