B McCurdy

A two-step algorithm for predicting portal dose images in arbitrary detectors.

Recently, portal imagingsystems have been successfully demonstrated in dosimetric treatment verification applications, where measured and predicted images are quantitatively compared. To advance this approach to dosimetric verification, a two-step model which predicts dose deposition in arbitrary portal image detectors is presented. The algorithm requires patient CT data, source–detector distance, and knowledge of the incident beam fluence. The first step predicts the fluence entering a portal imaging detector located behind the patient. Primary fluence is obtained through ray-tracing techniques, while scatter fluence prediction requires a library of Monte Carlo-generated scatter fluence kernels. These kernels allow prediction of basic radiation transport parameters characterizing the scatteredphotons, including fluence and mean energy. The second step of the algorithm involves a superposition of Monte Carlo-generated pencil beam kernels, describing dose deposition in a specific detector, with the predicted incident fluence. This process is performed separately for primary and scatter fluence, and yields a predicted doseimage. A small but noticeable improvement in prediction is obtained by explicitly modeling the off-axis energy spectrum softening due to the flattening filter. The algorithm is tested on a slab phantom and a simple lung phantom (6 MV). Furthermore, an anthropomorphic phantom is utilized for a simulated lung treatment (6 MV), and simulated pelvis treatment (23 MV). Data were collected over a range of air gaps (10–80 cm). Detectors incorporating both low and high atomic number buildup are used to measure portal image profiles. Agreement between predicted and measured portal dose is better than 3% in areas of low dose gradient (<30%/cm) for all phantoms, air gaps, beam energies, and detector configurations tested here. It is concluded that this portal dose prediction algorithm is fast, accurate, allows separation of primary and scatterdose, and can model arbitrary detectors.

Photon scatter in portal images: physical characteristics of pencil beam kernels generated using the EGS Monte Carlo code.

Pencil beam kernels describing scattered photon fluence behind homogeneous water slabs at various air gap distances were generated using the EGS Monte Carlo code. Photon scatter fluence was scored in separate bins based on the particles history: singly scattered, multiply scattered, and bremsstrahlung and positron annihilation photons. Simultaneously, the mean energy and mean angle with respect to the incident photon pencil beam were tallied. Kernels were generated for incident photon pencil beams exhibiting monoenergetic spectra of 2.0 and 10.0 MeV, and polyenergetic spectra representative of 6 and 24 MV beams. Reciprocity was used to generate scatter fractions on the central axis for various field sizes, phantom thicknesses, and air gaps. The scatter kernels were further characterized by full width at half-maximum estimates. Modulation transfer functions were calculated, providing theoretical estimates of the limit of performance of portal imaging systems due to the intrinsic scattering of photon radiation through the patient.

Comprehensive fluence model for absolute portal dose image prediction

Amorphous silicon (a-Si) electronic portal imaging devices (EPIDs) continue to be investigated as treatment verification tools, with a particular focus on intensity modulated radiation therapy (IMRT). This verification could be accomplished through a comparison of measured portal images to predicted portal dose images. A general fluence determination tailored to portal dose image prediction would be a great asset in order to model the complex modulation of IMRT. A proposed physics-based parameter fluence model was commissioned by matching predicted EPID images to corresponding measured EPID images of multileaf collimator (MLC) defined fields. The two-source fluence model was composed of a focal Gaussian and an extrafocal Gaussian-like source. Specific aspects of the MLC and secondary collimators were also modeled (e.g., jaw and MLC transmission factors, MLC rounded leaf tips, tongue and groove effect, interleaf leakage, and leaf offsets). Several unique aspects of the model were developed based on the results of detailed Monte Carlo simulations of the linear accelerator including (1) use of a non-Gaussian extrafocal fluence source function, (2) separate energy spectra used for focal and extrafocal fluence, and (3) different off-axis energy spectra softening used for focal and extrafocal fluences. The predicted energy fluence was then convolved with Monte Carlo generated, EPID-specific dose kernels to convert incident fluence to dose delivered to the EPID. Measured EPID data were obtained with an a-Si EPID for various MLC-defined fields (from 1 x 1 to 20 x 20 cm2) over a range of source-to-detector distances. These measured profiles were used to determine the fluence model parameters in a process analogous to the commissioning of a treatment planning system. The resulting model was tested on 20 clinical IMRT plans, including ten prostate and ten oropharyngeal cases. The model predicted the open-field profiles within 2%, 2 mm, while a mean of 96.6% of pixels over all IMRT fields was in agreement with the 2%, 3 mm criteria. This model demonstrates accuracy commensurate to existing methods for IMRT pretreatment verification with portal dose image prediction of complex clinical examples (< 2%, 3 mm).

Photon scatter in portal images: Accuracy of a fluence based pencil beam superposition algorithm

The accuracy of a pencil beam algorithm to predict scattered photon fluence into portal imaging systems was studied. A data base of pencil beam kernels describing scattered photon fluence behind homogeneous water slabs (1-50 cm thick) at various air gap distances (0-100 cm) was generated using the EGS Monte Carlo code. Scatter kernels were partitioned according to particle history: singly-scattered, multiply-scattered, and bremsstrahlung and positron annihilation photons. Mean energy and mean angle with respect to the incident photon pencil beam were also scored. This data allows fluence, mean energy, and mean angular data for each history type to be predicted using the pencil beam algorithm. Pencil beam algorithm predictions for 6 and 24 MV incident photon beams were compared against full Monte Carlo simulations for several inhomogeneous phantoms, including approximations to a lateral neck, and a mediastinum treatment. The accuracy of predicted scattered photon fluence, mean energy, and mean angle was investigated as a function of air gap, field size, photon history, incident beam resolution, and phantom geometry. Maximum errors in mean energies were 0.65 and 0.25 MeV for the higher and lower energy spectra, respectively, and 15 degrees for mean angles. The ability of the pencil beam algorithm to predict scatter fluence decreases with decreasing air gap, with the largest error for each phantom occurring at the exit surface. The maximum predictive error was found to be 6.9% with respect to the total fluence on the central axis. By maintaining even a small air gap (approximately 10 cm), the error in predicted scatter fluence may be kept under 3% for the phantoms and beam energies studied here. It is concluded that this pencil beam algorithm is sufficiently accurate (using International Commission on Radiation Units and Measurements Report No. 24 guidelines for absorbed dose) over the majority of clinically relevant air gaps, for further investigation in a portal dose prediction algorithm.

Automatic conformal prescription of very selective saturation bands for in vivo1H-MRSI of the prostate

An important step in the implementation of three‐dimensional in vivo proton magnetic resonance spectroscopic imaging (1H‐MRSI) of the prostate is the placement of spatial saturation pulses around the region of interest (ROI) for the removal of unwanted contaminating signals from peripheral tissue. The present study demonstrates the use of a technique called conformal voxel magnetic resonance spectroscopy (CV‐MRS). This method automates the placement, orientation, timing and flip angle of very selective saturation (VSS) pulses around an irregularly‐shaped, user‐defined ROI. The method employs a user adjustable number of automatically positioned VSS pulses (20 used in the present study) which null the signal from periprostatic lipids while closely conforming the shape of the excitation voxel to the shape of the prostate. A standard endorectal coil in combination with a torso‐phased array coil was used for all in vivo prostate studies. Three‐dimensional in vivo prostate 1H‐MRSI data were obtained using the proposed semi‐automated CV‐MRS technique, and compared with a standard point resolved spectroscopy (PRESS) technique at TE = 130 ms using manual placement of saturation pulses. The in vivo prostate 1H‐MRSI data collected from 12 healthy subjects using the CV‐MRS method showed significantly reduced lipid contamination throughout the prostate, and reduced baseline distortions. On average there was a 50 ± 17% (range 12% – 68%) reduction in lipids throughout the prostate. A voxel‐by‐voxel benchmark test of over 850 voxels showed that there were 63% more peaks fitted using the LCModel when using a Cramer‐Rao Lower Bound (CRLB) cut‐off of 40% when using the optimized conformal voxel technique in comparison to the manual placement approach. The evaluation of this CV‐MRS technique has demonstrated the potential for easy automation of the graphical prescription of saturation bands for use in 1H‐MRSI. Copyright

SU-GG-T-150: Commissioning and Validation of a Novel Measurement-Based IMRT QA Method, Incorporating Dose Recalculation On Patient CT Data

Purpose: A novel measurement‐based IMRT QA method was tested which provides an accurate reconstruction of the 3D dose distribution in the patient model. This approach is a significant improvement over current QA methods since it allows direct and independent comparison of the doses calculated by the treatment planning system (TPS), including the 3D spatial dose distribution overlaid on CT data and contoured structures, as well as DVHs. Method and Materials: The challenging RPC Head and Neck phantom was used for initial evaluation. A 6 MV, 7 field, 79 segment, step and shoot plan was developed satisfying required dose metrics. A 2D‐array of dose chambers (MatriXX, IBA Dosimetry) was mounted on a linear accelerator. This device captured the delivered IMRT plan fluence in a pretreatment QA context. The measurement data were read directly by the control software (COMPASS, IBA Dosimetry), which also provides the ability to import patient plan data from the TPS. The COMPASS software also includes a dose calculation engine and head fluence model. Beam commissioning procedures analogous to those of a TPS were required. Reconstructed dose and DVHs were compared to those calculated by the TPS. Results: The beam model in the COMPASS software was able to predict percentage depth dose and X and Y profiles (Dmax, 5, 10, 20 cm depths) for MLC‐defined apertures ranging from 1×1–20×20 cm∧2 to within 1.5% (percentage depth‐dose), 2.0% (in‐field profiles), and 2.5% (out‐of‐field profiles). The reconstructed doses in the RPC Head & Neck phantom were within −3 to +4% of those in the treatment planning system. DVHs compared to within 1%. Conclusion: A novel measurement‐based IMRT QA method was tested. Reconstructed doses were overlaid on CT data and contoured structures, to enable a clinically relevant understanding of delivered under‐ or over‐doseages as compared to the TPS plan. Research partially sponsored by IBA Dosimetry.

A model-based 3D patient-specific pre-treatment QA method for VMAT using the EPID

This study reports the development and validation of a model-based, 3D patient dose reconstruction method for pre-treatment quality assurance using EPID images. The method is also investigated for sensitivity to potential MLC delivery errors. Each cine-mode EPID image acquired during plan delivery was processed using a previously developed back-projection dose reconstruction model providing a 3D dose estimate on the CT simulation data. Validation was carried out using 24 SBRT-VMAT patient plans by comparing: (1) ion chamber point dose measurements in a solid water phantom, (2) the treatment planning system (TPS) predicted 3D dose to the EPID reconstructed 3D dose in a solid water phantom, and (3) the TPS predicted 3D dose to the EPID and our forward predicted reconstructed 3D dose in the patient (CT data). AAA and AcurosXB were used for TPS predictions. Dose distributions were compared using 3%/3 mm (95% tolerance) and 2%/2 mm (90% tolerance) γ-tests in the planning target volume (PTV) and 20% dose volumes. The average percentage point dose differences between the ion chamber and the EPID, AcurosXB, and AAA were 0.73  ±  1.25%, 0.38  ±  0.96% and 1.06  ±  1.34% respectively. For the patient (CT) dose comparisons, seven (3%/3 mm) and nine (2%/2 mm) plans failed the EPID versus AAA. All plans passed the EPID versus Acuros XB and the EPID versus forward model γ-comparisons. Four types of MLC sensitive errors (opening, shifting, stuck, and retracting), of varying magnitude (0.2, 0.5, 1.0, 2.0 mm), were introduced into six different SBRT-VMAT plans. γ-comparisons of the erroneous EPID dose and original predicted dose were carried out using the same criteria as above. For all plans, the sensitivity testing using a 3%/3 mm γ-test in the PTV successfully determined MLC errors on the order of 1.0 mm, except for the single leaf retraction-type error. A 2%/2 mm criteria produced similar results with two more additional detected errors.

Determination of equivalent photon fields through integrated 1D convolution kernels.

The equivalent fields concept has been studied using a variation of the convolution dose calculation method. This allows a better understanding of this classic, widely-used concept and its relation to modern dose calculation techniques. Total scatter energy contribution as a function of field size was computed by radially integrating three-dimensional point interaction scatter kernels representing cobalt-60, 6 MV and 24 MV photon beam spectra and utilizing reciprocity. For arbitrarily shaped treatment fields, equivalent square or circular fields are chosen based on a conservation of energy approach. Associated depth dose curves are generated via convolution of primary fluence with pre-integrated scatter kernels. For the energy spectra used, the resulting equivalent circle to square relationships agree with the empirical equation recommended in the British Journal of Radiology Supplement 25 (BJR 25) (to within 2%). Tables converting rectangular fields to equivalent squares and circles have been generated and most (approximately 63%) of the equivalent square (and circular) field sizes are within 0.5 cm of the BJR 25 tables. This agreement provides an explanation for the successful use of a single relationship for all therapeutic energies. Fields of different shape will be approximately equivalent only when the scatter kernels are similar in shape and total energy content. The equivalent field concept inevitably results in an optimal match of depth dose curves at a single depth only. These ideas are demonstrated for a highly elongated (2 x 30 cm2) field and its equivalent square field. The invalidity of the equivalent fields concept off-axis is demonstrated by examining predicted dose deposition for several points in and around a 15 x 15 cm2 cobalt-60 beam. Modern convolution techniques are shown to offer a unique approach to the classic equivalent fields method. The appropriate use of the BJR 25 equivalent fields tables is recommended for all photon energies.

SU‐E‐T‐210: Precise Gantry Angle Determination for EPID Images during Rotational IMRT

Purpose: Utilization of an aSi EPID to develop an in vivo patient dose verification system for rotational IMRT (rIMRT) delivery requires accurate knowledge of gantry angle as a function of time. Currently the accuracy of the gantry angle stamp in the header of the EPIDimage is limited to approximately +/−3 degrees. This work investigates several unique methods for a more accurate determination of the gantry angle during rIMRT. Methods: Gantry angles were determined using: (1) an incremental rotary encoder attached to the rotational axis of the gantry, (2) a direct analogue‐to‐digital measurement of the gantry potentiometer, and (3) through EPIDimage analyses of an in‐house phantom (manufactured at sub‐millimeter precision). The phantom consists of a cylindrical acrylic frame with one wire wrapped helically around its surface and one straight wire traversing its central axis. This design creates EPIDimages with unique and identifiable wire intersection points as a function of gantry orientation. Analysis of the treatment console log files was compared to the above methods. Results: The gantry potentiometer is considered the most accurate gantry angle but is unavailable during treatment. The ClinacLog produced discrepancies of up to ±2 degrees, the DynaLog up to ±1 degrees, and the encoder up to ±0.5 degrees with respect to the potentiometer. Preliminary analysis comparing our phantom‐determined gantry angles with the encoder gantry angles showed agreement within ±0.5 degrees of each other for 85% of the data and differed at most by 1.3 degrees from each other. Conclusions: We have developed several techniques to determine gantry angle as a function of time during rIMRT. We have shown a strong agreement in gantry determination by our phantom and encoder. This investigation of gantry angle is critical to develop an accurate in vivo patient dose verification system for rIMRT delivery.

Poster — Thur Eve — 51: An Investigation of Geometry Issues for EPID Dosimetry during Rotational IMRT

INTRODUCTION: Amorphous‐silicon electronic portal imaging devices(EPIDs) have been established as useful tools for dosimetry. To accurately reconstruct the patient dose delivered during rotational IMRT, one must acquire time‐resolved EPIDimages as a function of gantry‐angle. Dose reconstruction accuracy is directly impacted by the accuracy of the geometry of the imaging system, including the gantry‐angle readout (i.e. source geometry) and the EPID support‐arm sag (i.e. imager geometry). This work investigates these two factors. METHODS: The EPID support‐arm sag was investigated through measurements performed on Varian E‐arm and R‐arm models at two institutes and employing two different analysis methods. One method imaged an isocentric ball‐bearing whose position was tracked over all gantry‐angles. The second method involved analysing field edges to obtain the field centre location of all images. Gantry‐angle accuracy was examined by comparing the gantry‐angle indicated at the treatment console readout to the gantry‐angle written to the EPID DICOM header. We developed a method of measuring gantry‐angle directly from the gantry‐angle potentiometer. RESULTS: The E‐arm showed maximum displacement of roughly 0.6mm (cross‐plane) and 0.8mm (in‐plane). R‐arm results were significantly worse, estimated at 8.5mm (cross‐plane) and 5.0mm (in‐plane). Gantry‐angle analysis demonstrated approximately 2 degrees of uncertainty in the gantry‐angle contained in the EPIDimage. A direct measurement of the gantry angle potentiometer was demonstrated. CONCLUSIONS: Two main factors affecting patient dose reconstruction using EPIDdosimetry have been investigated. EPID support‐arm sag can be measured (and corrected). Near real‐time gantry‐angle measurement can be performed through directly monitoring the potentiometer signal.