- Email: [email protected]
379 oral VOXEL BASED METHOD TO GENERATE CT EQUIVALENT IMAGES FROM MR IMAGES FOR TREATMENT PLANNING PURPOSES
W EDNESDAY, M AY 11, 2011
Physics Proffered Papers 9: Challenges in Treatment Planning 377 oral BENCHMARK OF A NEW ALGORITHM FOR DOSE CALCULATION IN RT USING A GRID-BASED BOLTZMANN EQUATION SOLVER M. B. Kyed Jørgensen1 , L. Hoffmann1 , H. S. Rønde2 , J. Petersen1 1
A ARHUS U NIVERSITY H OSPITAL, Department of Medical Physics, Aarhus C, Denmark 2 V EJLE H OSPITAL, Department of Medical Physics, Vejle, Denmark
Purpose: The Acuros XB advanced dose calculation algorithm (Acuros XB) was developed to improve the accuracy and speed of external photon beam treatment planning. Acuros XB uses a grid-based technique to solve the Linear Boltzmann transport equation and directly accounts for the effects of heterogeneities in patient dose calculations. The aim of this study is to test the accuracy of the Acuros XB algorithm for heterogeneous media (lung, bone and tissue) and homogeneous media (water) by comparison with film measurements and measurements in water. The calculations in heterogeneous media were also performed with the Anisotropic Analytical Algorithm (AAA). Materials: All fields and test plans were calculated in Eclipse (Varian Oncology Systems, Ca) for both 6 MV and 15 MV photons and compared to measurements performed at a Varian accelerator.In connection with the implementation of Eclipse, output factors, profiles and depth doses for square symmetric fields were measured in a PTW water tank with a diamond detector (PTW, Freiburg, Germany) and a RK ion chamber (Precitron, Sweden). Furthermore, various asymmetric fields, fields with different MLC configurations and various source skin distances (SSD) (80 cm to 130 cm) were measured in the water tank for comparison with the Acuros XB algorithm. Eight test plans were calculated on a CIRS Thorax Phantom (CIRS, Va) consisting of tissue, lung and bone equivalent plastic materials. Dose distributions within the phantom were measured for selected transversal slices with Gafchromic EBT dosimetry films (ISP, NJ). The 2D dose distributions were evaluated in VeriSoft (PTW Freiburg, Germany) with a Gamma criterion of 3 % in dose and 3 mm in distance to agreement (DTA), G(3,3). Results: The accuracy of the Acuros XB algorithm for profiles at five different depths is within 1%, 1mm except outside the field. For the depth dose curves, the mean value of the deviations is within 1 mm/1 % for 6MV and 2mm/1% for 15MV. The output factors are within 1%. Similar results were obtained for output factors of asymmetric fields, fields with different MLC configuration and fields with various SSDs.Overall, the measured and calculated 2D dose distributions of the CIRS Thorax Phantom showed good agreement. The number of points meeting the G(3,3) criterion was higher for the plans calculated with the Acuros XB algorithm than with the AAA algorithm, with both algorithms failing at low doses (< 0,1 Gy) and on gradients. On average, the number of points meeting the G(3,3) criterion was 96 % for the Acuros XB algorithm. Conclusions: The dose calculations performed with the Acuros XB algorithm in both homogeneous and heterogeneous media display a good agreement with measurement. Film measurement of the eight test plans showed that the Acuros XB algorithm is superior to the AAA algorithm in heterogeneous media. 378 oral FUNCTIONAL FORMS FOR PHOTON SPECTRA OF CLINICAL LINACS E. Ali1 , D. Rogers1 1
C ARLETON U NIVERSITY, Physics, Ottawa, Canada
Purpose: In a clinical photon beam, the spectrum is a pre-requisite for dose calculations in treatment planning systems and for calculations of other spectrum-averaged dosimetric quantities. Describing spectra using a functional form with a few free parameters is desirable because it is compact, it facilitates virtual source modeling, and it tames the classic ill-conditioned problem of unfolding spectra from depth dose or transmission measurements. The literature has scattered parameterization models which are not rigorously benchmarked or compared to each other. This study thoroughly evaluates existing functional forms and proposes a new form that addresses existing limitations. Materials: Nine different functional forms published between 1989 and 2008 are evaluated, and a new functional form is proposed. The proposed form has five parameters, including the maximum energy and the magnitude of the 511 keV annihilation peak. The proposed function uses general features of linac heads and the common terms in thin target formulae from the classic bremsstrahlung review article of Koch and Motz. Fits are done for the energy fluence, not the fluence, because the energy fluence has a smaller range of variation, reveals better the discrepancies at the higher end of the spectrum, and is a more direct indicator of dose. Fitting is done using the standard LevenbergMarquardt least squares algorithm. A library of about 70 high resolution Monte Carlo spectra is used for testing all functional forms.
P ROFFERED PAPER
S 149
The library includes spectra from Varian, Siemens and Elekta machines (all with and without a flattening filter, on- and off-axis), Tomotherapy (on- and offaxis, treatment and imaging modes), Cyberknife and research spectra (NRC Vickers linac and NPL linac). Results: Existing functional forms are found to have one or more of the following limitations: too simple to accommodate spectra from all current linac beams, unjustifiably too complex to be practical, not based on physics and thus prone to yielding non-physical spectra when used in spectral unfolding, or have tails and thus are inaccurate in maximum energy estimation. When tested against the full library of spectra, the proposed functional form shows an average absolute deviation of ~1 %. Estimates of the maximum energy, peak energy and fluence-weighted mean energy were, on average, within 4 %, 3 % and 0.8 % of their true values, respectively. Sample fits are shown in the figure. In separate studies we show that the proposed functional form is well-behaved (and thus not over-parameterized) when used to unfold spectra from depth dose or transmission measurements.
Conclusions: The proposed functional form is accurate within 1 %. The function is simple, physics-based, has reasonably few parameters, and is flexible enough to accommodate photon spectra from all currently used linacs. 379 oral VOXEL BASED METHOD TO GENERATE CT EQUIVALENT IMAGES FROM MR IMAGES FOR TREATMENT PLANNING PURPOSES T. Nyholm1 , A. Johansson1 , M. Karlsson1 1
U MEÅ U NIVERSITY, Department of Radiation Sciences, Umeå, Sweden
Purpose: The voxel intensities in CT images are directly related to the electron density. CT is therefore a natural choice as a basis for dose calculations and patient positioning in external radiotherapy. There is however an emerging interest in replacing the CT with MR because of the superior soft tissue contrast obtained with magnetic resonance imaging. The main challenge is the extremely short T2 relaxation time of bone, 0.40.5 ms, making discrimination between bone and air difficult using traditional MR sequences. Therefore, ultra short echo time (UTE) sequences can be used to acquire signal from bone.The aim of the present work was to develop a voxel based method for generating a substitute CT (s-CT) image from an MRI examination. Materials: We made use of five clinical datasets with MRI and CT data of the head. Each of the five patients was imaged with CT and three MR sequences: two dual echo UTE sequences (TE = 0.07 and 3.76 ms) with different flip angles (10◦ and 60◦ ), and one conventional T2 weighted sequence. All MR images were acquired on a Siemens Espree 1.5 T scanner. For each patient the MR and CT images were co-registered and for each MR image two new images were derived by calculating the mean value and standard deviation of the voxels in a 27-voxel neighborhood around each voxel. The distribution of voxels in this 16-dimensional (5 MR images + CT) intensity space was approximated by a mixture of 20 multivariate Gaussians using the Expectation Maximization (EM) algorithm with k-means initialization. The model was trained on the MR and CT data from four patients at a time and applied to the MR data of the remaining patient to generate a substitute CT image. This procedure was repeated for all five combinations of training and validation data in a leave-one-out cross-validation (LOOCV) procedure. The substitute CT was compared to the real CT for each patient by calculating the mean absolute error (MAE) in Hounsfield units (HU). Results:
S 150
P ROFFERED PAPER
W EDNESDAY, M AY 11, 2011
out using pre validated PHSP files and the DOSXYZnrc code. VSM calculations were carried out using DOSXYZrt, a modified version of DOSXYZnrc which includes support for histogram-based particle sampling. Results: Our study has shown that for a non bow-tie filtered beam, 92% of the photons exiting the CBCT unit last interacted in the X-Ray target, 3% in the filtration cone and 5% elsewhere. For bow-tie filtered beams, 85% of the photons last interacted in the target, 3% in the filtration cone, 10% in the bow-tie filter and 5% elsewhere. For each source in the VSM, two planes were used to generate particles: one origin plane and one sampling plane. For both target and filtration cone, the origin plane was set to the X-ray focal spot and the sampling plane at 100 cm from the target. For the bow-tie filter source, the origin plane was set at the back of the filter. The particle’s energy spectrum was modelled and sampled separately. Figure 1 depicts the agreement between full MC and VSM calculations for the dose deposition in water of a clinical CBCT beam using large pelvis CBCT scans (120 kV, L20, F0).
The LOOCV MAE for the all the s-CT images was 141 HU. A real CT is seen along with a substitute CT in fig. 1 - Real CT (a, c, e, g) and a substitute CT derived from MR images of the same patient (b, d, f, h). Differences between real and substitute CT images were primarily found at the patient surface and at bonetissue and airtissue interfaces inside the head. The method did very accurately distinguish bone from air, which has been a problem for other MR based methods. All structures commonly used for patient positioning were clearly visable in the DDR’s based on the s-CT. Conclusions: Fig. 1. The fixation used during CT imaging is also seen in the real CT images.The s-CT concept is very promising both to generate a MR based basis for dose calculations and to serve as a reference for patient positioning. As the method is voxel based, i.e. the transformation from MR to s-CT is performed voxel by voxel, the method is not sensitive to untypical anatomy, which is a large problem for atlas based methods.
Figure 1. Percentage depth dose (A) and lateral dose profile (B) for full MC (solid line) and VSM (symbol). Conclusions: In this work we fully characterised the radiation output of a kV CBCT unit. We demonstrated that it is possible to reconstruct a diagnostic CBCT X-ray beam using a VSM. We built a generalised VSM for target, filtration cone and bow-tie filter and validated the VSM against full MC calculations. This is a first step towards the incorporation of concomitant dose calculation in commercial treatment planning systems.References: 1. Med Phys. 2009 Jan;36(1):127-36. 2. Phys Med Biol. 2009 54(7):N93-N100. 3. Med Phys. 2009 36(9):4156-67. 4. Med Phys. 2004 31(11):3106-21.
380 oral 381 oral A VIRTUAL SOURCE MODEL FOR CONE BEAM CT DOSE DEPOSITION E. Spezi1 , M. Fix2 , W. Volken2 , D. Frei2 1 V ELINDRE C ANCER C ENTRE, Department of Medical Physics, Cardiff, United Kingdom 2 I NSELSPITAL , U NIVERSITÄT B ERN, Division of Medical Radiation Physics, Bern, Switzerland
FEASIBILITY OF CBCT-BASED DOSE CALCULATION: COMPARISON OF THREE HU ADJUSTMENT TECHNIQUES I. Fotina1 , J. Hopfgartner1 , M. Stock1 , T. Steininger2 1 , D. Georg1 1 M EDICAL U NIVERSITY V IENNA , AKH, Department of Radiotherapy, Vienna, Austria 2 V IENNA U NIVERSITY OF T ECHNOLOGY, Vienna, Austria
Purpose: The purpose of this work is to study the main source components of kV X-ray beams generated by an on-board cone beam CT (CBCT) unit and to develop a virtual source model (VSM) that can be used for the calculation of concomitant dose due to the use of CBCT in IGRT. Materials: A previously built Monte Carlo (MC) model of the Elekta XVI unit [1-3] has been used to generate phase space (PHSP) files containing position, direction cosines, energy, weight, charge and position of the last interaction (ZLAST) for each particle exiting the CBCT unit. The PHSP files were separated accordingly to ZLAST and sub sources were generated for target, filtration cone, and bow-tie filter using an enhanced version of the beampp MC code [3]. Particles which last interacted in the remaining components of the unit were neglected. After a detailed analysis of the position, direction and energy spectrum characteristics of the sub sources, a histogram-based VSM was built, following an approach previously used in MV external beam radiotherapy [4]. The VSM was used to sample particles’ position, direction and energy for subsequent calculations of dose deposition in phantoms and validated against full MC simulations. Reference MC calculations were carried
Purpose: Decisions in adaptive radiotherapy can be performed on conebeam CT (CBCT)-based dose assessment. However, CBCT scans do not provide correct Hounsfield Units (HU) and cannot be used directly for the dose calculation. The aim of this study was to compare the accuracy of three different methods for HU adjustment in dose calculation on CBCT for different patient geometries and planning techniques. Materials: CBCT images of five prostate cases and five stereotactic lung cases were acquired on the XVI Synergy system (Elekta, Sweden). A four field box (4F-box) and 7 beam IMRT plans were created on planning CT for prostate while a 7 beam coplanar conformal technique was used for SBRT. Dose calculation was performed with a Monte Carlo-based algorithm using the iPlan treatment planning system (Brainlab, Germany). Structure sets and plans were transferred from planning CT to fused CBCT image sets and dose was recalculated. For the first approach dedicated HU conversion curves (CC) based on the Catphan phantom and determined from abdomen or thorax acquisition protocol, were applied to CBCT images. The second method called Water-Air-Bone (WAB) approach overrides the structures with fixed HU
Physics Proffered Papers 9: Challenges in Treatment Planning 377 oral BENCHMARK OF A NEW ALGORITHM FOR DOSE CALCULATION IN RT USING A GRID-BASED BOLTZMANN EQUATION SOLVER M. B. Kyed Jørgensen1 , L. Hoffmann1 , H. S. Rønde2 , J. Petersen1 1
A ARHUS U NIVERSITY H OSPITAL, Department of Medical Physics, Aarhus C, Denmark 2 V EJLE H OSPITAL, Department of Medical Physics, Vejle, Denmark
Purpose: The Acuros XB advanced dose calculation algorithm (Acuros XB) was developed to improve the accuracy and speed of external photon beam treatment planning. Acuros XB uses a grid-based technique to solve the Linear Boltzmann transport equation and directly accounts for the effects of heterogeneities in patient dose calculations. The aim of this study is to test the accuracy of the Acuros XB algorithm for heterogeneous media (lung, bone and tissue) and homogeneous media (water) by comparison with film measurements and measurements in water. The calculations in heterogeneous media were also performed with the Anisotropic Analytical Algorithm (AAA). Materials: All fields and test plans were calculated in Eclipse (Varian Oncology Systems, Ca) for both 6 MV and 15 MV photons and compared to measurements performed at a Varian accelerator.In connection with the implementation of Eclipse, output factors, profiles and depth doses for square symmetric fields were measured in a PTW water tank with a diamond detector (PTW, Freiburg, Germany) and a RK ion chamber (Precitron, Sweden). Furthermore, various asymmetric fields, fields with different MLC configurations and various source skin distances (SSD) (80 cm to 130 cm) were measured in the water tank for comparison with the Acuros XB algorithm. Eight test plans were calculated on a CIRS Thorax Phantom (CIRS, Va) consisting of tissue, lung and bone equivalent plastic materials. Dose distributions within the phantom were measured for selected transversal slices with Gafchromic EBT dosimetry films (ISP, NJ). The 2D dose distributions were evaluated in VeriSoft (PTW Freiburg, Germany) with a Gamma criterion of 3 % in dose and 3 mm in distance to agreement (DTA), G(3,3). Results: The accuracy of the Acuros XB algorithm for profiles at five different depths is within 1%, 1mm except outside the field. For the depth dose curves, the mean value of the deviations is within 1 mm/1 % for 6MV and 2mm/1% for 15MV. The output factors are within 1%. Similar results were obtained for output factors of asymmetric fields, fields with different MLC configuration and fields with various SSDs.Overall, the measured and calculated 2D dose distributions of the CIRS Thorax Phantom showed good agreement. The number of points meeting the G(3,3) criterion was higher for the plans calculated with the Acuros XB algorithm than with the AAA algorithm, with both algorithms failing at low doses (< 0,1 Gy) and on gradients. On average, the number of points meeting the G(3,3) criterion was 96 % for the Acuros XB algorithm. Conclusions: The dose calculations performed with the Acuros XB algorithm in both homogeneous and heterogeneous media display a good agreement with measurement. Film measurement of the eight test plans showed that the Acuros XB algorithm is superior to the AAA algorithm in heterogeneous media. 378 oral FUNCTIONAL FORMS FOR PHOTON SPECTRA OF CLINICAL LINACS E. Ali1 , D. Rogers1 1
C ARLETON U NIVERSITY, Physics, Ottawa, Canada
Purpose: In a clinical photon beam, the spectrum is a pre-requisite for dose calculations in treatment planning systems and for calculations of other spectrum-averaged dosimetric quantities. Describing spectra using a functional form with a few free parameters is desirable because it is compact, it facilitates virtual source modeling, and it tames the classic ill-conditioned problem of unfolding spectra from depth dose or transmission measurements. The literature has scattered parameterization models which are not rigorously benchmarked or compared to each other. This study thoroughly evaluates existing functional forms and proposes a new form that addresses existing limitations. Materials: Nine different functional forms published between 1989 and 2008 are evaluated, and a new functional form is proposed. The proposed form has five parameters, including the maximum energy and the magnitude of the 511 keV annihilation peak. The proposed function uses general features of linac heads and the common terms in thin target formulae from the classic bremsstrahlung review article of Koch and Motz. Fits are done for the energy fluence, not the fluence, because the energy fluence has a smaller range of variation, reveals better the discrepancies at the higher end of the spectrum, and is a more direct indicator of dose. Fitting is done using the standard LevenbergMarquardt least squares algorithm. A library of about 70 high resolution Monte Carlo spectra is used for testing all functional forms.
P ROFFERED PAPER
S 149
The library includes spectra from Varian, Siemens and Elekta machines (all with and without a flattening filter, on- and off-axis), Tomotherapy (on- and offaxis, treatment and imaging modes), Cyberknife and research spectra (NRC Vickers linac and NPL linac). Results: Existing functional forms are found to have one or more of the following limitations: too simple to accommodate spectra from all current linac beams, unjustifiably too complex to be practical, not based on physics and thus prone to yielding non-physical spectra when used in spectral unfolding, or have tails and thus are inaccurate in maximum energy estimation. When tested against the full library of spectra, the proposed functional form shows an average absolute deviation of ~1 %. Estimates of the maximum energy, peak energy and fluence-weighted mean energy were, on average, within 4 %, 3 % and 0.8 % of their true values, respectively. Sample fits are shown in the figure. In separate studies we show that the proposed functional form is well-behaved (and thus not over-parameterized) when used to unfold spectra from depth dose or transmission measurements.
Conclusions: The proposed functional form is accurate within 1 %. The function is simple, physics-based, has reasonably few parameters, and is flexible enough to accommodate photon spectra from all currently used linacs. 379 oral VOXEL BASED METHOD TO GENERATE CT EQUIVALENT IMAGES FROM MR IMAGES FOR TREATMENT PLANNING PURPOSES T. Nyholm1 , A. Johansson1 , M. Karlsson1 1
U MEÅ U NIVERSITY, Department of Radiation Sciences, Umeå, Sweden
Purpose: The voxel intensities in CT images are directly related to the electron density. CT is therefore a natural choice as a basis for dose calculations and patient positioning in external radiotherapy. There is however an emerging interest in replacing the CT with MR because of the superior soft tissue contrast obtained with magnetic resonance imaging. The main challenge is the extremely short T2 relaxation time of bone, 0.40.5 ms, making discrimination between bone and air difficult using traditional MR sequences. Therefore, ultra short echo time (UTE) sequences can be used to acquire signal from bone.The aim of the present work was to develop a voxel based method for generating a substitute CT (s-CT) image from an MRI examination. Materials: We made use of five clinical datasets with MRI and CT data of the head. Each of the five patients was imaged with CT and three MR sequences: two dual echo UTE sequences (TE = 0.07 and 3.76 ms) with different flip angles (10◦ and 60◦ ), and one conventional T2 weighted sequence. All MR images were acquired on a Siemens Espree 1.5 T scanner. For each patient the MR and CT images were co-registered and for each MR image two new images were derived by calculating the mean value and standard deviation of the voxels in a 27-voxel neighborhood around each voxel. The distribution of voxels in this 16-dimensional (5 MR images + CT) intensity space was approximated by a mixture of 20 multivariate Gaussians using the Expectation Maximization (EM) algorithm with k-means initialization. The model was trained on the MR and CT data from four patients at a time and applied to the MR data of the remaining patient to generate a substitute CT image. This procedure was repeated for all five combinations of training and validation data in a leave-one-out cross-validation (LOOCV) procedure. The substitute CT was compared to the real CT for each patient by calculating the mean absolute error (MAE) in Hounsfield units (HU). Results:
S 150
P ROFFERED PAPER
W EDNESDAY, M AY 11, 2011
out using pre validated PHSP files and the DOSXYZnrc code. VSM calculations were carried out using DOSXYZrt, a modified version of DOSXYZnrc which includes support for histogram-based particle sampling. Results: Our study has shown that for a non bow-tie filtered beam, 92% of the photons exiting the CBCT unit last interacted in the X-Ray target, 3% in the filtration cone and 5% elsewhere. For bow-tie filtered beams, 85% of the photons last interacted in the target, 3% in the filtration cone, 10% in the bow-tie filter and 5% elsewhere. For each source in the VSM, two planes were used to generate particles: one origin plane and one sampling plane. For both target and filtration cone, the origin plane was set to the X-ray focal spot and the sampling plane at 100 cm from the target. For the bow-tie filter source, the origin plane was set at the back of the filter. The particle’s energy spectrum was modelled and sampled separately. Figure 1 depicts the agreement between full MC and VSM calculations for the dose deposition in water of a clinical CBCT beam using large pelvis CBCT scans (120 kV, L20, F0).
The LOOCV MAE for the all the s-CT images was 141 HU. A real CT is seen along with a substitute CT in fig. 1 - Real CT (a, c, e, g) and a substitute CT derived from MR images of the same patient (b, d, f, h). Differences between real and substitute CT images were primarily found at the patient surface and at bonetissue and airtissue interfaces inside the head. The method did very accurately distinguish bone from air, which has been a problem for other MR based methods. All structures commonly used for patient positioning were clearly visable in the DDR’s based on the s-CT. Conclusions: Fig. 1. The fixation used during CT imaging is also seen in the real CT images.The s-CT concept is very promising both to generate a MR based basis for dose calculations and to serve as a reference for patient positioning. As the method is voxel based, i.e. the transformation from MR to s-CT is performed voxel by voxel, the method is not sensitive to untypical anatomy, which is a large problem for atlas based methods.
Figure 1. Percentage depth dose (A) and lateral dose profile (B) for full MC (solid line) and VSM (symbol). Conclusions: In this work we fully characterised the radiation output of a kV CBCT unit. We demonstrated that it is possible to reconstruct a diagnostic CBCT X-ray beam using a VSM. We built a generalised VSM for target, filtration cone and bow-tie filter and validated the VSM against full MC calculations. This is a first step towards the incorporation of concomitant dose calculation in commercial treatment planning systems.References: 1. Med Phys. 2009 Jan;36(1):127-36. 2. Phys Med Biol. 2009 54(7):N93-N100. 3. Med Phys. 2009 36(9):4156-67. 4. Med Phys. 2004 31(11):3106-21.
380 oral 381 oral A VIRTUAL SOURCE MODEL FOR CONE BEAM CT DOSE DEPOSITION E. Spezi1 , M. Fix2 , W. Volken2 , D. Frei2 1 V ELINDRE C ANCER C ENTRE, Department of Medical Physics, Cardiff, United Kingdom 2 I NSELSPITAL , U NIVERSITÄT B ERN, Division of Medical Radiation Physics, Bern, Switzerland
FEASIBILITY OF CBCT-BASED DOSE CALCULATION: COMPARISON OF THREE HU ADJUSTMENT TECHNIQUES I. Fotina1 , J. Hopfgartner1 , M. Stock1 , T. Steininger2 1 , D. Georg1 1 M EDICAL U NIVERSITY V IENNA , AKH, Department of Radiotherapy, Vienna, Austria 2 V IENNA U NIVERSITY OF T ECHNOLOGY, Vienna, Austria
Purpose: The purpose of this work is to study the main source components of kV X-ray beams generated by an on-board cone beam CT (CBCT) unit and to develop a virtual source model (VSM) that can be used for the calculation of concomitant dose due to the use of CBCT in IGRT. Materials: A previously built Monte Carlo (MC) model of the Elekta XVI unit [1-3] has been used to generate phase space (PHSP) files containing position, direction cosines, energy, weight, charge and position of the last interaction (ZLAST) for each particle exiting the CBCT unit. The PHSP files were separated accordingly to ZLAST and sub sources were generated for target, filtration cone, and bow-tie filter using an enhanced version of the beampp MC code [3]. Particles which last interacted in the remaining components of the unit were neglected. After a detailed analysis of the position, direction and energy spectrum characteristics of the sub sources, a histogram-based VSM was built, following an approach previously used in MV external beam radiotherapy [4]. The VSM was used to sample particles’ position, direction and energy for subsequent calculations of dose deposition in phantoms and validated against full MC simulations. Reference MC calculations were carried
Purpose: Decisions in adaptive radiotherapy can be performed on conebeam CT (CBCT)-based dose assessment. However, CBCT scans do not provide correct Hounsfield Units (HU) and cannot be used directly for the dose calculation. The aim of this study was to compare the accuracy of three different methods for HU adjustment in dose calculation on CBCT for different patient geometries and planning techniques. Materials: CBCT images of five prostate cases and five stereotactic lung cases were acquired on the XVI Synergy system (Elekta, Sweden). A four field box (4F-box) and 7 beam IMRT plans were created on planning CT for prostate while a 7 beam coplanar conformal technique was used for SBRT. Dose calculation was performed with a Monte Carlo-based algorithm using the iPlan treatment planning system (Brainlab, Germany). Structure sets and plans were transferred from planning CT to fused CBCT image sets and dose was recalculated. For the first approach dedicated HU conversion curves (CC) based on the Catphan phantom and determined from abdomen or thorax acquisition protocol, were applied to CBCT images. The second method called Water-Air-Bone (WAB) approach overrides the structures with fixed HU