Kazunari Hioki

Absorbed dose measurements for kV-cone beam computed tomography in image-guided radiation therapy.

In this study, we develope a novel method to directly evaluate an absorbed dose-to-water for kilovoltage-cone beam computed tomography (kV-CBCT) in image-guided radiation therapy (IGRT). Absorbed doses for the kV-CBCT systems of the Varian On-Board Imager (OBI) and the Elekta X-ray Volumetric Imager (XVI) were measured by a Farmer ionization chamber with a (60)Co calibration factor. The chamber measurements were performed at the center and four peripheral points in body-type (30 cm diameter and 51 cm length) and head-type (16 cm diameter and 33 cm length) cylindrical water phantoms. The measured ionization was converted to the absorbed dose-to-water by using a (60)Co calibration factor and a Monte Carlo (MC)-calculated beam quality conversion factor, kQ, for (60)Co to kV-CBCT. The irradiation for OBI and XVI was performed with pelvis and head modes for the body- and the head-type phantoms, respectively. In addition, the dose distributions in the phantom for both kV-CBCT systems were calculated with MC method and were compared with measured values. The MC-calculated doses were calibrated at the center in the water phantom and compared with measured doses at four peripheral points. The measured absorbed doses at the center in the body-type phantom were 1.96 cGy for OBI and 0.83 cGy for XVI. The peripheral doses were 2.36-2.90 cGy for OBI and 0.83-1.06 cGy for XVI. The doses for XVI were lower up to approximately one-third of those for OBI. Similarly, the measured doses at the center in the head-type phantom were 0.48 cGy for OBI and 0.21 cGy for XVI. The peripheral doses were 0.26-0.66 cGy for OBI and 0.16-0.30 cGy for XVI. The calculated peripheral doses agreed within 3% in the pelvis mode and within 4% in the head mode with measured doses for both kV-CBCT systems. In addition, the absorbed dose determined in this study was approximately 4% lower than that in TG-61 but the absorbed dose by both methods was in agreement within their combined uncertainty. This method is more robust and accurate compared to the dosimetry based on a conventional air-kerma calibration factor. Therefore, it is possible to be used as a standard dosimetry protocol for kV-CBCT in IGRT.

Dosimetric impact of Lipiodol in stereotactic body radiation therapy on liver after trans‐arterial chemoembolization

Purpose: Stereotactic body radiation therapy (SBRT) combining trans‐arterial chemoembolization (TACE) with Lipiodol is expected to improve local control. This study is aimed to estimate the dose enhancement in Lipiodols proximity and to evaluate the dose calculation accuracy of the Acuros XB (AXB) algorithm and anisotropic analytical algorithm (AAA) in the Eclipse treatment planning system (TPS) (ver. 11, Varian Medical Systems, Palo Alto, USA), compared with that of the Monte Carlo (MC) calculation (using BEAMnrc/DOSXYZnrc code) for a virtual phantom and a treatment plan for liver SBRT after TACE. Methods: The MC calculation accuracy was validated by comparing its results with the percent depth dose (PDD) and the off‐axis ratio (OAR) measured using a water‐equivalent phantom containing Lipiodol. The dose difference in Lipiodols proximity and the inhomogeneity correction accuracies of the AAA, AXB algorithm, and MC calculation were evaluated by calculating the PDDs and OARs for the virtual phantom with Lipiodol and the lateral profile for the clinical plan data. Results: The measured data and the MC results agreed within 3%. The average dose in the Lipiodol uptake region was higher by 8.1% for the virtual phantom and 6.0% for the clinical case compared to that in regions without Lipiodol uptake. For the virtual phantom, compared with the MC calculation, the AAA and the AXB algorithm underestimated the doses immediately upstream of the Lipiodol region by 5.0% and 4.2%, in the Lipiodol region by 7.4% and 9.8%, and downstream of the Lipiodol region by 5.5% and 3.9% respectively. These discrepancy between the AXB and MC calculations were due to the incorrect assignment of Lipiodol material properties. Namely, the bone material was assigned automatically by the AXB algorithm as the materials for the AXB algorithm do not contain iodine, which is the main constituent of Lipiodol. Conclusions: The MC calculation indicated a larger and more accurate dose increase in Lipiodol compared with the TPS algorithms. The observed dose enhancement in the tumor area could be clinically significant.

New absorbed dose measurement with cylindrical water phantoms for multidetector CT

The aim of this study was to develop new dosimetry with cylindrical water phantoms for multidetector computed tomography (MDCT). The ionization measurement was performed with a Farmer ionization chamber at the center and four peripheral points in the body-type and head-type cylindrical water phantoms. The ionization was converted to the absorbed dose using a (60)Co absorbed-dose-to-water calibration factor and Monte Carlo (MC) -calculated correction factors. The correction factors were calculated from MDCT (Brilliance iCT, 64-slice, Philips Electronics) modeled with GMctdospp (IMPS, Germany) software based on the EGSnrc MC code. The spectrum of incident x-ray beams and the configuration of a bowtie filter for MDCT were determined so that calculated photon intensity attenuation curves for aluminum (Al) and calculated off-center ratio (OCR) profiles in air coincided with those measured. The MC-calculated doses were calibrated by the absorbed dose measured at the center in both cylindrical water phantoms. Calculated doses were compared with measured doses at four peripheral points and the center in the phantom for various beam pitches and beam collimations. The calibration factors and the uncertainty of the absorbed dose determined using this method were also compared with those obtained by CTDIair (CT dose index in air). Calculated Al half-value layers and OCRs in air were within 0.3% and 3% agreement with the measured values, respectively. Calculated doses at four peripheral points and the centers for various beam pitches and beam collimations were within 5% and 2% agreement with measured values, respectively. The MC-calibration factors by our method were 44-50% lower than values by CTDIair due to the overbeaming effect. However, the calibration factors for CTDIair agreed within 5% with those of our method after correction for the overbeaming effect. Our method makes it possible to directly measure the absorbed dose for MDCT and is more robust and accurate than the CTDIair measurement.

Development of multi-planar dose verification by use of a flat panel EPID for intensity-modulated radiation therapy

Our purpose in this study was to evaluate the accuracy of a new multi-planar dose measurement method. The multi-planar dose distributions were reconstructed at each depth by convolution of EPID fluence and dose kernels with the use of EPIDose software (SunNuclear). The EPIDose was compared with EPID, MapCHECK (SunNuclear), EDR2 (Kodak), and Monte Carlo-calculated dose profiles. The EPIDose profiles were almost in agreement with Monte Carlo-calculated dose profiles and MapCHECK for test plans. The dose profiles were in good agreement with EDR2 at the penumbra region. For dose distributions, EPIDose, EDR2, and MapCHECK agreed with that of the treatment-planning system at each depth in the gamma analysis. In comparisons of clinical IMRT plans, EPIDose had almost the same accuracy as MapCHECK and EDR2. Because EPIDose has a wide dynamic range and high resolution, it is a useful tool for the complicated IMRT verification. Furthermore, EPIDose can also evaluate the absolute dose.

SU-G-BRB-17: Dosimetric Evaluation of the Respiratory Interplay Effect During VMAT Delivery Using IPAGAT Polymer Gel Dosimeter

PURPOSE To evaluate the dosimetric impact of the interplay effect between multileaf collimator (MLC) movement and tumor respiratory motion during delivery of volumetric modulate arc therapy (VMAT) by using customized polymer gel dosimeter. METHODS Polyacrylamide-based gel dosimeter contained magnesium chloride as a sensitizer (iPAGAT) was used in this study. An excellent gas barrier PAN (BAREX) techno bottle (φ8 cm, 650 mL) filled with iPAGAT was set to the QUASAR™ respiratory motion phantom, and was moved with motion amplitudes of 1 and 2 cm with a 4 second period during VMAT delivery by the Novalis Tx linear accelerator (Varian/BrainLAB). Two spherical tumors with a 2 cm diameter (GTV1 and GTV2) were defined, and ITV1 (GTV1+1 cm) and ITV2 (GTV2+2 cm) with expansion in the superior-inferior (S-I) direction were also defined with simulated respiratory motion. PTV margin was 2 mm around the ITV considering the setup uncertainty. Two single arc VMAT plans with 30 Gy at 3 Gy per fraction (GTV: D98>100%, PTV: D95=100%) were generated by the Varian Eclipse treatment planning system. Three-dimensional dose distribution in iPAGAT was read out by the Signa 1.5T MRI system (GE), and was evaluated by dose-volume histogram (DVH) using in-house developed software. RESULTS According to DVH analysis by iPAGAT, D98 of GTV1 and GTV2 were more than 100% of the prescribed dose. In contrast, D95 of PTV1 and PTV2 were about 85% and 65%, respectively. Furthermore, low-to-intermediate dose was widespread with motion amplitude of 2 cm. CONCLUSION DVH analysis using iPAGAT polymer gel dosimeter was performed in this study. As a result, interplay effect was negligible, since dose coverage of GTV was sufficient during VMAT delivery with simulated respiratory motion. However, the dose reduction of PTV and the spread of low-to-intermediate dose compared to the planned dose require scrupulous attention for large tumor respiratory motion.

SU-F-T-630: Energy Spectral Study On Lipiodol After Trans-Arterial Chemoembolization Using the Flattened and Unflattened Photon Beams

PURPOSE SBRT combining transarterial chemoembolization with Lipiodol is expected to improve local control. Our showed that the dose enhancement effect in the Lipiodol with 10X flattening filter free (FFF) was inserted. This study was to investigate the energy fluence variations of electron in the Lipiodol using flattened (FF) and FFF beams. METHODS FF and FFF for 6X and 10X beams by TrueBeam were used in this study. The Lipiodol (3 X 3 X 3 cm3 ) was located at the depth of 5 cm in water, the dose enhancement factor (DEF) and energy fluence were calculated by Monte Carlo (MC) calculations (PHITS). RESULTS DEFs with FF and FFF of 6X were 17.1% and 24.3% at rebuild-up region in the Lipiodol (5.3cm depth), 7.0% and 17.0% at the center of Lipiodol (6.5cm depth), and -13.2% and -8.2% at behind Lipiodol (8.3cm depth). DEFs with FF and FFF of 10X were 21.7% and 15.3% at rebuild-up region, 8.2% and 10.5% at the center of Lipiodol, and -14.0% and -8.6% at behind Lipiodol. Spectral results showed that the FFF beam contained more low-energy (0-0.3MeV) component of electrons than FF beam, and FF beam contained more high-energy (over 0.3MeV) electrons than FFF beam in Lipiodol. Behind the Lipiodol, build-down effect with FF beam was larger than FFF beam because FF beam contained more high energy electrons. The difference of DEFs between FFF and FF beams for 6X were larger than for 10X. This is because 10X beam contained more high-energy electrons. CONCLUSION It was found that the 6XFFF beam gives the largest change of energy fluence and the largest DEF in this study. These phenomena are mainly caused by component of low-energy electrons, and this energy is almost correspond to the boundary of photo electronic dominant and Compton scattering dominant region for photon beams.

SU-E-T-313: The Accuracy of the Acuros XB Advanced Dose Calculation Algorithm for IMRT Dose Distributions in Head and Neck

PURPOSE To investigate the accuracy of the Acuros XB version 11 (AXB11) advanced dose calculation algorithm by comparing with Monte Caro (MC) calculations. The comparisons were performed with dose distributions for a virtual inhomogeneity phantom and intensity-modulated radiotherapy (IMRT) in head and neck. METHODS Recently, AXB based on Linear Boltzmann Transport Equation has been installed in the Eclipse treatment planning system (Varian Medical Oncology System, USA). The dose calculation accuracy of AXB11 was tested by the EGSnrc-MC calculations. In additions, AXB version 10 (AXB10) and Analytical Anisotropic Algorithm (AAA) were also used. First the accuracy of an inhomogeneity correction for AXB and AAA algorithms was evaluated by comparing with MC-calculated dose distributions for a virtual inhomogeneity phantom that includes water, bone, air, adipose, muscle, and aluminum. Next the IMRT dose distributions for head and neck were compared with the AXB and AAA algorithms and MC by means of dose volume histograms and three dimensional gamma analysis for each structure (CTV, OAR, etc.). RESULTS For dose distributions with the virtual inhomogeneity phantom, AXB was in good agreement with those of MC, except the dose in air region. The dose in air region decreased in order of MC<AXB11<AXB10. This may be caused by the difference of the electron cut-off energy for algorithms, ie: 0.700 MeV for MC, 0.711 MeV for AXB11, and 1.011 MeV for AXB 10. Since the AAA algorithm is based on the dose kernel of water, the doses in regions for air, bone, and aluminum considerably became higher than those of AXB and MC. The pass rates of the gamma analysis for IMRT dose distributions in head and neck were similar to those of MC in order of AXB11<AXB10<AAA. CONCLUSION The dose calculation accuracy of AXB11 was almost equivalent to the MC dose calculation.

SU‐E‐T‐515: Clinical Evaluation of Dose Calculation Algorithms in Stereotactic Radiation Therapy for Lung Cancer

Purpose: To evaluate the dose calculation accuracy of the model‐based algorithms and a new grid‐based Boltzmann equation solver by comparing with Monte Carlo‐based algorithm in stereotactic lung irradiation with photon beams. Methods: The stereotactic body radiation therapy (SBRT) for 20 clinical lung cancer cases was planned using Eclipse (Varian Medical Systems) treatment planning system (TPS). The prescribed dose at isocenter was 48 Gy with 4 fractions and six non‐coplanar fields. First, the dose distributions were calculated with analytical anisotropic algorithm (AAA). Next, pencil beam convolution (PBC) and Acuros XB (AXB) based on a new grid‐based Boltzmann equation in Eclipse, and Voxel Monte Carlo algorithm (VMC) in iPlan (BrainLAB) were implemented under identical planning conditions as AAA. The dose distributions and dose volume histograms (DVHs) for SBRT were compared among algorithms. Similarly, the dose indices and monitor unit (MU) for the planning target volume (PTV) were also compared. The grid size for all dose calculations was 2.5×2.5×2.5 mm3. Results: AAA tended to underestimate the PTV dose compared to VMC. In contrast, PBC overestimated the PTV dose in almost all plans. The discrepancies in the D95 evaluation of AAA and PBC for VMC were up to 4.4% and 16.1%, respectively. AXB tended to slightly overestimate the PTV dose compared to VMC but the discrepancy was within 3% in D95. This discrepancy was attributed to differences in material assignments, material voxelization methods, and an energy cut‐off for electron interactions. The differences in MU of AXB, AAA and PBC for VMC were up to 1.7%, 3.5%, and 7.8%, respectively. Conclusion: The dose distributions in lung regions significantly differ based on the principle of the calculation algorithms. The dose calculation accuracy of AXB is comparable to VMC and is more suitable than AAA.

TU‐G‐103‐08: Monte Carlo‐Calculated Patient Dose Distributions From KV‐Cone Beam CT for IGRT

PURPOSE To calculate patient dose distributions from kV-cone beam CT (CBCT) for image-guided radiation therapy (IGRT) with the Monte Carlo (MC) method and to evaluate quantitatively organ doses from dose-volume histograms (DVHs). METHODS The Varian On-Board Imager (OBI) and the Elekta X-ray Volumetric Imager (XVI) systems were modeled using the EGSnrc/BEAMnrc cord system. MC-calculated doses for both kV-CBCT were calibrated by converting ion chamber readings measured using a CT water phantom into absolute doses. The chamber measurements were performed by a Farmer chamber with 60Co absorbed dose-to-water calibration factor. Then MC-dose distributions for kV-CBCT were calculated using patient CT images. In this study, the organ doses of pelvis and head were evaluated from DVHs obtained by the MC-dose distributions. The beam setting for the pelvis with full scan of 360o was 125 kV and 680 mAs with a half-bowtie filter for OBI, and 120 kV and 1056 mAs with a full-bowtie filter for XVI. For the head with half scan of 200o, the beam setting was 100 kV and 145 mAs with a full-bowtie filter for OBI, and 100 kV and 73.2 mAs without the bowtie filter for XVI. RESULTS The calculated mean doses for prostate, rectum, and bladder for OBI and XVI were 2.6 and 0.9 cGy, 2.6 and 0.9 cGy, 3.1 and 1.0 cGy, respectively. The absorbed dose in pelvis for OBI was three times higher than XVI. Similarly, mean doses for eye lens, brain stem, and spinal code for OBI and XVI were 0.08 and 0.02 cGy, 0.34 and 0.12 cGy, 0.19 and 0.15 cGy, respectively. The dose for bone increased up to three or four times compared to that of soft tissue. CONCLUSION Monte Carlo-calculated dose distributions are useful to evaluate quantitatively patient doses from kV-CBCT for IGRT.