Treatment Planning Dose Calculations Research Articles

Profile Evaluation for Carbon Pencil Beam in Matter by Selective Acquisition of Nuclear Fragments

Profile Evaluation for Carbon Pencil Beam in Matter by Selective Acquisition of Nuclear Fragments

Verification Of A Novel Pencil-beam Based Dose Calculation Algorithm With Heterogeneity Correction For Situations Of Lateral Electron Disequilibrium And Comparison With Monte Carlo Calculations

Purpose/Objective(s): Pencil-beam (PB) based dose calculation for treatment planning is limited by inaccuracies in regions of tissue inhomogeneities, particularly in situations with lateral electron disequilibrium as is present at tissue/lung interfaces. To overcome these limitations, a new ’’lateral disequilibrium inclusive’’ (LDI) PB based dose calculation algorithm was introduced. In this study we evaluated the accuracy of the new model by film and ionization chamber measurements and Monte Carlo simulations. Materials/Methods: To validate the performance of the new LDI algorithm implemented in Corvus 09, eight test plans were generated on inhomogeneous thorax and pelvis phantoms. In addition, three plans were calculated with a simple effective path length (EPL) algorithm on the inhomogeneous thorax phantom. To simulate homogeneous tissues, four test plans were evaluated in homogeneous phantoms (Homogeneous dose calculation (HOM)). Results:Themeanpixelpassrates andstandarddeviationsofthegamma4%/4mmtestfor thefilmmeasurementswere(96±3)% for the plans calculated with LDI, (70 ± 5)% for the plans calculated with EPL and (99 ± 1)% for the homogeneous plans. Ionization chamber measurements and Monte Carlo simulations confirmed the high accuracy of the new algorithm (dose deviations smaller or equal 4%, gamma 3%/3mm pass rates larger or equal 96%).

Comparison of bulk electron density and voxel‐based electron density treatment planning

The use of magnetic resonance imaging (MRI) alone for radiation planning is limited by the lack of electron density for dose calculations. The purpose of this work is to evaluate the dosimetric accuracy of using bulk electron density as a substitute for computed tomography (CT)‐derived electron density in intensity‐modulated radiation therapy (IMRT) treatment planning of head and neck (HN) cancers. Ten clinically‐approved, CT‐based IMRT treatment plans of HN cancer were used for this study. Three dose distributions were calculated and compared for each treatment plan. The first calculation used CT‐derived density and was assumed to be the most accurate. The second calculation used a homogeneous patient density of 1 g/cm3. For the third dose calculation, bone and air cavities were contoured and assigned a uniform density of 1.5 g/cm3 and 0 g/cm3, respectively. The remaining tissues were assigned a density of 1 g/cm3. The use of homogeneous anatomy resulted in up to 4%–5% deviations in dose distribution as compared to CT‐derived electron density calculations. Assigning bulk density to bone and air cavities significantly improved the accuracy of the dose calculations. All parameters used to describe planning target volume coverage were within 2% of calculations based on CT‐derived density. For organs at risk, most of the parameters were within 2%, with the few exceptions located in low‐dose regions. The data presented here show that if bone and air cavities are overridden with the proper density, it is feasible to use a bulk electron density approach for accurate dose calculation in IMRT treatment planning of HN cancers. This may overcome the problem of the lack of electron density information should MRI‐only simulation be performed.PACS number: 87.55.D‐

SU‐D‐220‐03: Inter‐ and Intra‐Fraction Verification with Integrated On‐Board Xray CBCT and 3D Ultrasound Imaging: A Feaibility Study

Purpose: CBCT and 3D Ultrasound (3DUS) provide complementary information on bony landmarks and soft tissues, particularly for abdominal sites. Furthermore, 3DUS can potentially track intra‐fractional motion of soft tissues. In this study, we investigate the feasibility of integrating these two modalities to provide a comprehensive image guidance procedure that corrects for both inter‐and intra‐fractional motion. Methods: The ultrasound probe will be operated under robotic control to enhance reproducible placement. We investigated the effect of potential CT artifacts caused by the probe and the subsequent effect on treatment plan dose calculation. We then measured the effect on ultrasound image quality due to an 18 MV radiation beam in close proximity to the probe. A phantom was constructed by imbedding markers in gel. The reproducibility of the marker positions under controlled repeat compressions, mimicking robotic placement, was measured. The effect of removing a beam to allow probe placement during treatment was studied by comparing optimized liver plans with or without the specific beam. Results: CT artifacts associated with probe placement affected treatment planning dose by 20%. The artifacts can be avoided with a CT‐compatible model probe. Radiation dose delivered 1 cm from the probe did not affect the ultrasound image quality. When the same compression was used to compress the gel phantom, the positions of the fiducial markers were reproducible to within 0.3 mm. The reoptimized liver plan with the omission of one beam was not noticeably different from the original plan. Conclusions: On‐board x‐ray CBCT is highly effective for patient setup based on bony landmark. 3DUS is a non‐invasive, non‐ionizing means to continuously monitor soft tissue motion. Our studies indicate that it is feasible to combine both systems during treatment. Their integration, enhanced by robotic repositioning, provides a highly complementary approach for IGRT of soft tissue sites in the abdomen.Martin Lachaine is the Director of Research at Resonant Medical.

SU-E-T-736: Dosimetric Evaluation with Heterogeneity in Eclipse Treatment Planning Dose Calculation Algorithms

Purpose: Dose calculation accuracy utilizing anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) with multiple inhomogeneous phantoms was evaluated with measurements. Methods: A five layer virtual phantom was created in the Eclipse planning system. Each 5 cm layer was assigned in terms of water, air, water, bone, and water materials. Plans with a 6MV beam in different field sizes were calculated in AAA_8615 (with heterogeneity correction) and PBC_8615 (with and without heterogeneity correction). Combinations of solid water, PVC, and Styrofoam were then manufactured to acquire doses with parallel plate chamber and EDR2 films. Next, a seven field IMRT plan in PBC without heterogeneity was created within a Rando phantom. A cylindrical target volume was defined and 95% target was covered by prescription doses. Plans were then regenerated by recalculating this optimized base plan with keeping identical beam parameters but switching algorithms. The base plan was delivered to the phantom with TLDs and films. Measurements were compared with all calculations at same locations. Results: AAA shows a better dose prediction in all materials from measurements (Fig. 1–Fig.4). Discrepancies in air and bone regions due to lacks of lateral scatters and beam hardening were observed. In IMRT, both PBC with heterogeneity and AAA mean calculated target doses were <1% compared to TLD measurements; and PBC without heterogeneity values were lower by 11%. AAA produced better matches in profiles (Fig.5) as well. Conclusions: The findings suggest that AAA is more appropriate in dose predictions especially when lung and bone are involved.

Accurate calibration of a polymer gel dosimeter with a plastic scintillation detector

Three dimensional dose polymer gel dosimetry measurements provide unique information on sophisticated dose distributions. In this study, the authors propose a novel method to improve the accuracy of polymer gel dosimeters by inserting a plastic scintillation detector (PSD) to provide a dose reference. PSD dosimeters were calibrated using chromatic deconvolution and then inserted into polyacrylanide gel (PAG) dosimeters. The gel and the PSDs were immersed into water and irradiated with 6 MV wedge filtered beams to obtain a wide range of dose variation. Calibration vials containing the same gel were also irradiated to generate a standard calibration curve. The distribution of magnetic nuclear transverse relaxation rate (R2) values of the gel was determined with a multislice multiecho MRI sequence at 1.5 T. Another calibration curve was obtained by assigning the R2 values in the gel surrounding the scintillators to the dose determined by the PSDs. A reference calibration point from a PSD located in a low dose gradient area served to correct the standard calibration method yielding three novel calibration methods. The results were compared with EBT2 GAFCHROMIC film measurements acquired in the same condition and with the Pinnacle3 treatment planning dose calculations, The mean absolute error of the standard calibration method ranged from 6.1 to 12.4%. The corresponding gamma index (3%/3 mm distance to agreement) criterion was satisfied for only 56% of the pixels in the middle slice of the gel compared to Pinnacle3 dose calculations and to EBT2 film measurements in the center part of the field. Calibration methods using a PSD reduced the mean absolute error to less than 4%; this value was under 2.6% for one of the three methods. In that case, 98% of the pixels satisfied the gamma index criterion. The accuracy of PAG dosimeters may be highly improved using one reference dose point measurement using a plastic scintillation detector. The best calibration procedure corrected the slope of the calibration curve derived from the calibration vials to match the R2 value around a PSD calibration, while keeping the R2 value at 0 Gy constant.

Dose specification for radiation therapy: dose to water or dose to medium?

The Monte Carlo method enables accurate dose calculation for radiation therapy treatment planning and has been implemented in some commercial treatment planning systems. Unlike conventional dose calculation algorithms that provide patient dose information in terms of dose to water with variable electron density, the Monte Carlo method calculates the energy deposition in different media and expresses dose to a medium. This paper discusses the differences in dose calculated using water with different electron densities and that calculated for different biological media and the clinical issues on dose specification including dose prescription and plan evaluation using dose to water and dose to medium. We will demonstrate that conventional photon dose calculation algorithms compute doses similar to those simulated by Monte Carlo using water with different electron densities, which are close (<4% differences) to doses to media but significantly different (up to 11%) from doses to water converted from doses to media following American Association of Physicists in Medicine (AAPM) Task Group 105 recommendations. Our results suggest that for consistency with previous radiation therapy experience Monte Carlo photon algorithms report dose to medium for radiotherapy dose prescription, treatment plan evaluation and treatment outcome analysis.

Volumetric-Modulated Arc Therapy: Effective and Efficient End-to-End Patient-Specific Quality Assurance

To explore an effective and efficient end-to-end patient-specific quality-assurance (QA) protocol for volumetric modulated arc radiotherapy (VMAT) and to evaluate the suitability of a stationary radiotherapy QA device (two-dimensional [2D] ion chamber array) for VMAT QA. Three methods were used to analyze 39 VMAT treatment plans for brain, spine, and prostate: ion chamber (one-dimensional absolute, n = 39), film (2D relative, coronal/sagittal, n = 8), and 2D ion chamber array (ICA, 2D absolute, coronal/sagittal, n = 39) measurements. All measurements were compared with the treatment planning system dose calculation either via gamma analysis (3%, 3- to 4-mm distance-to-agreement criteria) or absolute point dose comparison. The film and ion chamber results were similarly compared with the ICA measurements. Absolute point dose measurements agreed well with treatment planning system computed doses (ion chamber: median deviation, 1.2%, range, -0.6% to 3.3%; ICA: median deviation, 0.6%, range, -1.8% to 2.9%). The relative 2D dose measurements also showed good agreement with computed doses (>93% of pixels in all films passing gamma, >90% of pixels in all ICA measurements passing gamma). The ICA relative dose results were highly similar to those of film (>90% of pixels passing gamma). The coronal and sagittal ICA measurements were statistically indistinguishable by the paired t test with a hypothesized mean difference of 0.1%. The ion chamber and ICA absolute dose measurements showed a similar trend but had disparities of 2-3% in 18% of plans. After validating the new VMAT implementation with ion chamber, film, and ICA, we were able to maintain an effective yet efficient patient-specific VMAT QA protocol by reducing from five (ion chamber, film, and ICA) to two measurements (ion chamber and single ICA) per plan. The ICA (Matrixx®, IBA Dosimetry) was validated for VMAT QA, but ion chamber measurements are recommended for absolute dose comparison until future developments correct the ICA angular dependence.

CT-Based Brachytherapy Treatment Planning using Monte Carlo Simulation Aided by an Interface Software

Introduction: In brachytherapy, radioactive sources are placed close to the tumor, therefore, small changes in their positions can cause large changes in the dose distribution. This emphasizes the need for computerized treatment planning. The usual method for treatment planning of cervix brachytherapy uses conventional radiographs in the Manchester system. Nowadays, because of their advantages in locating the source positions and the surrounding tissues, CT and MRI images are replacing conventional radiographs. In this study, we used CT images in Monte Carlo based dose calculation for brachytherapy treatment planning, using an interface software to create the geometry file required in the MCNP code. The aim of using the interface software is to facilitate and speed up the geometry set-up for simulations based on the patient’s anatomy. This paper examines the feasibility of this method in cervix brachytherapy and assesses its accuracy and speed. Material and Methods: For dosimetric measurements regarding the treatment plan, a pelvic phantom was made from polyethylene in which the treatment applicators could be placed. For simulations using CT images, the phantom was scanned at 120 kVp. Using an interface software written in MATLAB, the CT images were converted into MCNP input file and the simulation was then performed. Results: Using the interface software, preparation time for the simulations of the applicator and surrounding structures was approximately 3 minutes; the corresponding time needed in the conventional MCNP geometry entry being approximately 1 hour. The discrepancy in the simulated and measured doses to point A was 1.7% of the prescribed dose. The corresponding dose differences between the two methods in rectum and bladder were 3.0% and 3.7% of the prescribed dose, respectively. Comparing the results of simulation using the interface software with those of simulation using the standard MCNP geometry entry showed a less than 1% difference of the prescribed dose at 67% of the studied points (minimum <0.01%, maximum 4.8%). Discussion and Conclusions: Using the interface software reduces the overall simulation time. Comparison of the results of the measurements, simulation using the interface software and simulation using standard MCNP geometry entry shows that the use of tomographic images and transforming them into MCNP input file for dose calculation in brachytherapy is feasible and reasonably accurate.

A novel lateral disequilibrium inclusive (LDI) pencil‐beam based dose calculation algorithm: Evaluation in inhomogeneous phantoms and comparison with Monte Carlo calculations

Pencil-beam (PB) based dose calculation for treatment planning is limited by inaccuracies in regions of tissue inhomogeneities, particularly in situations with lateral electron disequilibrium as is present at tissue/lung interfaces. To overcome these limitations, a new "lateral disequilibrium inclusive" (LDI) PB based calculation algorithm was introduced. In this study, the authors evaluated the accuracy of the new model by film and ionization chamber measurements and Monte Carlo simulations. To validate the performance of the new LDI algorithm implemented in Corvus 09, eight test plans were generated on inhomogeneous thorax and pelvis phantoms. In addition, three plans were calculated with a simple effective path length (EPL) algorithm on the inhomogeneous thorax phantom. To simulate homogeneous tissues, four test plans were evaluated in homogeneous phantoms (homogeneous dose calculation). The mean pixel pass rates and standard deviations of the gamma 4%/4 mm test for the film measurements were (96 +/- 3)% for the plans calculated with LDI, (70 +/- 5)% for the plans calculated with EPL, and (99 +/- 1)% for the homogeneous plans. Ionization chamber measurements and Monte Carlo simulations confirmed the high accuracy of the new algorithm (dose deviations < or = 4%; gamma 3%/3 mm > or = 96%). LDI represents an accurate and fast dose calculation algorithm for treatment planning.

Alanine/EPR dosimetry applied to the verification of a total body irradiation protocol and treatment planning dose calculation using a humanoid phantom

To avoid complications in total body irradiation (TBI), it is important to achieve a homogeneous dose distribution throughout the body and to deliver a correct dose to the lung which is an organ at risk. The purpose of this work was to validate the TBI dose protocol and to check the accuracy of the 3D dose calculations of the treatment planning system. Dosimetry based on alanine/electron paramagnetic resonance (EPR) was used to measure dose at numerous locations within an anthropomorphic phantom (Alderson) that was irradiated in a clinical TBI beam setup. The alanine EPR dosimetry system was calibrated against water calorimetry in a Co-60 beam and the absorbed dose was determined by the use of "dose-normalized amplitudes" A(D). The dose rate of the TBI beam was checked against a Farmer ionization chamber. The phantom measurements were compared to 3D dose calculations from a treatment planning system (Pinnacle) modeled for standard dose calculations. Alanine dosimetry allowed accurate measurements which were in accordance with ionization chamber measurements. The combined relative standard measurement uncertainty in the Alderson phantom was U(r)(A(D))=0.6%. The humanoid phantom was irradiated to a reference dose of 10 Gy, limiting the lung dose to 7.5 Gy. The ratio of the average measured dose midplane in the craniocaudal direction to the reference dose was 1.001 with a spread of +/- 4.7% (1 sd). Dose to the lung was measured in 26 locations and found, in average, 1.8% lower than expected. Lung dose was homogeneous in the ventral-dorsal direction but a dose gradient of 0.10 Gy cm(-1) was observed in the craniocaudal direction midline within the lung lobe. 3D dose calculations (Pinnacle) were found, in average, 2% lower compared to dose measurements on the body axis and 3% lower for the lungs. The alanine/EPR dosimetry system allowed accurate dose measurements which enabled the authors to validate their TBI dose protocol. Dose calculations based on a collapsed cone convolution dose algorithm modeled for regular treatments are accurate within 3% and can further be improved when the algorithm is modeled for TBI.

Experimental validation of a commercial 3D dose verification system for intensity-modulated arc therapies

We validate the dosimetric performance of COMPASS®, a novel 3D quality assurance system for verification of volumetric-modulated arc therapy (VMAT) treatment plans that can correlate the delivered dose to the patient's anatomy, taking into account the tissue inhomogeneity. The accuracy of treatment delivery was assessed by the COMPASS® for 12 VMAT plans, and the resulting assessments were evaluated using an ionization chamber and film measurements. Dose–volume relationships were evaluated by the COMPASS® for three additional treatment plans and these were used to verify the accuracy of treatment planning dose calculations. The results matched well between COMPASS® and measurements for the ionization chamber (⩽3%) and film (73–99% for gamma(3%/3 mm) < 1 and 98–100% for gamma(5%/5 mm) < 1) for the phantom plans. Differences in dose–volume statistics for the average dose to the PTV were within 2.5% for three treatment plans. For the structures located in the low-dose region, a maximum difference of <9% was observed. In its current implementation, the system could measure the delivered dose with sufficient accuracy and could project the 3D dose distribution directly on the patient's anatomy. Slight deviations were found for large open fields. These could be minimized by improving the COMPASS® in-built beam model.

Poster — Thur Eve — 36: Lung SBRT: Dosimetric Evaluation of 4DCT Based Treatment Planning in Presence of Respiratory Motion

The respiratory related dosimetric uncertainties in the 4DCT based lung stereotactic body radiotherapy (SBRT) treatment planning and treatment delivery are evaluated by performing density assignment correction (DAC) in the ITV. Siemens 40‐slice CT scanner was used to acquire 8 phases of 4DCT images and a free breathing CT scan using a CIRS Dynamic Thorax Phantom. The ITV was created from 4DCT scan and treatment planning was done in free breathing CT scan. The ITV dose coverage was measured by CIRS phantom with ionization chamber moving together with the tumor during the beam delivery under free breathing treatment, which considers the time and positional averaged distribution of tissue heterogeneities within the ITV. The measured results are compared with the treatment planning with and without DAC in the ITV. The ITV dose coverage from the measurement is 1.9% higher than the planned ITV mean dose. The ITV received adequate dose due to the presence of effective density of the tumor within the ITV when the treatment is delivered under free breathing. The plan accounting for respiratory motion by assigning an average tumor density into the treatment planning had better uniform dose than the plan without DAC. By incorporating the ITV tumor density override into treatment planning dose calculation, the ITV dose coverage becomes more uniform and the mean dose of ITV increases and more closely matches the measured values than the plan without DAC in the ITV.

The Monte Carlo code MCPTV—Monte Carlo dose calculation in radiation therapy with carbon ions

The Monte Carlo code MCPTV is presented. MCPTV is designed for dose calculation in treatment planning in radiation therapy with particles and especially carbon ions. MCPTV has a voxel-based concept and can perform a fast calculation of the dose distribution on patient CT data. Material and density information from CT are taken into account. Electromagnetic and nuclear interactions are implemented. Furthermore the algorithm gives information about the particle spectra and the energy deposition in each voxel. This can be used to calculate the relative biological effectiveness (RBE) for each voxel. Depth dose distributions are compared to experimental data giving good agreement. A clinical example is shown to demonstrate the capabilities of the MCPTV dose calculation.

SU-GG-T-590: Delta-Functional Multi Segmented Pencil Beam Algorithm for Highly Accurate Proton Dose Calculation in Heterogeneous Body

Purpose: A pencil beam algorithm (PBA), which is the most widely used dose calculation method for proton therapy, could make a considerable error in the case of complicated structure. This is because it utilizes the one‐dimensional density scaling along the central axis of the pencil beam and neglects the off‐axis density heterogeneity. In order to obtain accurate dose distribution in heterogeneous materials in a patient body, we have developed a novel dose calculation algorithm, delta‐functional multi segmented pencil beam algorithm (DMS‐PBA). In this study, the superiority of DMS‐PBA to conventional PBA was verified by using phantom experiment. Method and Materials: An L‐shaped polyethylene block, representing lateral heterogeneity, was set above polyethylene plates. Three dimensional dose distributions were measured with a simulated using two algorithms: DMS‐PBA and conventional PBA. The accuracy of the calculation was evaluated by comparing the measured and calculated dose distributions. Results: In the measurement, hot and cold spots were detected at z = 123, 142, 162 mm‐WEL. Conventional PBA method could reproduce none of these spots. The difference between PBA results and the measured dose amounted to 12 %. On the other hand, DMS‐PBA and the measured dose generally agreed well. The hot and cold spots may be produced by the proton trajectories in different angles around the border between polyethylene and air. DMS‐PBA could reproduce the detour and overreaching of proton by using “narrow” pencil beams with laterally delta‐functional initial beam emittance. In addition, the computing time by DMS‐PBA method in this situation was reasonable due to the less overlapped narrow pencil beams. Conclusions: We showed that our proposed method, DMS‐PBA, could improve the accuracy of dose calculation for treatment planning of proton therapy.

The Effect of Intravenous Contrast on Photon Radiation Therapy Dose Calculations for Lung Cancer

The aim of this study was to evaluate the effect of intravenous contrast-enhanced computed tomography (CT) scans on the photon radiation dose calculations for lung cancer treatment planning. Nonionic iodinated intravenous contrast (Iohexol) was administered during the treatment planning CT scan of 9 patients with node-positive non-small-cell lung cancer (NSCLC). The potential effect of intravenous contrast was studied by changing the density of the contrast-enhanced vessels. A total of 9 patients were treated in this study: 5 patients with intensity-modulated radiation therapy (IMRT), and 4 patients with three-dimensional (3D) conformal radiation therapy. A treatment plan was generated from an unmanipulated "normal contrast" planning scan. The same planning parameters were then applied to a "no contrast" planning scan. The effect of intravenous contrast was quantified by calculating the percent change of dose in a variety of target and normal structures. To evaluate a worst-case scenario, the comparison between "normal contrast" and "no contrast" planning scans was repeated, assigning each vessel the artificial high density of 1.3 g/cm. Dose differences between the planning image set using intravenous contrast and the image set without contrast were less than 2.5% for planning target volumes. A worst-case scenario in which normal contrast was overridden with an artificially high density of 1.3 g/cm led to small dose differences of less than 3%. Planning lung radiation therapy treatment using CT scans that contain intravenous contrast does not result in clinically significant errors in dose delivery.

Optimizing an analytical dose calculation algorithm for fast 2D calculations

Previously, an analytical dose calculation algorithm for MLC-based radiotherapy was developed and commissioned, which includes a detailed model of various MLC effects as a unique feature [1]. The algorithm was originally developed as an independent verification of the treatment planning system's dose calculation and it explicitly modeled spatial and depth dependent MLC effects such as interleaf transmission, the tongue-and-groove effect, rounded leaf ends, MLC scatter, beam hardening, and divergence of the beam, which in turn resulted in a gradual MLC transmission fall-off with increasing off-axis distance. Originally the algorithm was implemented in Mathematica™ (Wolfram). To speed up the calculation time and to be able to calculate high resolution 2D dose distributions within a reasonable time frame (<2 s) the algorithm needs to be optimized and to be embedded in a user friendly environment. To achieve this goal, the dose calculation model is implemented in VisualBasic 6.0, which decreases the calculation time moderately. More importantly, the numerical algorithm for dose calculation is changed at two levels: the dose contributions are split into their x- and y-contributions and the calculation is aperture- rather than as originally point-based. Implementing these three major changes, the calculation time is reduced considerably without loosing accuracy. The time for a typical IMRT field with about 2500 calculation points decreased from 2387 seconds to 0.624 seconds (a factor of about 3800). The mean agreement of the optimized and the not optimized calculation algorithm at the isocenter for a fairly complex IMRT plan with 23 fields is better than 1% relative to the local dose at the measuring point.

Treatment planning system and dose delivery accuracy in extracranial stereotactic radiotherapy using Elekta body frame

Treatment planning system and dose delivery accuracy in extracranial stereotactic radiotherapy using Elekta body frame The purpose of this study was to measure the photon beam transmission through the Elekta Stereotactic Body Frame (ESBF) and treatment couch, to determine the dose calculations accuracy of the MasterPlan Treatment Planning System (TPS) using Pencil Beam (PBA) and Collapsed Cone (CCA) algorithms during the use of Elekta Stereotactic Body Frame (ESBF), and to demonstrate a simple calculation method to put this transmission into account during the treatment planning dose calculations. The dose was measured at the center of an in-house custom-built inhomogeneous PMMA thorax phantom with and without ‘the frame + treatment couch’. The phantom was CT-imaged inside the ESBF and planned with multiple 3D-CRT fields using PBA and CCA for photon beams of energies 6 MV and 10 MV. There were two treatment plans for dose calculations. In the first plan, the ‘frame + couch’ were included in the body contour and, therefore, included in the TPS dose calculations. In the second plan, the ‘frame + couch’ were not included in the body contour and, therefore, not included in the calculations. Transmission of the ‘frame + couch’ was determined by the ratio of the dose measurements with the ‘frame + couch’ to the measurements without them. To validate the accuracy of the calculation model, plans with and without the ‘frame + couch’ surrounding the phantoms were compared with their corresponding measurements. The transmission of the ‘frame + couch’ varies from 90.23-97.54% depending on the energy, field size, the angle of the beams and whether the beams also intercept them. The validation accuracy of the Pencil Beam (PBA) and Collapsed Cone (CCA) algorithms were within 5.33% and 4.04% respectively for the individual measurements for all gantry angles under this study. The results showed that both PBA and CCA algorithms can calculate the dose to the target within 4.25% and 1.95% of the average measured value. The attenuation caused by the ESBF and couch must be accounted into the planning process. For MasterPlan, the ‘frame + couch’ should be contoured and included in all calculations. This can be done easily and accurately.

Comparing the accuracy of four‐dimensional photon dose calculations with three‐dimensional calculations using moving and deforming phantoms

Four-dimensional (4D) dose calculation algorithms, which explicitly incorporate respiratory motion in the calculation of doses, have the potential to improve the accuracy of dose calculations in thoracic treatment planning; however, they generally require greater computing power and resources than currently used for three-dimensional (3D) dose calculations. The purpose of this work was to quantify the increase in accuracy of 4D dose calculations versus 3D dose calculations. The accuracy of each dose calculation algorithm was assessed using measurements made with two phantoms. Specifically, the authors used a rigid moving anthropomorphic thoracic phantom and an anthropomorphic thoracic phantom with a deformable lung insert. To incorporate a clinically relevant range of scenarios, they programed the phantoms to move and deform with two motion patterns: A sinusoidal motion pattern and an irregular motion pattern that was extracted from an actual patient's breathing profile. For each combination of phantom and motion pattern, three plans were created: A single-beam plan, a multiple-beam plan, and an intensity-modulated radiation therapy plan. Doses were calculated using 4D dose calculation methods as well as conventional 3D dose calculation methods. The rigid moving and deforming phantoms were irradiated according to the three treatment plans and doses were measured using thermoluminescent dosimeters (TLDs) and radiochromic film. The accuracy of each dose calculation algorithm was assessed using measured-to-calculated TLD doses and a gamma analysis. No significant differences were observed between the measured-to-calculated TLD ratios among 4D and 3D dose calculations. The gamma results revealed that 4D dose calculations had significantly greater percentage of pixels passing the 5%/3 mm criteria than 3D dose calculations. These results indicate no significant differences in the accuracy between the 4D and the 3D dose calculation methods inside the gross tumor volume. On the other hand, the film results demonstrated that the 4D dose calculations provided greater accuracy than 3D dose calculations in heterogeneous dose regions. The increase in accuracy of the 4D dose calculations was evident throughout the planning target volume.

SU‐FF‐J‐12: HU and Dosimetric Comparison of Six Cone‐Beam CT Acquisition Techniques for Radiation Therapy Planning

Purpose: It has been shown that cone‐beam computed tomography (CBCT) can be used in dose calculation for treatment planning. This study investigates the HU and dosimetric properties of six CBCT techniques in comparison with conventional CT to determine which techniques are proper for planning. Method and Materials: HU of material disks in Catphan and HU profiles for homogeneous and inhomogeneous phantoms for six different CBCT techniques (Low‐Dose Head, Standard‐Dose Head, High Quality Head, Pelvis Spotlight, Pelvis, and Low‐Dose Thorax) were compared to CT images. Plans with a single photon beam based on CBCT techniques and conventional CT of phantoms were compared using dose values at the isocenter, isodose distributions, and dose volume histograms. Results: HU values of all the CBCT techniques are very close to those of CT in Catphan and in homogeneous phantoms except for Low‐Dose Thorax (LDTH) technique, which overestimates HU by 85. However, the LDTH CBCT showed the HU profile in the lung area matched well with the CT compared to the other five techniques. The dose distribution based on all CBCT techniques in the homogeneous phantoms matched well with the CT except for LDTH, which underestimated the dose. For the lung area, the LDTH dose distribution matched well with CT and the other CBCT techniques did not. For the spine area, the dose distribution of the Pelvis technique matched best with CT, but the other techniques matched relatively well with CT too except for LDTH. Conclusion: This study illustrates HU values and dose calculation based on six different CBCT techniques compared to conventional CT for the purpose of radiation therapy treatment planning. The dose calculation accuracy depends on combination of CBCT acquisition technique and treatment site. A CBCT technique should be properly selected for a treatment site if CBCT‐based dose calculation is to be performed.