Small static radiosurgery field dosimetry with small volume ionization chambers

Defining field output factors in small fields based on dose area product measurements: A feasibility study

BACKGROUND

Dose area product in water (DAPw) in small fields relies on the use of detectors with a sensitive area larger than the irradiation field. This quantity has recently been used to establish primary standards down to 5 mm field size, with an uncertainty smaller than 0.7%. It has the potential to decrease the uncertainty related to field output factors, but is not currently integrated into treatment planning systems.

PURPOSE

This study aimed to explore the feasibility of converting DAPw into a point dose in small fields by determining the volume averaging correction factor. By determining the field output factors, a comparison between the so-called "DAPw to point dose" approach and the IAEA TRS483 methodology was performed.

METHOD

Diodes, microdiamonds, and a micro ionization chamber were used to measure field output factors following the IAEA TRS483 methodology on two similar linacs equipped with circular cones down to 6 mm diameter. For the "DAPw to point dose" approach, measurements were performed with a dedicated and built-in-house 3 cm diameter plane-parallel ionization chamber calibrated in terms of DAPw in the French Primary Dosimetry Standards Laboratory LNE-LNHB. Beam profile measurements were performed to generate volume averaging correction factors enabling the conversion of an integral DAPw measurement into a point dose and the determination of the field output factors. Both sets of field output factors were compared.

RESULTS

According to the IAEA TRS483 methodology, field output factors were within ±3% for all detectors on both linacs. Large variations were observed for the volume averaging correction factors with a maximum spread between the detectors of 26% for the smallest field size. Consequently, deviations of up to 15% between the "IAEA TRS483" and the "DAPw to point dose" methodologies were found for the field output factor of the smallest field size. This was attributed to the difficulty in accurately determining beam profiles in small fields.

CONCLUSION

Although primary standards associated with small uncertainties can be established in terms of DAPw in a primary laboratory, the "DAPw to point dose" methodology requires volume averaging correction to derive a field output factor from DAPw measurements. None of the point detectors studied provided satisfactory results, and additional work using other detectors, such as film, is still required to allow the transfer of a DAP primary standard to users in terms of absorbed point dose.

Field output correction factors of small static field for IBA razor nanochamber

Purpose.The goal of this work is present results of field output factors (OF) using an IBA CC003 (Razor NanoChamber) and compare these results with PTW 60019 (MicroDiamond) and IBA Razor Diode. The experimental results for IBA CC003 were also compared with Monte Carlo (MC) Simulation, using Penelope and Ulysses programs. In addition, field output correction factors (kQclin,Qmsrfclin,fmsr) for IBA CC003 were derived with three different methods: (1) using PTW 60019 and IBA Razor as reference detectors; (2) comparison between MC and experimental measurements; and (3) using only MC.Material and Methods. The beam collimation included in this study were (1) square field size between 10 × 10 and 0.5 × 0.5 cm2defined by the MLC and jaws and (2) cones of different diameters. For IBA CC003 it was determined the polarity and ion collection efficiency correction factors in parallel and perpendicular orientation.Results.The results indicate (1) the variation of polarity effect with the field size is relevant for the determination of OF using IBA CC003, especially for parallel orientation; (2) there is no significant variation of the ion collection efficiency with the field size using IBA CC003 in parallel orientation; (3) OF differences between IBA CC003 and PTW 60019/IBA Razor, and experimental and MC results, increase with decreasing field size;ThekQclin,Qmsrfclin,fmsrresults indicate (1) using the first and second method,kQclin,Qmsrfclin,fmsrincrease with decreasing field size, which can be related with the influence of the volume effect and (2) using the third method,kQclin,Qmsrfclin,fmsrdecrease with decreasing field size, which can be explained by the perturbation effect.Conclusions. Our results demonstrate the need of applyingkQclin,Qmsrfclin,fmsrfor IBA CC003 forSclin≤1 cm, to compensate for volume averaging and perturbations effects.

Optimization of the Eclipse TPS beam configuration parameters for small field dosimetry using Monte Carlo simulations and experimental measurements

PURPOSE

To evaluate the impact of tuning the beam configurations parameters on the Analytical Anisotropic Algorithm (AAA) and the Acuros XB (AXB) algorithm for small fields using Monte Carlo simulations and measurements.

METHODS

The TrueBeam STx with the high-definition 120 multi-leaf collimator (HD120-MLC) was modeled with Geant4 application for emission tomography (GATE) Monte Carlo platform and validated against measurements. The impact of varying the effective spot size (ESS) and dosimetric leaf gap (DLG) on AAA and AXB calculations was carried out for small MLC-fields ranging from 0.5×0.5 cm2 to 3 × 3 cm2. Beam penumbras, field sizes and output factors calculated by AAA and AXB were compared to GATE calculations and measurements.

RESULTS

The beam penumbra comparisons showed that the best ESS value for AXB was about 1.0 mm in the crossplane direction and 0.5 mm in the inplane direction. By optimizing the ESS values, AXB could provide output factor results almost within 2% of GATE calculations and measurements for fields down to 0.5×0.5 cm2. For AAA, significant output factor differences were observed for all ESS values and tuning the DLG in addition to the ESS optimization resulted in an absorbed dose difference of less than 2.5% for MLC-fields down to 1 × 1 cm2.

CONCLUSION

By optimizing the ESS values, AXB can achieve accurate output factors in the case of small MLC-fields without the need of DLG tuning. Nevertheless, compromises between the output factor, DLG and ESS values were found necessary for AAA calculations. A MLC model improvement would allow to avoid the complexity related to tuning the configuration parameters.

Monte-Carlo calculation of output correction factors for ionization chambers, solid-state detectors, and EBT3 film in small fields of high-energy photons

High-energy accelerators are often used in oncological practice, but the information on the small-field dosimetry for the photon beams with nominal energy above 10 MV is limited. The goal of the present work was to determine the values of the output correction factor ( k Q clin , Q ref f clin , f ref $k_{{Q}_{{\rm{clin}}},{Q}_{{\rm{ref}}}}^{{f}_{{\rm{clin}}},{f}_{{\rm{ref}}}}$ ) for solid-state detectors (Diode E, PTW 60017; microDiamond, PTW 60019), EBT3 film, and ionization chambers (Semiflex, PTW 31010; Semiflex 3D, PTW 31021; PinPoint, PTW 31015; PinPoint 3D, PTW 31016) in the small fields formed by 10, 15, 18, and 20 MV photon beams. The output correction factors were calculated by Monte-Carlo method using EGSnrc toolkit for six field sizes (from 0.5 × 0.5 cm 2 $0.5 \times 0.5\ {\rm{cm}}^2$ to 10 × 10 cm 2 $10 \times 10\ {\rm{cm}}^2$ ) for isocentric and constant source-to-surface distance (SSD) techniques. The decrease in the field size led to an increase in k Q clin , Q ref f clin , f ref $k_{{Q}_{{\rm{clin}}},{Q}_{{\rm{ref}}}}^{{f}_{{\rm{clin}}},{f}_{{\rm{ref}}}}$ for ionization chambers, while for solid-state detectors and radiochromic film, k Q clin , Q ref f clin , f ref $k_{{Q}_{{\rm{clin}}},{Q}_{{\rm{ref}}}}^{{f}_{{\rm{clin}}},{f}_{{\rm{ref}}}}$ were less than unity at the smallest field size. A larger sensitive volume of ionization chamber corresponded to a stronger deviation of output correction factor from unity: 1.847 (125 mm³ PTW 31010) versus up to 1.183 (16 mm³ PTW 31016) at the smallest field of 10 MV beam. The calculated output correction factors were used to correct the output factors for PTW 60017, PTW 60019, and EBT3. The deviation of the corrected output factor from the results of Monte-Carlo simulation did not exceed 3% in the fields from 1.0 × 1.0 cm 2 $1.0 \times 1.0\ {\rm{cm}}^2$ to 4.0 × 4.0 cm 2 $4.0 \times 4.0\ {\rm{cm}}^2$ for 10 and 18 MV beams. Thus, Diode E, microDiamond, and EBT3 film can be recommended for small-field dosimetry of high-energy photons.