The impact of corrected field output factors based on IAEA/AAPM code of practice on small-field dosimetry to the calculated monitor unit in eclipse™ treatment planning system

Applicability evaluation of the TRS-483 protocol for the determination of small-field output factors using different multi-leaf collimator and field-shaping types

PURPOSE

To evaluate the applicability of TRS-483 output correction factors (CFs) for small-field output factors (OFs) using different multi-leaf collimators (MLC) and field-shaping types.

METHODS

All measurements were performed on TrueBeam, TrueBeam STx, and Halcyon using 6 MV flattening filter-free energy. Four detectors, including CC01, CC04, microDiamond, and EDGE, were used. Nominal field sizes ranging from 1 × 1 to 4 × 4, and 10 × 10 cm2 were used to measure small-field OFs at source-to-axis distance of 100 cm with a 0° gantry angle in a 3D water phantom. Further, the field-shaping types were defined using jaw collimator or MLC (five different configurations). A field size of 10 × 10 cm2 was used as the reference for calculation of OFs obtained as ratio of detector readings (OFdet). The percentage difference and coefficient of variation of OFdet and OFdet corrected by applying CF were compared for each field size and configuration.

RESULTS

For OFdet corrected by applying CF, the ranges of percentage difference and coefficient of variation in all configurations for ≥ 2 × 2 cm2 fields were reduced from 1.2-2.2 to 0.8-1.3 percentage points (%pt) and from 0.5-1.0 to 0.4-0.7%, respectively. For 1 × 1 cm2 field, the ranges of percentage difference and coefficient of variation were reduced from 3.3-5.7 to 1.2-2.2 %pt and from 2.2-3.7 to 0.8-1.1%, respectively.

CONCLUSIONS

The CFs described in TRS-483 dosimetry protocol have broad applicability in reducing OF variations between detectors under different MLC and field-shaping types.

Evaluation of International Atomic Energy Agency Technical Report Series-483 Detector-specific Output Correction Factor for Various Collimator Systems

Aim

In this study, a 6MV flattening filter (FF) and 6MV FF Free (FFF) photon beam small-field output factors (OF) were measured with various collimators using different detectors. The corrected OFs were compared with the treatment planning system (TPS) calculated OFs.

Materials and Methods

OF measurements were performed with four different types of collimators: Varian Millennium multi-leaf collimator (MLC), Elekta Agility MLC, Apex micro-MLC (mMLC) and a stereotactic cone. Ten detectors (four ionization chambers and six diodes) were used to perform the OF measurements at a depth of 10 cm with a source-to-surface distance of 90 cm. The corrected OF was calculated from the measurements. The corrected OFs were compared with existing TPS-generated OFs.

Results

The use of detector-specific output correction factor (OCF) in the PTW diode P detector reduced the OF uncertainty by <4.1% for 1 cm × 1 cm Sclin. The corrected OF was compared with TPS calculated OF; the maximum variation with the IBA CC01 chamber was 3.75%, 3.72%, 1.16%, and 0.90% for 5 mm stereotactic cone, 0.49 cm × 0.49 cm Apex mMLC, 1 cm × 1 cm Agility MLC, and 1 cm × 1 cm Millennium MLC, respectively.

Conclusion

The technical report series-483 protocol recommends that detector-specific OCF should be used to calculate the corrected OF from the measured OF. The implementation of OCF in the TPS commissioning will reduce the small-field OF variation by <3% for any type of detector.

Planning issues on linac-based stereotactic radiotherapy

This work aims to summarize and evaluate the current planning progress based on the linear accelerator in stereotactic radiotherapy (SRT). The specific techniques include 3-dimensional conformal radiotherapy, dynamic conformal arc therapy, intensity-modulated radiotherapy, and volumetric-modulated arc therapy (VMAT). They are all designed to deliver higher doses to the target volume while reducing damage to normal tissues; among them, VMAT shows better prospects for application. This paper reviews and summarizes several issues on the planning of SRT to provide a reference for clinical application.

Effect of inaccurate small field output factors on brain SRS plans

External beam radiotherapy often includes the use of field sizes 3 × 3 cm²or less, which can be defined as small fields. Dosimetry is a difficult, yet important part of the radiotherapy process. The dosimetry of small fields has additional challenges, which can lead to treatment inconsistencies if not done properly. Most important is the use of an appropriate detector, as well as the application of the necessary corrections. The International Atomic Energy Agency and the American Association of Physicists in Medicine provide the International Code of Practice (CoP) TRS-483 for the dosimetry of small static fields used in external MV photon beams. It gives guidelines on how to apply small-field correction factors for small field dosimetry. The purpose of this study was to evaluate the impact of inaccurate small-field output factors on clinical brain stereotactic radiosurgery plans with and without applying the small-field correction factors as suggested in the CoP. Small-field correction factors for a Varian TrueBeam linear accelerator were applied to uncorrected relative dose factors. Uncorrected and corrected clinical plans were created with two different beam configurations, 6 MV with a flattening filter (6 WFF) and 6 MV without a flattening filter (6 FFF). For the corrected plans, the planning target volume mean dose was 1.6 ± 0.9% lower with p < 0.001 for 6 WFF and 1.8 ± 1.5% lower with p < 0.001 for 6 FFF. For brainstem, a major organ at risk, the corrected plans had a dose that was 1.6 ± 0.9% lower with p = 0.03 for 6 WFF and 1.8 ± 1.5% lower with p = 0.10 for 6 FFF. This represents a systematic error that should and can be corrected.

Investigation of field output factors using IAEA-AAPM TRS-483 code of practice recommendations and Monte Carlo simulation for 6 MV photon beams

Introduction:

This study aims to experimentally determine field output factors using the methodologies suggested by the IAEA-AAPM TRS-483 for small field dosimetry and compare with the calculation from Monte Carlo (MC) simulation.

Methods:

The IBA-CC01, Sun Nuclear EDGE and IBA-SFD detectors were employed to determine the uncorrected and the corrected field output factors for 6 MV photon beams. Measurements were performed at 100 cm source to axis distance, 10 cm depth in water, and the field sizes ranged from 1 × 1 to 10 × 10 cm2. The use of field output correction factors proposed by the TRS-483 was utilised to determine field output factors. The measured field output factors were compared to that calculated using the egs_chamber user code.

Results:

The decrease in the percentage standard deviation of the measured three detectors was observed after applying the field output correction factors. Measured field output factors using CC01 and EDGE detectors agreed with MC values within 3% for field sizes down to 1 × 1 cm2, except the SFD detector.

Conclusions:

The corrected field output factors agree with the calculation from MC, except the SFD detector. CC01 and EDGE are suitable for determining field output factors, while the SFD may need more implementation of the intermediate field method.

Evaluation of Acuros® XB accuracy for static small fields dose calculations based on the IAEA/AAPM TRS-483 recommendation

PURPOSE

Evaluate Acuros® XB dose calculation accuracy following TRS-483 recommendations in small static fields for flattened and un-flattened 6 MV X-ray beams.

METHODS

Field output factors were measured following TRS-483 recommendations using four radiation detectors. Two sets of field output factors were measured. One set was used to configure the beam model into Acuros® XB down to a jaw-defined field size of 1.0 cm × 1.0 cm. The second set was used to evaluate the differences between calculated and measured field output factors for MLC-fields down to a field size of 0.5 cm × 0.5 cm.

RESULTS

Acuros® XB showed an accuracy within 1.5% down to an MLC-field of 1.0 cm × 1.0 cm, for a focal spot size of 1.0 and 0.0 mm in the cross and in-plane directions. For an MLC-field of 0.5 cm × 0.5 cm, an agreement was found within 3% between calculated and measured field output factors. These results were addressed by optimizing the focal spot size to minimize the differences between calculated and measured dose profiles.

CONCLUSIONS

By optimizing the focal spot size, Acuros® XB showed an acceptable agreement within 3% down to an MLC-field of 0.5 cm × 0.5 cm. The results of this work suggest that if static and modulated delivery of very small targets is planned, then a field output factor table down to a field size of 1.0 cm is required in the beam configuration model.

Implementation and Challenges of International Atomic Energy Agency/American Association of Physicists in Medicine TRS 483 Formalism for Field Output Factors and Involved Uncertainties Determination in Small Fields for TomoTherapy

PURPOSE

International Atomic Energy Agency published TRS-483 to address the issues of small field dosimetry. Our study calculates the output factor in the small fields of TomoTherapy using different detectors and dosimetric conditions. Furthermore, it estimates the various components of uncertainty and presents challenges faced during implementation.

MATERIALS AND METHODS

Beam quality TPR20,10(10) at the hypothetical field size of 10 cm × 10 cm was calculated from TPR20,10(S). Two ionization chambers based on the minimum field width required to satisfy the lateral charge particle equilibrium and one unshielded electron field diode (EFD) were selected. Output factor measurements were performed in various dosimetric conditions.

RESULTS

Beam quality TPR20,10(10) has a mean value of 0.627 ± 0.001. The maximum variation of output factor between CC01 chamber and EFD diode at the smallest field size was 11.80%. In source to surface setup, the difference between water and virtual water was up to 9.68% and 8.13%, respectively, for the CC01 chamber and EFD diode. The total uncertainty in the ionization chamber was 2.43 times higher compared to the unshielded EFD diode at the smallest field size.

CONCLUSIONS

Beam quality measurements, chamber selection procedure, and output factors were successfully carried out. A difference of up to 10% in output factor can occur if density scaling for electron density in virtual water is not considered. The uncertainty in output correction factors dominates, while positional and meter reading uncertainty makes a minor contribution to total uncertainty. An unshielded EFD diode is a preferred detector in small fields because of lower uncertainty in measurements compared to ionization chambers.