The impact of ion chamber components onkB,Qfor reference dosimetry in MRgRT.

For reference dosimetry in MRgRT,kB,Qis used to correct for the impact of the magnetic field on the chamber calibration coefficient. It has been demonstrated that for accurate simulation ofkB,Qthe dead volume (DV) must be considered. This work goes one step further by analysing the contribution of secondary electrons generated in the various chamber components tokB,Q. The Farmer-type chamber PTW 30013 geometry was modelled for two different DVs. Monte Carlo simulations were performed for a60Co source and a 7 MV MRI-linac and the model was validated against measurements. Both parallel (α = 0° or 180°) and perpendicular (α = 90° or 270°) orientations of the chamber and the magnetic (B) field were considered, and severalB-field strengths between 0 T and 1.5 T. To study the dose contribution to the reduced volume (RV = cavity - DV) from the secondary electrons produced in certain components of the chamber the labelling of the particles was implemented in the PENELOPE user code PENMAIN. A separate model with each solid component of the chamber modelled as liquid water was used to investigate the impact of material choice onkB,Q. Results show that simulatedkB,Qvalues agree better with the measuredkB,Qwhen the DV is considered. It is demonstrated that small components of the chamber impactkB,Qconsiderably, since the contribution to the RV-dose from the bodies closer to the RV is higher than withoutB. Moreover, it is seen that the impact to the dose in the RV is reduced when the material of each component is modelled as liquid water. Therefore, chamber design and, to a lesser extent, choice of material affectkB,Q, and an accurate geometrical model of the chamber components and its further validation are important for correct calculations ofkB,Q.

Direct measurement of ion chamber correction factors, k Q and k B, in a 7 MV MRI-linac.

The output of MRI-integrated photon therapy (MRgXT) devices is measured in terms of absorbed dose to water, D w. Traditionally this is done with reference type ion chambers calibrated in a beam quality Q 0 without magnetic field. To correct the ion chamber response for the application in the magnetic field, a factor needs to be applied that corrects for both beam quality Q and the presence of the magnetic field B, k Q,B. This can be expressed as the product of k Q, without magnetic field, and ion chamber magnetic field correction, k B. k B depends on the magnetic field strength and its direction, the direction of the beam and the orientation and type of the ion chamber. In this study, for the first time, both k Q and k B were measured directly for six waterproof ion chambers (3 × PTW 30013 and 3 × IBA FC65-G) in a pre-clinical 7 MV MRI-linac at 0 T and at 1.5 T. Measurements were done with the only available primary standard built for this purpose, a water calorimeter. Resulting k Q factors for PTW and IBA chambers were 0.985(5) and 0.990(4), respectively. k B factors were measured with the chambers in antiparallel direction to the magnetic field (|| 180°), and perpendicular direction (⊥ -90°). k B|| and k B⊥ for the PTW chambers were 0.985(6) and 0.963(4), respectively and for IBA chambers 0.995(4) and 0.956(4). Agreement with the available literature values was shown, partly caused by the relatively large standard deviation (SD) in those values. The values in this study are currently the only available measured values for k Q and k B in an MRI-linac that are directly linked to the international traceability framework for the quantity absorbed dose to water, D w.

Commissioning of a water calorimeter as a primary standard for absorbed dose to water in magnetic fields.

MRI guided radiotherapy devices are currently in clinical use. Detector responses are affected by the magnetic field and need to be characterized in terms of absorbed dose to water, D w, against primary standards under these conditions. The aim of this study was to commission a water calorimeter, accepted as the Dutch national standard for D w in MV photons and to validate its claimed standard uncertainty of 0.37% in the 7 MV photon beam of a pre-clinical MRI-linac in a 1.5 T magnetic field. To evaluate the primary standard on a fundamental basis, realisation of D w at 1.5 T was evaluated parameter by parameter. A thermodynamic description was given to demonstrate potential temperature effects due to the magneto-caloric effect (MCE). Methods were developed for measurement of depth, variation in detector distance and beam output in the bore of the MRI-linac. This resulted in D w measurements with a magnetic field of 1.5 T and, after ramp-down, without magnetic field. It was shown that the measurement of ΔT w and calorimeter corrections are either independent of or can be determined in a magnetic field. The chemical heat defect, h, was considered zero within its stated uncertainty, as for 0 T. Evaluation of the MCE and measurements done during magnet ramp-down, indicated no changes in the specific heat capacity of water. However, variations of the applied monitor system increased the uncertainty on beam output normalization. This study confirmed that the uncertainty for measurement of D w with a water calorimeter in a 1.5 T magnetic field is estimated to be the same as under conventional reference conditions. The VSL water calorimeter can be applied as a primary standard for D w in magnetic fields and is currently the only primary standard operable in a magnetic field that provides direct access to the international traceability framework.

Comparison of k Q factors measured with a water calorimeter in flattening filter free (FFF) and conventional flattening filter (cFF) photon beams.

Recently flattening filter free (FFF) beams became available for application in modern radiotherapy. There are several advantages of FFF beams over conventional flattening filtered (cFF) beams, however differences in beam spectra at the point of interest in a phantom potentially affect the ion chamber response. Beams are also non-uniform over the length of a typical reference ion chamber and recombination is usually larger. Despite several studies describing FFF beam characteristics, only a limited number of studies investigated their effect on k Q factors. Some of those studies predicted significant discrepancies in k Q factors (0.4% up to 1.0%) if TPR20,10 based codes of practice (CoPs) were to be used. This study addresses the question to which extent k Q factors, based on a TPR20,10 CoP, can be applied in clinical reference dosimetry. It is the first study that compares k Q factors measured directly with an absorbed dose to water primary standard in FFF-cFF pairs of clinical photon beams. This was done with a transportable water calorimeter described elsewhere. The measurements corrected for recombination and beam radial non-uniformity were performed in FFF-cFF beam pairs at 6 MV and 10 MV of an Elekta Versa HD for a selection of three different Farmer-type ion chambers (eight serial numbers). The ratio of measured k Q factors of the FFF-cFF beam pairs were compared with the TPR20,10 CoPs of the NCS and IAEA and the %dd(10) x CoP of the AAPM. For the TPR20,10 based CoPs differences less than 0.23% were found in k Q factors between the corresponding FFF-cFF beams with standard uncertainties smaller than 0.35%, while for the %dd(10) x these differences were smaller than 0.46% and within the expanded uncertainty of the measurements. Based on the measurements made with the equipment described in this study the authors conclude that the k Q factors provided by the NCS-18 and IAEA TRS-398 codes of practice can be applied for flattening filter free beams without additional correction. However, existing codes of practice cannot be applied ignoring the significant volume averaging effect of the FFF beams over the ion chamber cavity. For this a corresponding volume averaging correction must be applied.

An on-site dosimetry audit for high-energy electron beams.

BACKGROUND AND PURPOSE: External dosimetry audits are powerful quality assurance instruments for radiotherapy. The aim of this study was to implement an electron dosimetry audit based on a contemporary code of practice within the requirements for calibration laboratories performing proficiency tests. This involved the determination of suitable acceptance criteria based on thorough uncertainty analyses. MATERIALS AND METHODS: Subject of the audit was the determination of absorbed dose to water, D w, and the beam quality specifier, R 50,dos. Fifteen electron beams were measured in four institutes according to the Belgian-Dutch code of practice for high-energy electron beams. The expanded uncertainty (k = 2) for the D w values was 3.6% for a Roos chamber calibrated in 60Co and 3.2% for a Roos chamber cross-calibrated against a Farmer chamber. The expanded uncertainty for the beam quality specifier, R 50,dos, was 0.14 cm. The audit acceptance levels were based on the expanded uncertainties for the comparison results and estimated to be 2.4%. RESULTS: The audit was implemented and validated successfully. All D w audit results were satisfactory with differences in D w values mostly smaller than 0.5% and always smaller than 1%. Except for one, differences in R 50,dos were smaller than 0.2 cm and always smaller than 0.3 cm. CONCLUSIONS: An electron dosimetry audit based on absorbed dose to water and present-day requirements for calibration laboratories performing proficiency tests was successfully implemented. It proved international traceability of the participants value with an uncertainty better than 3.6% (k = 2).

A water calorimeter for on-site absorbed dose to water calibrations in (60)Co and MV-photon beams including MRI incorporated treatment equipment.

In reference dosimetry the aim is to establish the absorbed dose to water, D w, under reference conditions. However, existing dosimetry protocols are not always applicable for rapidly emerging new treatment modalities. For primary standard dosimetry laboratories it is generally not feasible to acquire such modalities. Therefore it is strongly desired that D w measurements with primary standards can be performed on-site in clinical beams for the new treatment modalities in order to characterize and calibrate detectors. To serve this need, VSL has developed a new transportable water calorimeter serving as a primary D w standard for (60)Co and MV-photons including MRI incorporated treatment equipment. Special attention was paid to its operation in different beam geometries and beam modalities including the application in magnetic fields. The new calorimeter was validated in the VSL (60)Co beam and on-site in clinical MV-photon beams. Excellent agreement of 0.1% was achieved with previous (60)Co field calibrations, i.e. well within the uncertainty of the previous calorimeter, and with measurements performed in horizontal and vertical MV-photon beams. k Q factors, determined for two PTW 30013 ionization chambers, agreed very well with available literature data. The relative combined standard uncertainty (k = 1) for D w measurements in (60)Co and MV-photons is 0.37%. Calibrations are carried out with a standard uncertainty of 0.42% and k Q -factors are determined with a relative standard uncertainty of 0.40%.