Characterization and evaluation of 2.5 MV electronic portal imaging for accurate localization of intra- and extracranial stereotactic radiosurgery

A convolution-superposition fluence model for the Siemens HD120 multi leaf collimator with application to a 3D VMAT dose engine

Purpose. To construct a fast-calculating fluence modelfor the Siemens HD120 multi leaf collimator (MLC) using convolution-superposition techniques, and to develop a 3D VMAT dose engine using this fluence model.  This work offers analternative to time-consuming open-source Monte Carlo simulations for thosedeveloping in-house dose-calculating software for research or clinical needs.Methods. EPID-acquired images of sweeping-window and sweeping-checker field profiles were used to commission transmission, 2 Dinterleaf leakage, and tongue-and-groove maps specific to the HD120 MLC.  These maps, along with a 2D head-scattermodel were incorporated into a convolution-superposition algorithm to provide a fluence model for the HD120 MLC. This fluence model was used to develop a 3D VMAT dose engine, where 3D pre-computed 6MV dose kernels (EGSnrc) and a 3D fluence curvature-correction map were incorporatedto calculate 3D VMAT doses in a 22 cm diameter cylindrical phantom. Four VMAT patient plans witha large range of PTV sizes (36 cc to 604 cc) were chosen to test the fluence model and dose engine.Results. Excellent agreement was observed between the simulated commissioning fields and measured EPID-responses. 2D 2%/2 mm gamma analysis yielded a 98.9% pass rate for 1 cm, 2 cm, and 4 cm sweeping-window fields. 2D 2%/2mm gamma analysis for outer/inner MLC leaves yielded 89.1%/77.0% and 95.2%/91.1% pass rates from 1 cm and 2 cm sweeping-checker fields. Mean 3%/3 mm gamma analysis showed excellent agreement between our dose engine and Eclipse (Acuros) regardless of PTV size: 98.7% pass rate, with 95.1% pass rate in the high-dose volume.  Fluence calculation times were13.6 seconds per dynamic MLC field and 1.4 minutes/arc for 3D VMAT dose on a standard PC. Conclusions. A fast-calculating convolution-superposition fluence model has been commissioned for the Siemens HD120 MLC and incorporatedinto a 3D VMAT dose engine.  This work can be used to facilitate the development of fast in-house dose-calculating software.

The effects of different photon beam energies in stereotactic radiosurgery with cones

BACKGROUND

Stereotactic radiosurgery (SRS) relies on small fields to ablate lesions. Currently, linac based treatment is delivered via circular cones using a 6 MV beam. There is interest in both lower energy photon beams, which can offer steeper dose fall off as well as higher energy photon beams, which have higher dose rates, thus reducing radiation delivery times. Of interest in this study is the 2.5 MV beam developed for imaging applications and both the 6 and 10 MV flattening-filter-free (FFF) beams, which can achieve dose rates up to 2400 cGy/min.

PURPOSE

This study aims to assess the benefit and feasibility among different energy beams ranging from 2.5 to 10 MV beams by evaluating the dosimetric effects of each beam and comparing the dose to organs-at-risk (OARs) for two separate patient plans. One based on a typical real patient tremor utilizing a 4 mm cone and the other a typical brain metastasis delivered with a 10 mm cone.

METHODS

The Monte Carlo codes BEAMnrc/DOSXYZnrc were used to generate beams of 2.5 MV, 6 MV-FFF, 6 MV-SRS, 6 MV, 10 MV-FFF, and 10 MV from a Varian TrueBeam except 6 MV-SRS, which is taken from a Varian TX model linear accelerator. Each beam's energy spectrum, mean energy, %dd curve, and dose profile were obtained by analyzing the simulated beams. Calculated patient dose distributions were compared among six different energy beam configurations based on a realistic treatment plan for thalamotomy and a conventional brain metastasis plan. Dose to OARs were evaluated using dose-volume histograms for the same target dose coverage.

RESULTS

The mean energies of photons within the primary beam projected area were insensitive to cone sizes and the values of percentage depth-dose curves (%dd) at d = 5 cm and SSD = 95 cm for a 4 mm (10 mm) cone ranges from 62.6 (64.4) to 82.2 (85.7) for beam energy ranging from 2.5 to 10 MV beams, respectively. Doses to OARs were evaluated among these beams based on real treatment plans delivering 15 000 and 2200 cGy to the target with a 4 and 10 mm cone, respectively. The maximum doses to the brainstem, which is 10 mm away from the isocenter, was found to be 434 (300), 632 (352), 691 (362), 733 (375), 822 (403), and 975 (441) cGy for 2.5 MV, 6 MV-FFF, 6 MV-SRS, 6 MV, 10 MV-FFF, and 10 MV beams delivering 15 000 (2200) cGy target dose, respectively.

CONCLUSION

Using the 6 MV-SRS as reference, changes of the maximum dose (691 cGy) to the brain stem are -37%, -9%, +6%, +19%, and 41% for 2.5 MV, 6 MV-FFF, 6 MV, 10 MV-FFF, and 10 MV beams, respectively, based on the thalamotomy plan, where the "-" or "+" signs indicate the percentage decrease or increase. Changes of the maximum dose (362 cGy) to brain stem, based on the brain metastasis plan are much less for respective beam energies. The sum of 21 arcs beam-on time was 39 min on our 6 MV-SRS beam with 1000 cGy/min for thalamotomy. The beam-on time can be reduced to 16 min with 10 MV-FFF.

A step closer to automation: kilovoltage and Megavoltage Planar Imaging Quality Assurance, baseline, tolerance and action levels definition and exploration

PURPOSE

To establish automated quality assurance (QA) procedures for the kilovoltage (kV) and the Megavoltage (MV) imagers of two linear accelerators (LINACS) using a commercial software.

METHODS

SNC Machine™ phantoms and software were used and the baseline values, tolerance and action levels for various image quality parameters were defined. Scaling, spatial resolution, contrast, uniformity and noise were considered, explored and evaluated utilizing the appropriate phantoms and the accompanying software. kV and MV planar radiographic images, for 6MV and 10MV beams were obtained for each LINAC. For both kV and MV QA tasks, the baseline values for spatial resolution, contrast, uniformity and noise were defined.

RESULTS

Subsequent measurements performed were highly reproducible and within tolerance and action levels, while noise showed variations. The calculated tolerance and action levels for noise were looser compared to the other image quality metrics.

CONCLUSIONS

An automated QA workflow of the kV and MV planar radiographic mode of LINAC imagers' was established and appears to be time effective.

Stability and reproducibility comparisons between deep inspiration breath-hold techniques for left-sided breast cancer patients: A prospective study

PURPOSE

Deep inspiration breath-hold (DIBH) is crucial in reducing the lung and cardiac dose for treatment of left-sided breast cancer. We compared the stability and reproducibility of two DIBH techniques: Active Breathing Coordinator (ABC) and VisionRT (VRT).

MATERIALS AND METHODS

We examined intra- and inter-fraction positional variation of the left lung. Eight left-sided breast cancer patients were monitored with electronic portal imaging during breath-hold (BH) at every fraction. For each patient, half of the fractions were treated using ABC and the other half with VRT, with an equal amount starting with either ABC or VRT. The lung in each portal image was delineated, and the variation of its area was evaluated. Intrafraction stability was evaluated as the mean coefficient of variation (CV) of the lung area for the supraclavicular (SCV) and left lateral (LLat) field over the course of treatment. Reproducibility was the CV for the first image of each fraction. Daily session time and total imaging monitor units (MU) used in patient positioning were recorded.

RESULTS

The mean intrafraction stability across all patients for the LLat field was 1.3 ± 0.7% and 1.5 ± 0.9% for VRT and ABC, respectively. Similarly, this was 1.5 ± 0.7% and 1.6 ± 0.8% for VRT and ABC, respectively, for the SCV field. The mean interfraction reproducibility for the LLat field was 11.0 ± 3.4% and 14.9 ± 6.0% for VRT and ABC, respectively. Similarly, this was 13.0 ± 2.5% and 14.8 ± 9% for VRT and ABC, respectively, for the SCV. No difference was observed in the number of verification images required for either technique.

CONCLUSIONS

The stability and reproducibility were found to be comparable between ABC and VRT. ABC can have larger interfractional variation with less feedback to the treating therapist compared to VRT as shown in the increase in geometric misses at the matchline.

Measurement of dose exposure of image guidance in external beam accelerated partial breast irradiation: Evaluation of different techniques and linear accelerators

INTRODUCTION

Verifying the patient position is always an essential part of the treatment process, especially in hypofractionated treatments such as accelerated partial breast irradiation (APBI). The purpose of the study was to compare five image guidance techniques with respect to imaging dose and image quality.

METHODS AND MATERIALS

We chose five types of imaging methods applicable for APBI and measured their dose exposure on four different accelerators (Synergy, TrueBeam, Artiste and CyberKnife). Absorbed dose was measured with ionization chamber in thorax phantom. Besides dose exposure image quality was also compared.

RESULTS

The lowest dose exposure was measured with kV-kV planar imaging followed by kV-CBCT, MV-MV pair and MV-CBCT in ascending order. Average phantom dose with kV-kV image pair on CyberKnife was 0.01 cGy as the lowest and with MV-CBCT on Artiste was 7.11 cGy as the highest. Average dose exposures of MV-MV images with TrueBeam, Synergy and Artiste were 1.18 cGy, 2.13 cGy and 1.61 cGy, respectively, with similar image quality. For the same machines the doses of kV-CT imaging were comparable: 0.65 cGy, 0.65 cGy and 0.52 cGy, with some differences in image quality. MV-CBCT technique resulted in the highest dose and poorest image quality.

CONCLUSIONS

In APBI the position of the patient and tumour bed can be verified with many tools. When fiducials are available, often 2D imaging is enough to achieve appropriate positioning and the kV-kV method is recommended. Imaging with 2.5MV can also be a good solution instead of 6MV. Without fiducials 3D images should be acquired and the recommended method is the kV-CBCT.

Characteristics of 2.5 MV beam and imaging dose to patients

PURPOSE

This work provides the beam characteristics and evaluates the imaging dose to patients for a 2.5 MV portal imaging beam.

METHOD AND MATERIALS

The Monte Carlo technique has been used to simulate the 2.5 MV imaging beam. Beam characteristics have been analyzed including the energy spectra and the fluence distributions as a function of position away from the beam central axis. The accuracy of a simulated beam was validated through comparisons between the Monte Carlo calculated and measured dose distributions in a water phantom. The simulated 2.5 MV beam was also used to obtain the absorbed-dose beam quality conversion factor, k_Q, for absorbed dose calibration. The simulated beams were then used to evaluate the imaging dose to patients compared with that from a conventional therapeutic 6 MV beam.

RESULTS

The mean energies of photons and electrons in the 2.5 MV beam are 0.48 MeV and 0.37 MeV respectively. The photon fluence decreases at 20 cm away from the central axis by only up to 30% for this flattening-filter free beam. The values of %dd curves at depth = 10 cm are 53% and 63% for 10 × 10 cm² and 40 × 40 cm² fields respectively. Portal imaging doses (D50 of the DVHs) to the eyes, heart and bladder from representative pairs of 2.5 MV (or 6 MV) setup images are 1.8 cGy (3.5 cGy), 1.1 cGy (2.5 cGy) and 1.0 cGy (2.4 cGy) for head, thorax and pelvis image acquisitions respectively.

CONCLUSION

We provide dosimetric data, as well as estimates of organ imaging doses, for this 2.5 MV beam. When clinical default imaging protocols are used, the imaging dose from the 2.5 MV beam is about 50% of that from a 6 MV beam. The information can be used to select image procedures and to estimate organ dose from imaging procedures.