Inter-institutional variability of small-field-dosimetry beams among HD120™ multileaf collimators: a multi-institutional analysis

A multi-detector comparison to determine convergence of measured relative output factors for small field dosimetry

The TRS-483 Code of Practice (CoP) provides generic relative output correction factors, [Formula: see text], for a range of detectors and beam energies as used in small field dosimetry. In this work, the convergence of the relative output factors (ROFs) for 6 MV X-ray beams with and without flattening filters was investigated under different combinations of beam collimation and published detector correction factors. The SFD, PFD and CC04 (IBA) were used to measure ROFs of a TrueBeam STx linear accelerator with small fields collimated by the high-definition MLC, which has 2.5 and 5.0 mm projected leaves. Two configurations were used for the collimators: (1) fixed jaws at 10 × 10 cm2 and (2) with a 2 mm offset from the MLC edge, in line with the recommended geometry from IROC-H as part of their auditing program and published dataset. The [Formula: see text] factors for the three detectors were taken from the TRS483 CoP and other published works. The average differences of ROFs measured by detectors under MLC fields with fixed jaws and with 2 mm jaws offset for the 6 MV-WFF beam are 1.4% and 1.9%, respectively. Similarly, they are 2.3% and 2.4% for the 6MV-FFF beam. The relative differences between the detector-average ROFs and the corresponding IROC-H dataset are 2.0% and 3.1% for the 6 MV-WFF beam, while they are 2.4% and 3.2% for the 6MV-FFF beam at the smallest available field size of 2 × 2 cm2. For smaller field sizes, the average ROFs of the three detectors and corresponding results from Akino and Dufreneix showed the largest difference to be 6.6% and 6.2% under the 6 MV-WFF beam, while they are 3.4% and 3.6% under the 6 MV-WFF beam at the smallest field size of 0.5 × 0.5 cm2. Some well-published specific output correction factors for different small field detector types give better convergence in the calculation of the relative output factor in comparison with the generic data provided by the TRS-483 CoP. Relative output factor measurements should be performed as close as possible to the clinical settings including a combination of collimation systems, beam types and using at least three different types of small field detector for more accurate computation of the treatment planning system. The IROC-H dataset is not available for field size smaller than 2 × 2 cm2 for double checks and so that user should carefully check with other publications with the same setting.

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.

Mitigating the uncertainty in small field dosimetry by leveraging machine learning strategies

Small field dosimetry is significantly different from the dosimetry of broad beams due to loss of electron side scatter equilibrium, source occlusion, and effects related to the choice of detector. However, use of small fields is increasing with the increase in indications for intensity-modulated radiation therapy and stereotactic body radiation therapy, and thus the need for accurate dosimetry is ever more important. Here we propose to leverage machine learning (ML) strategies to reduce the uncertainties and increase the accuracy in determining small field output factors (OFs). Linac OFs from a Varian TrueBeam STx were calculated either by the treatment planning system (TPS) or measured with a W1 scintillator detector at various multi-leaf collimator (MLC) positions, jaw positions, and with and without contribution from leaf-end transmission. The fields were defined by the MLCs with the jaws at various positions. Field sizes between 5 and 100 mm were evaluated. Separate ML regression models were generated based on the TPS calculated or the measured datasets. Accurate predictions of small field OFs at different field sizes (FSs) were achieved independent of jaw and MLC position. A mean and maximum % relative error of 0.38 ± 0.39% and 3.62%, respectively, for the best-performing models based on the measured datasets were found. The prediction accuracy was independent of contribution from leaf-end transmission. Several ML models for predicting small field OFs were generated, validated, and tested. Incorporating these models into the dose calculation workflow could greatly increase the accuracy and robustness of dose calculations for any radiotherapy delivery technique that relies heavily on small fields.

Report of AAPM Task Group 155: Megavoltage photon beam dosimetry in small fields and non-equilibrium conditions

Small-field dosimetry used in advance treatment technologies poses challenges due to loss of lateral charged particle equilibrium (LCPE), occlusion of the primary photon source, and the limited choice of suitable radiation detectors. These challenges greatly influence dosimetric accuracy. Many high-profile radiation incidents have demonstrated a poor understanding of appropriate methodology for small-field dosimetry. These incidents are a cause for concern because the use of small fields in various specialized radiation treatment techniques continues to grow rapidly. Reference and relative dosimetry in small and composite fields are the subject of the International Atomic Energy Agency (IAEA) dosimetry code of practice that has been published as TRS-483 and an AAPM summary publication (IAEA TRS 483; Dosimetry of small static fields used in external beam radiotherapy: An IAEA/AAPM International Code of Practice for reference and relative dose determination, Technical Report Series No. 483; Palmans et al., Med Phys 45(11):e1123, 2018). The charge of AAPM task group 155 (TG-155) is to summarize current knowledge on small-field dosimetry and to provide recommendations of best practices for relative dose determination in small megavoltage photon beams. An overview of the issue of LCPE and the changes in photon beam perturbations with decreasing field size is provided. Recommendations are included on appropriate detector systems and measurement methodologies. Existing published data on dosimetric parameters in small photon fields (e.g., percentage depth dose, tissue phantom ratio/tissue maximum ratio, off-axis ratios, and field output factors) together with the necessary perturbation corrections for various detectors are reviewed. A discussion on errors and an uncertainty analysis in measurements is provided. The design of beam models in treatment planning systems to simulate small fields necessitates special attention on the influence of the primary beam source and collimating devices in the computation of energy fluence and dose. The general requirements for fluence and dose calculation engines suitable for modeling dose in small fields are reviewed. Implementations in commercial treatment planning systems vary widely, and the aims of this report are to provide insight for the medical physicist and guidance to developers of beams models for radiotherapy treatment planning systems.

PENELOPE simulations and experiment for 6 MV clinac iX accelerator for standard and small static fields

The goal of this work was to produce accurate data for use as a 'gold standard' and a valid tool for measurements in reference dosimetry for standard/small static field sizes from 0.5 × 0.5 to 10 × 10 cm². It is based on the accuracy of the phase space files (PSFs) as a key quantity. Because the IAEA general public database provides few PSFs for the Varian iX, we simulated the head through Monte Carlo (MC) simulations and calculated validated PSFs for 12 square field sizes including seven for small static fields. The resulting dosimetric calculations allowed us to reach a good level of agreement in comparison to our relative and absolute dose measurements performed on a Varian iX in water phantom. Measured and MC calculated output factors were investigated for different detectors. Based on the TRS 483 formalism and MC (PENELOPE/penEasy), we calculated output correction factors for the unshielded Diode-E (T60017) and the PinPoint-3D (T31016) micro-chamber according to manufacturers' blueprints. Our MC results were in agreement with the recommended data; they compete with recent measurements and MC simulations and in particular the TRS 483 MC data obtained from similar simulations. Moreover, our MC results provide supplemental data in comparison to TRS 483 data in particular for the PinPoint-3D (T31016). We suggest our MC output correction factors as new datasets for future TRS compilations. The work was substantial, used different robust MC strategies depending on the scoring regions, and led in most cases to uncertainties of less than 1%.

The influence of small field output factors simulated uncertainties on the calculated dose in VMAT plans for brain metastases: a multicentre study

OBJECTIVES

This multicentric study was carried out to investigate the impact of small field output factors (OFs) inaccuracies on the calculated dose in volumetric arctherapy (VMAT) radiosurgery brain plans.

METHODS

Nine centres, realised the same five VMAT plans with common planning rules and their specific clinical equipment Linac/treatment planning system commissioned with their OFs measured values (OFbaseline). In order to simulate OFs errors, two new OFs sets were generated for each centre by changing only the OFs values of the smallest field sizes (from 3.2 × 3.2 cm² to 1 × 1 cm²) with well-defined amounts (positive and negative). Consequently, two virtual machines for each centre were recommissioned using the new OFs and the percentage dose differences ΔD (%) between the baseline plans and the same plans recalculated using the incremented (OFup) and decremented (OFdown) values were evaluated. The ΔD (%) were analysed in terms of planning target volume (PTV) coverage and organs at risk (OARs) sparing at selected dose/volume points.

RESULTS

The plans recalculated with OFdown sets resulted in higher variation of doses than baseline within 1.6 and 3.4% to PTVs and OARs respectively; while the plans with OFup sets resulted in lower variation within 1.3% to both PTVs and OARs. Our analysis highlights that OFs variations affect calculated dose depending on the algorithm and on the delivery mode (field jaw/MLC-defined). The Monte Carlo (MC) algorithm resulted significantly more sensitive to OFs variations than all of the other algorithms.

CONCLUSION

The aim of our study was to evaluate how small fields OFs inaccuracies can affect the dose calculation in VMAT brain radiosurgery treatments plans. It was observed that simulated OFs errors, return dosimetric calculation accuracies within the 3% between concurrent plans analysed in terms of percentage dose differences at selected dose/volume points of the PTV coverage and OARs sparing.

ADVANCES IN KNOWLEDGE

First multicentre study involving different Planning/Linacs about undetectable errors in commissioning output factor for small fields.

Field output factors for small fields: A large multicentre study

PURPOSE

The determination of output factors in small field dosimetry is a crucial point, especially when implementing stereotactic radiotherapy (SRT). Herein, a working group of the French medical physicist society (SFPM) was created to collect small field output factors. The objective was to gather and disseminate information on small field output factors based on different detectors for various clinical SRT equipment and measurement configurations.

METHOD

Participants were surveyed for information about their SRT equipment, including the type of linear particle accelerator (linac), collimator settings, measurement conditions for the output factors and the detectors used. Participants had to report both the ratio of detector readings and the correction factors applied as described in the IAEA TRS-483 code of practice for nominal field sizes smaller or equal to 3 cm. Mean field output factors and their associated standard deviations were calculated when data from at least 3 linacs were available.

RESULTS

23 centres were enrolled in the project. Standard deviations of the mean field output factors were systematically smaller than 1.5% for field sizes larger or equal to 1 cm and reached 5% for the smallest field size (0.5 cm). Deviations with published data were smaller than 2% except for the 0.5 cm circular fixed aperture collimator of the CyberKnife where it reached 3.5%.

CONCLUSION

These field output factor values obtained via a large multicentre study can be considered as an external cross verification for any radiotherapy centre starting a SRT program and should help minimize systematic errors when determining small field output factors.

Effect of intensity-modulated radiation therapy on sciatic nerve injury caused by echinococcosis

Conventional radiotherapy has a good killing effect on femoral echinococcosis. However, the sciatic nerve around the lesion is irreversibly damaged owing to bystander effects. Although intensity-modulated radiation therapy shows great advantages for precise dose distribution into lesions, it is unknown whether intensity-modulated radiation therapy can perfectly protect the surrounding sciatic nerve on the basis of good killing of femoral echinococcosis foci. Therefore, this study comparatively analyzed differences between intensity-modulated radiation therapy and conventional radiotherapy on the basis of safety to peripheral nerves. Pure-breed Meriones meridiani with bilateral femoral echinococcosis were selected as the research object. Intensity-modulated radiation therapy was used to treat left femoral echinococcosis of Meriones meridianus, while conventional radiotherapy was used to treat right femoral echinococcosis of the same Meriones meridianus. The total radiation dose was 40 Gy. To understand whether intensity-modulated radiation therapy and conventional radiotherapy can kill femoral echinococcosis, trypan blue staining was used to detect pathological changes of bone Echinococcus granulosus and protoscolex death after radiotherapy. Additionally, enzyme histochemical staining was utilized to measure acid phosphatase activity in the protoscolex after radiotherapy. One week after radiotherapy, the overall structure of echinococcosis in bilateral femurs of Meriones meridiani treated by intensity-modulated radiation therapy disappeared. There was no significant difference in the mortality rate of protoscoleces of Echinococcus granulosus between the bilateral femurs of Meriones meridiani. Moreover, there was no significant difference in acid phosphatase activity in the protoscolex of Echinococcus granulosus between bilateral femurs. To understand the injury of sciatic nerve surrounding the foci of femoral echinococcosis caused by intensity-modulated radiation therapy and conventional radiotherapy, the ultrastructure of sciatic nerves after radiotherapy was observed by transmission electron microscopy. Additionally, apoptosis of neurons was examined using a terminal-deoxynucleotidyl transferase-mediated dUTP nick end labeling assay, and expression of Bcl-2 and Bax in sciatic nerve tissue was detected by immunohistochemical staining and western blot assay. Our results showed that most neurons in the left sciatic nerve of Meriones meridiani with echinococcosis treated by intensity-modulated radiation therapy had reversible injury, and there was no obvious apoptosis. Compared with conventional radiotherapy, the number of apoptotic cells and Bax expression in sciatic nerve treated by intensity-modulated radiation therapy were significantly decreased, while Bcl-2 expression was significantly increased. Our findings suggest that intensity-modulated radiation therapy has the same therapeutic effect on echinococcosis as conventional radiotherapy, and can reduce apoptosis of the sciatic nerve around foci caused by radiotherapy. Experiments were approved by the Animal Ethics Committee of People’s Hospital of Xinjiang Uygur Autonomous Region, China (Approval No. 20130301A41) on March 1, 2013.

Collection and analysis of photon beam data for Varian C-series linear accelerators: a potential reference beam data set

This study aimed to collect and analyze photon beam data for the Varian C-series linear accelerators (Varian Medical Systems, Palo Alto, CA, USA). We evaluated the potential of the average data to be used as reference beam data for the radiotherapy treatment planning system commissioning verification. We collected 20 data sets for 4 and 6 MV photon beams, and 40 data sets for a 10 MV photon beam generated by the Varian C-series machines, which contained the percent depth dose (PDD), off-center ratio (OCR), and output factor (OPF) from 20 institutions. The average for each of the data types was calculated across the 20 machines. Dose differences from the average for PDD at the dose fall-off region were less than 1.0%. Relative differences from the average for the OPF data were almost within 1.0% for all energies and field sizes. For OCR data in the flat regions, the standard deviation of the dose differences from the average was within 1.0%, excluding that of the 30 × 30 mm² field size being approximately 1.5%. For all energies and field sizes, the distance to agreement from the average in the OCR penumbra regions was less than 1.0 mm. The average data except for the small field size found in this study can be used as reference beam data for verifying users' commissioning results.

Characterization of a microSilicon diode detector for small-field photon beam dosimetry

This study characterized a new unshielded diode detector, the microSilicon (model 60023), for small-field photon beam dosimetry by evaluating the photon beams generated by a TrueBeam STx and a CyberKnife. Temperature dependence was evaluated by irradiating photons and increasing the water temperature from 11.5 to 31.3°C. For Diode E, microSilicon, microDiamond and EDGE detectors, dose linearity, dose rate dependence, energy dependence, percent-depth-dose (PDD), beam profiles and detector output factor (OFdet) were evaluated. The OFdet of the microSilicon detector was compared to the field output factors of the other detectors. The microSilicon exhibited small temperature dependence within 0.4%, although the Diode E showed a linear variation with a ratio of 0.26%/°C. The Diode E and EDGE detectors showed positive correlations between the detector reading and dose rate, whereas the microSilicon showed a stable response within 0.11%. The Diode E and microSilicon demonstrated negative correlations with the beam energy. The OFdet of microSilicon was the smallest among all the detectors. The maximum differences between the OFdet of microSilicon and the field output factors of microDiamond were 2.3 and 1.6% for 5 × 5 mm2 TrueBeam and 5 mm φ CyberKnife beams, respectively. The PDD data exhibited small variations in the dose fall-off region. The microSilicon and microDiamond detectors yielded similar penumbra widths, whereas the other detectors showed steeper penumbra profiles. The microSilicon demonstrated favorable characteristics including small temperature and dose rate dependence as well as the small spatial resolution and output factors suitable for small field dosimetry.

Inter-unit variability of multi-leaf collimator parameters for IMRT and VMAT treatment planning: a multi-institutional survey

Modern treatment machines have shown small inter-unit variability regarding beam data. Recently, vendor-provided average beam data, such as the Representative Beam Data (RBD) of the TrueBeam (Varian Medical Systems, Palo Alto, CA, USA), has been used for modeling of the Eclipse (Varian Medical Systems) treatment planning system. However, RBD does not provide multi-leaf collimator (MLC) parameters, such as MLC leaf transmission factor (LTF) and dosimetric leaf gap (DLG). We performed a web-based multi-institutional survey to investigate these parameters as well as the measurement protocols and customization of the parameters for intensity-modulated radiotherapy (IMRT) and/or volumetric modulated radiotherapy (VMAT) commissioning. We collected 69 sets of linear accelerator (linac) data from 58 institutions. In order to measure MLC parameters, most institutions used farmer-type ionization chambers with a sensitive volume of 0.6 cm3, water phantoms, source surface distance of 90 cm with 10 cm depth, and a vendor-provided plan. The LTF showed small inter-unit variabilities, although the DLG showed large variations. For optimization of the parameters for IMRT/VMAT calculations, DLG values were upwardly adjusted at many institutions, whereas the LTF values were modestly changed. We clarified that MLC parameters were measured under the same conditions at more than half of the facilities. Most institutions customized parameters in a similar manner for IMRT/VMAT. The median measured and customized values obtained in our study will be valuable to verify MLC installation accuracy and to shorten the iterative processes of finding the optimal values.

Small-field dosimetry of TrueBeamTM flattened and flattening filter-free beams: A multi-institutional analysis

PURPOSE

Detector-dependent interinstitutional variations of the beam data may lead to uncertainties of the delivered dose to patients. Here we evaluated the inter-unit variability of the flattened and flattening filter-free (FFF) beam data of multiple TrueBeam (Varian Medical Systems) linear accelerators focusing on the small-field dosimetry.

METHODS

The beam data of 6- and 10-MV photon beams with and without flattening filter measured for modeling of an iPLAN treatment planning system (BrainLAB) were collected from 12 institutions - ten HD120 Multileaf Collimator (MLC) and two Millennium120 MLC. Percent-depth dose (PDD), off-center ratio (OCR), and detector output factors (OFdet ) measured with different detectors were evaluated. To investigate the detector-associated effects, we evaluated the inter-unit variations of the OFdet before and after having applied the output correction factors provided by the International Atomic Energy Agency (IAEA) Technical Reports Series no. 483.

RESULTS

PDD measured with a field size of 5 × 5 mm2 showed that the data measured using an ionization chamber had variations exceeding 1% from the median values. The maximum difference from median value was 2.87% for 10 MV photon beam. The maximum variations of the penumbra width for OCR with 10 × 10 mm2 field size were 0.97 mm. The OFdet showed large variations exceeding 15% for a field size of 5 × 5 mm2 . When the output correction factors were applied to the OFdet , the variations were greatly reduced. The relative difference of almost all field output factors were within ± 5% from the median field output factors.

CONCLUSION

In this study, the inter-unit variability of small-field dosimetry was evaluated for TrueBeam linear accelerators. The variations were large at a field size of 5 × 5 mm2 , and most occurred in a detector-dependent manner.

Impact of detector selections on inter-institutional variability of flattening filter-free beam data for TrueBeam™ linear accelerators

This study evaluates the type of detector influencing the inter-institutional variability in flattening filter-free (FFF) beam-specific parameters for TrueBeam™ linear accelerators (Varian Medical Systems,Palo Alto, CA, USA). Twenty-four beam data sets, including the percent depth dose (PDD), off-center ratio (OCR), and output factor (OPF) for modeling within the Eclipse (Varian Medical Systems) treatment planning system, were collected from 19 institutions. Although many institutions collected the data using CC13 (IBA Dosimetry, Schwarzenbruck, Germany) or PTW31010 semiflex (PTW Freiburg, Freiburg, Germany) ionization chambers, some institutions used diode detectors, diamond detectors, and ionization chambers with smaller cavities. The OCR data included penumbra width, full width at half maximum (FWHM), and FFF beam-specific parameters, including unflatness and slope. The data measured by CC13/PTW31010 ionization chambers were compared with those measured by all other detectors. PDD data demonstrated the variations within ±1% at the dose fall-off region deeper than peak depth. The penumbra widths of the OCR measured with the CC13/PTW31010 detectors were significantly larger than those measured with all other detectors (P < 0.05). Especially the EDGE detector (Sun Nuclear Corp., Melbourne, FL, USA) and the microDiamond detectors (model 60019; PTW Freiburg) demonstrated much smaller penumbra values compared to those of the CC13/PTW31010 detectors for the 30 × 30 mm² field. There was no difference in the FWHM, unflatness, and slope parameters between the values for the CC13/PTW31010 detectors and all other detectors. OPF curves demonstrated small variations, and the relative difference from the mean value of each data point was almost within 1% for all field sizes. Although the penumbra region exhibited detector-dependent variations, all other parameters showed tiny interunit variations regardless of the detector type.

Feasibility of virtual starshot analysis providing submillimeter radiation isocenter accuracy: A long-term multi-institutional analysis

PURPOSE

We developed a technique to calculate the offset between room lasers and the radiation isocenter using a digital Winston-Lutz (WL) test with a starshot technique. We have performed isocenter localization quality assurance (QA) with submillimeter accuracy for a long period. Here we evaluated the feasibility and accuracy of this virtual starshot (VS) analysis for isocenter localization QA.

METHODS

A 6-MV photon beam with a square multileaf collimator field was used to irradiate a WL sphere positioned at the intersection of the room lasers. Images were acquired using an electronic portal imaging device. A four-field WL test was performed, and the path of each beam was calculated from the offset between the beam and sphere. Virtual starshot analysis was used to analyze the radiation isocenter, which calculates the center of the beam paths by using a least-squares method, similar to the starshot analysis. Then, eight coplanar and 12 noncoplanar beams were irradiated to evaluate isocenter localization accuracy.

RESULTS

Several VS analyses, using different WL spheres, were performed at three institutions, and the calculated accuracies were within 0.1 mm at all institutions. Long-term analysis showed that the isocenter localization accuracy was appropriately managed with three-dimensional accuracy within ± 0.5 mm for 90 months after the first laser adjustments. The offset between each beam and the room laser was within 0.6 mm and within 1.0 mm for eight coplanar and 12 noncoplanar beams, respectively, for 90 months. Cone-beam computed tomography images, acquired after verification beams, showed that the offset between the radiation isocenter and the imaging center was within 0.66 mm for 90 months. The isocenter localization accuracy within 1 mm was kept for long period at other four institutions.

CONCLUSIONS

Long-term analysis showed the feasibility of VS analysis for isocenter localization QA, including room laser re-alignment, noncoplanar irradiation verification, and image guidance accuracy.