Technical Note: Preferred dosimeter size and associated correction factors in commissioning high dose per pulse, flattening filter free x-ray beams

Determination of thePrpand radial dose correction factor in reference dosimetry

Objectives.In an addendum to AAPM TG-51 protocol, McEwenet al, (DOI:10.1118/1.4866223) introduced a new factorPrpto account for the radial dose distribution of the photon beam over the detector volume mainly in flattening filter free (FFF) beams.Prpand its extension to non-FFF beam reference dosimetry is investigated to see its impact in a clinical situation.Approches.ThePrpwas measured using simplified version of Sudhyadhomet al(DOI:10.1118/1.4941691) for Elekta and Varian FFF beams with two commonly used calibration detectors; PTW-30013 and Exradin-A12 ion chambers after acquiring high resolution profiles in detectors cardinal coordinates. For radial dose correction factor, the ion chambers were placed in a small water phantom and the central axis position was set to center of the sensitive volume on the treatment table and was studied by rotating the table by 15-degree interval from -90 to +90 degrees with respect to the initial (zero) position.Main results.The magnitude ofPrpvaries very little with machine, detector and beam energies to a value of 1.003 ± 0.0005 and 1.005 ± 0.0005 for 6FFF and 10FFF, respectively. The radial anisotropy for the Elekta machine with Exradin-A12 and PTW-30013 detector the magnitudes are in the range of (0.9995±0.0011 to 1.0015±0.0010) and (0.9998±0.0007 to 1.0015±0.0010), respectively. Similarly, for the Varian machine with Exradin-A12 and PTW-30013 ion chambers, the magnitudes are in the range of (1.0004±0.0010 to 1.0018±0.0018) and (1.0006±0.0009 to 1.0027±0.0007), respectively.Significance.ThePrpis ≤ 0.3% and 0.5% for 6FFF and 10FFF, respectively. The radial dose correction factor in regular beams also does not impact the dosimetry where the maximum magnitude is ±0.2% which is within experimental uncertainty.

Quantification of the role of lead foil in flattening filter free beam reference dosimetry

PURPOSE

To quantify the potential error in outputs for flattening filter free (FFF) beams associated with use of a lead foil in beam quality determination per the addendum protocol for TG-51, we examined differences in measurements of the beam quality conversion factor k_Q when using or not using lead foil.

METHODS

Two FFF beams, a 6 MV FFF and a 10 MV FFF, were calibrated on eight Varian TrueBeams and two Elekta Versa HD linear accelerators (linacs) according to the TG-51 addendum protocol by using Farmer ionization chambers [TN 30013 (PTW) and SNC600c (Sun Nuclear)] with traceable absorbed dose-to-water calibrations. In determining k_Q , the percentage depth-dose at 10 cm [PDD(10)] was measured with 10×10 cm² field size at 100 cm source-to-surface distance (SSD). PDD(10) values were measured either with a 1 mm lead foil positioned in the path of the beam [%dd(10)_Pb ] or with omission of a lead foil [%dd(10)]. The %dd(10)x values were then calculated and the k_Q factors determined by using the empirical fit equation in the TG-51 addendum for the PTW 30013 chambers. A similar equation was used to calculate k_Q for the SNC600c chamber, with the fitting parameters taken from a very recent Monte Carlo study. The differences in k_Q factors were compared for with lead foil vs. without lead foil.

RESULTS

Differences in %dd(10)x with lead foil and with omission of lead foil were 0.9 ± 0.2% for the 6 MV FFF beam and 0.6 ± 0.1% for the 10 MV FFF beam. Differences in k_Q values with lead foil and with omission of lead foil were -0.1 ± 0.02% for the 6 MV FFF and -0.1 ± 0.01% for the 10 MV FFF beams.

CONCLUSION

With evaluation of the lead foil role in determination of the k_Q factor for FFF beams. Our results suggest that the omission of lead foil introduces approximately 0.1% of error for reference dosimetry of FFF beams on both TrueBeam and Versa platforms.

AAPM WGTG51 Report 374: Guidance for TG-51 reference dosimetry

Practical guidelines that are not explicit in the TG-51 protocol and its Addendum for photon beam dosimetry are presented for the implementation of the TG-51 protocol for reference dosimetry of external high-energy photon and electron beams. These guidelines pertain to: (i) measurement of depth-ionization curves required to obtain beam quality specifiers for the selection of beam quality conversion factors, (ii) considerations for the dosimetry system and specifications of a reference-class ionization chamber, (iii) commissioning a dosimetry system and frequency of measurements, (iv) positioning/aligning the water tank and ionization chamber for depth ionization and reference dose measurements, (v) requirements for ancillary equipment needed to measure charge (triaxial cables and electrometers) and to correct for environmental conditions, and (vi) translation from dose at the reference depth to that at the depth required by the treatment planning system. Procedures are identified to achieve the most accurate results (errors up to 8% have been observed) and, where applicable, a commonly used simplified procedure is described and the impact on reference dosimetry measurements is discussed so that the medical physicist can be informed on where to allocate resources.

Dose accuracy improvement on head and neck VMAT treatments by using the Acuros algorithm and accurate FFF beam calibration

Background

The purpose of this study was to assess dose accuracy improvement and dosimetric impact of switching from the anisotropic analytical algorithm (AA) to the Acuros XB algorithm (AXB) when performing an accurate beam calibration in head and neck (H&N) FFF-VMAT treatments.

Materials and methods

Twenty H&N cancer patients treated with FFF-VMAT techniques were included. Calculations were performed with the AA and AXB algorithm (dose-to-water - AXB_w- and dose-to-medium - AXB_m-). An accurate beam calibration was used for AXB calculations. Dose prescription to the tumour (PTV70) and at-risk-nodal region (PTV58.1) were 70 Gy and 58.1 Gy, respectively. A PTV70_{_bone} including bony structures in PTV70 was contoured. Dose-volume parameters were compared between the algorithms. Statistical tests were used to analyze the differences in mean values and the correlation between compliance with the D95 > 95% requirement and occurrence of local recurrence.

Results

AA systematically overestimated the dose compared to AXB algorithm with mean dose differences within 1.3 Gy/2%, except for the PTV70_{_bone} (2.2 Gy/3.2%). Dose differences were significantly higher for AXB_m calculations when including accurate beam calibration (maximum dose differences up to 2.8 Gy/4.1% and 4.2 Gy/6.3% for PTV70 and PTV70_{_bone}, respectively). 80% of AA-calculated plans did not meet the D95 > 95% requirement after recalculation with AXB_m and accurate beam calibration. The reduction in D95 coverage in the tumour was not clinically relevant.

Conclusions

Using the AXB_m algorithm and carefully reviewing the beam calibration procedure in H&N FFF-VMAT treatments ensures (1) dose accuracy increase by approximately 3%; (2) a consequent dose increase in targets; and (3) a dose reporting mode that is consistent with the trend of current algorithms.

Assessment of ion recombination correction and polarity effects for specific ionization chambers in flattening-filter-free photon beams

PURPOSE

To investigate ion recombination correction and polarity effects in four ion chamber models in flattening-filter-free (FFF) beams to (1) evaluate their suitability for reference dosimetry; (2) assess the accuracy of the two-voltage technique (TVA) against the Bruggmoser formalism; and (3) examine the influence of the accelerator type on the recombination correction.

METHODS

Jaffé plots were created for a variety of microchambers, small-volume and Farmer-type chambers to obtain k_S, the recombination correction factor, using two different types of accelerators. These values were plotted against dose-per-pulse and Jaffé plots for opposite polarities were created to determine which chambers meet the AAPM TG-51 addendum recombination and polarity specifications.

RESULTS

Nearly all small-volume chambers exhibited reference-class behavior with respect to ion recombination and polarity effects. The microchambers exhibited anomalous recombination and polarity effects, precluding their use for reference dosimetry in FFF beams. For the reference-class chambers, agreement between TVA-determined k_S values and Jaffé and Bruggmoser formalisms-determined k_S values was within 0.1%. No significant differences were found between the k_S values obtained with the two different accelerators used in this work.

CONCLUSIONS

This study stresses the need to characterize ion recombination correction and polarity effects for small-volume chambers and microchambers on an individual chamber basis and with the more rigorous criteria of the AAPM TG-51 addendum. Furthermore, the study demonstrated the suitability of the TVA method for chambers that exhibit reference-class behavior in FFF beams. Finally, this work has shown that the recombination correction does not depend on the type of accelerator but on its dose-per-pulse.

Experimental and Monte Carlo-based determination of the beam quality specifier for TomoTherapyHD treatment units

Reference dosimetry by means of clinical linear accelerators in high-energy photon fields requires the determination of the beam quality specifier TPR_20,10, which characterizes the relative particle flux density of the photon beam. The measurement of TPR_20,10 has to be performed in homogenous photon beams of size 10×10cm² with a focus-detector distance of 100cm. These requirements cannot be fulfilled by TomoTherapy treatment units from Accuray. The TomoTherapy unit provides a flattening-filter-free photon fan beam with a maximum field width of 40cm and field lengths of 1.0cm, 2.5cm and 5.0cm at a focus-isocenter distance of 85cm. For the determination of the beam quality specifier from measurements under nonstandard reference conditions Sauer and Palmans proposed experiment-based fit functions. Moreover, Sauer recommends considering the impact of the flattening-filter-free beam on the measured data. To verify these fit functions, in the present study a Monte Carlo based model of the treatment head of a TomoTherapyHD unit was designed and commissioned with existing beam data of our clinical TomoTherapy machine. Depth dose curves and dose profiles were in agreement within 1.5% between experimental and Monte Carlo-based data. Based on the fit functions from Sauer and Palmans the beam quality specifier TPR_20,10 was determined from field sizes 5×5cm², 10×5cm², 20×5cm² and 40×5cm² based on dosimetric measurements and Monte Carlo simulations. The mean value from all experimental values of TPR_20,10 resulted in TPR_20,10¯=0.635±0.4%. The impact of the non-homogenous field due to the flattening-filter-free beam was negligible for field sizes below 20×5cm². The beam quality specifier calculated by Monte Carlo simulations was TPR_20,10=0.628 and TPR_20,10=0.631 for two different calculation methods. The stopping power ratio water-to-air s_w,a^Δ directly depends on the beam quality specifier. The value determined from all experimental TPR_20,10 data was s_w,a^Δ=1.126±0.1%, which is in excellent agreement with the value directly calculated by Monte Carlo simulations. The agreement is a good indication that the equations proposed by Sauer and Palmans are able to calculate the beam quality specifier under reference conditions from measurements in arbitrary photon field sizes with high accuracy and are applicable for the TomoTherapyHD treatment unit.

Technical Note: On the impact of the incident electron beam energy on the primary dose component of flattening filter free photon beams

PURPOSE

For commercially available linear accelerators (Linacs), the electron energies of flattening filter free (FFF) and flattened (FF) beams are either identical or the electron energy of the FFF beam is increased to match the percentage depth dose curve (PDD) of the FF beam (in reference geometry). This study focuses on the primary dose components of FFF beams for both kinds of settings, studied on the same Linac.

METHODS

The measurements were conducted on a VersaHD Linac (Elekta, Crawley, UK) for both FF and FFF beams with nominal energies of 6 and 10 MV. In the clinical setting of the VersaHD, the energy of FFFM (Matched) beams is set to match the PDDs of the FF beams. In contrast the incident electron beam of the FFFU beam was set to the same energy as for the FF beam. Half value layers (HVLs) and a dual parameter beam quality specifier (DPBQS) were determined.

RESULTS

For the 6 MV FFFM beam, HVL and DPBQS values were very similar compared to those of the 6 MV FF beam, while for the 10 MV FFFM and FF beams, only %dd(10)x and HVL values were comparable (differences below 1.5%). This shows that matching the PDD at one depth does not guarantee other beam quality dependent parameters to be matched. For FFFU beams, all investigated beam quality specifiers were significantly different compared to those for FF beams of the same nominal accelerator potential. The DPBQS of the 6 MV FF and FFFM beams was equal within the measurement uncertainty and was comparable to published data of a machine with similar TPR20,10 and %dd(10)x. In contrast to that, the DPBQS's two parameters of the 10 MV FFFM beam were substantially higher compared to those for the 10 MV FF beam.

CONCLUSIONS

PDD-matched FF and FFF beams of both nominal accelerator potentials were observed to have similar HVL values, indicating similarity of their primary dose components. Using the DPBQS revealed that the mean attenuation coefficient was found to be the same within the uncertainty of 0.8% for 6 MV FF and 6 MV FFFM beams, while for 10 MV beams, they differed by 6.4%. This shows that the DPBQS can provide a differentiation of photon beam characteristics that would remain hidden by the use of a single beam quality specifier, such as %dd(10)x or HVL.