Daisuke Kawahara

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.

Availability of applying diaphragm matching with the breath-holding technique in stereotactic body radiation therapy for liver tumors

PURPOSE Image-guided radiotherapy (IGRT) based on bone matching can produce large target-positioning errors because of expiration breath-hold reproducibility during stereotactic body radiation therapy (SBRT) for liver tumors. Therefore, the feasibility of diaphragm-based 3D image matching between planning computed tomography (CT) and pretreatment cone-beam CT was investigated. METHODS In 59 liver SBRT cases, Lipiodol uptake after transarterial chemoembolization was defined as a tumor marker. Further, the relative isocenter coordinate that was obtained by Lipiodol matching was defined as the reference coordinate. The distance between the relative isocenter coordinate and reference coordinate, which was obtained from diaphragm matching and bone matching techniques, was defined as the target positioning error. Furthermore, the target positioning error between liver matching and Lipiodol matching was evaluated. RESULTS The positioning errors in all directions by the diaphragm matching were significantly smaller than those obtained by using by the bone matching technique (p < 0.05). Further, the positioning errors in the A-P and C-C directions that were obtained by using liver matching were significantly smaller than those obtained by using bone matching (p < 0.05). The estimated PTV margins calculated by the formula proposed by van Herk for diaphragm matching, liver matching, and bone matching were 5.0 mm, 5.0 mm, and 11.6 mm in the C-C direction; 3.6 mm, 2.4 mm, and 6.9 mm in the A-P direction; and 2.6 mm, 4.1 mm, and 4.6 mm in the L-R direction, respectively. CONCLUSIONS Diaphragm matching-based IGRT may be an alternative image matching technique for determining liver tumor positions in patients.

Marginal prescription equivalent to the isocenter prescription in lung stereotactic body radiotherapy: preliminary study for Japan Clinical Oncology Group trial (JCOG1408)

A new randomized Phase III trial, the Japan Clinical Oncology Group (JCOG) 1408, which compares two dose fractionations (JCOG 0403 and JCOG 0702) for medically inoperable Stage IA NSCLC or small lung lesions clinically diagnosed as primary lung cancer, involves the introduction of a prescribed dose to the D95% of the planning target volume (PTV) using a superposition/convolution algorithm. Therefore, we must determine the prescribed dose in the D95% prescribing method to begin JCOG1408. JCOG 0702 uses density correction and the D95% prescribing method. However, JCOG 0403 uses no density correction and isocenter- prescribing method. The purpose of this study was to evaluate the prescribed dose to the D95% of the PTV equivalent to a dose of 48 Gy to the isocenter (JCOG 0403) using a superposition algorithm. The peripheral isodose line, which has the highest conformity index, and the D95% of the PTV were analyzed by considering the weighting factor, i.e. the inverse of the difference between the doses obtained using the superposition and Clarkson algorithms. The average dose at the isodose line of the highest conformity index and the D95% of the PTV were 41.5 ± 0.3 and 42.0 ± 0.3 Gy, respectively. The D95% of the PTV had a small correlation with the target volume (r2 = 0.0022) and with the distance between the scatterer and tumor volumes (r2 = 0.19). Thus, the prescribed dose of 48 Gy using the Clarkson algorithm (JCOG0403) was found to be equivalent to the prescribed dose of 42 Gy to the D95% of the PTV using the superposition algorithm.

Absorbed dose and image quality of Varian TrueBeam CBCT compared with OBI CBCT

PURPOSE Nowadays, patient positioning and target localization can be verified by using kilovolt cone beam computed tomography (kV-CBCT). There have been various studies on the absorbed doses and image qualities of different kV-CBCT systems. However, the Varian TrueBeam CBCT (TB CBCT) system has not been investigated so far. We assess the image quality and absorbed dose of TB CBCT through comparison with those of on-board imager (OBI) CBCT. METHODS The image quality was evaluated using two phantoms. A CATPHAN phantom measured the image quality parameters of the American Association of Physicists in Medicine Task Group 142 (AAPM TG-142) report. These factors are the pixel value stability and accuracy, noise, high-contrast resolution, low-contrast resolution, and image uniformity. A H2SO4 phantom was used to evaluate the image uniformity over a larger region than the CATPHAN phantom. In evaluating the absorbed dose, the radial dose profile and the patient organ doses at the prostate and rectum levels were evaluated. RESULTS The image quality parameters of AAPM TG-142 using TB CBCT are equal to or greater than those of OBI CBCT. In particular, the contrast-to-noise ratio with TB CBCT is 2.5 times higher than that with OBI CBCT. For the test of a large field uniformity, the maximum difference in the Hounsfield unit (HU) values between the centre and peripheral regions is within 30 HU with TB CBCT and 283 HU with OBI CBCT. The maximum absorbed dose with TB CBCT is decreased by 60%. CONCLUSIONS We find that the image quality improved and the absorbed dose decreased with TB CBCT in comparison to those with OBI CBCT. Its image uniformity is also superior over a larger scanning range.

Estimation of patient-specific imaging dose for real-time tumour monitoring in lung patients during respiratory-gated radiotherapy

PURPOSE To quantify the patient-specific imaging dose for real-time tumour monitoring in the lung during respiratory-gated stereotactic body radiotherapy (SBRT) in clinical cases using SyncTraX. METHODS AND MATERIALS Ten patients who underwent respiratory-gated SBRT with SyncTraX were enrolled in this study. The imaging procedure for real-time tumour monitoring using SyncTraX was simulated using Monte Carlo. We evaluated the dosimetric effect of a real-time tumour monitoring in a critical organ at risk (OAR) and the planning target volume (PTV) over the course of treatment. The relationship between skin dose and gating efficiency was also investigated. RESULTS For all patients, the mean D50 to the PTV, ipsilateral lung, liver, heart, spinal cord and skin was 118.3 (21.5-175.9), 31.9 (9.5-75.4), 15.4 (1.1-31.6), 10.1 (1.3-18.1), 25.0 (1.6-101.8), and 3.6 (0.9-7.1) mGy, respectively. The mean D2 was 352.0 (26.5-935.8), 146.4 (27.3-226.7), 90.7 (3.6-255.0), 42.2 (4.8-82.7), 88.0 (15.4-248.5), and 273.5 (98.3-611.6) mGy, respectively. The D2 of the skin dose was found to increase as the gating efficiency decreased. CONCLUSIONS The additional dose to the PTV was at most 1.9% of the prescribed dose over the course of treatment for real-time tumour monitoring. For OARs, we could confirm the high dose region, which may not be susceptible to radiation toxicity. However, to reduce the skin dose from SyncTraX, it is necessary to increase the gating efficiency.

Relative biological effectiveness study of Lipiodol based on microdosimetric-kinetic model

OBJECTIVES We examine the contrast agent Lipiodol effect on the relative biological effectiveness (RBE) values for flattening filter free (FFF) and flattening filter (FF) beams of 6 MV-Xray (6 MVX) and 10 MVX. METHODS Lipiodol was placed at 5 cm depth in water. According to the microdosimetric kinetic model, the RBE values for killing the human liver hepatocellular cells were calculated from dose and lineal energy (yd(y)) from Monte Carlo simulations. RBE200kVX and RBECo were defined as the ratios of dose using reference radiation (200 kVX, Co-ɤ) to the dose of test radiation (FFF and FF beams for 6 MV and 10 MV) to produce the same biological effects. The dose enhancement RBE (RBEDE) was defined as the ratios of a dose without Lipiodol to with Lipiodol using to produce the same biological effects. The dose needed to achieve 10% (D10%) and 1% cell survival (D1%) was evaluated by cell surviving fraction (SF) formula. RESULTS The deviation of mean y‾D values with and without Lipiodol were 3.9-4.8% for 6 MVX and 3.5-3.6% for 10 MVX. The RBE200kVX and RBECo with Lipiodol were larger than that without Lipiodol. The RBEDE was larger for FFF beam than for FF beam. The deviation of RBEDE for FFF and FF beams of 6 MVX was larger than that of 10 MVX. CONCLUSION The presence of Lipiodol seemed to locally increase the absorbed dose and to also cause an enhancement of the relative biological effectiveness.

Tolerance levels of CT number to electron density table for photon beam in radiotherapy treatment planning system

Abstract The accuracy of computed tomography number to electron density (CT‐ED) calibration is a key component for dose calculations in an inhomogeneous medium. In a previous work, it was shown that the tolerance levels of CT‐ED calibration became stricter with an increase in tissue thickness and decrease in the effective energy of a photon beam. For the last decade, a low effective energy photon beam (e.g., flattening‐filter‐free (FFF)) has been used in clinical sites. However, its tolerance level has not been established yet. We established a relative electron density (ED) tolerance level for each tissue type with an FFF beam. The tolerance levels were calculated using the tissue maximum ratio (TMR) and each corresponding maximum tissue thickness. To determine the relative ED tolerance level, TMR data from a Varian accelerator and the adult reference computational phantom data in the International Commission on Radiological Protection publication 110 (ICRP‐110 phantom) were used in this study. The 52 tissue components of the ICRP‐110 phantom were classified by mass density as five tissues groups including lung, adipose/muscle, cartilage/spongy‐bone, cortical bone, and tooth tissue. In addition, the relative ED tolerance level of each tissue group was calculated when the relative dose error to local dose reached 2%. The relative ED tolerances of a 6 MVFFF beam for lung, adipose/muscle, and cartilage/spongy‐bone were ±0.044, ±0.022, and ±0.044, respectively. The thicknesses of the cortical bone and tooth groups were too small to define the tolerance levels. Because the tolerance levels of CT‐ED calibration are stricter with a decrease in the effective energy of the photon beam, the tolerance levels are determined by the lowest effective energy in useable beams for radiotherapy treatment planning systems.

Effect of secondary electron generation on dose enhancement in Lipiodol with and without a flattening filter

Abstract Purpose Lipiodol, which was used in transcatheter arterial chemoembolization before liver stereotactic body radiation therapy (SBRT), remains in SBRT. Previous we reported the dose enhancement in Lipiodol using 10 MV (10×) FFF beam. In this study, we compared the dose enhancement in Lipiodol and evaluated the probability of electron generation (PEG) for the dose enhancement using flattening filter (FF) and flattening filter free (FFF) beams. Methods FF and FFF for 6 MV (6×) and 10× beams were delivered by TrueBeam. The dose enhancement factor (DEF), energy spectrum, and PEG was calculated using Monte Carlo (MC) code BEAMnrc and heavy ion transport code system (PHITS). Results DEFs for FF and FFF 6× beams were 7.0% and 17.0% at the center of Lipiodol (depth, 6.5 cm). DEFs for FF and FFF 10× beams were 8.2% and 10.5% at the center of Lipiodol. Spectral analysis revealed that the FFF beams contained more low‐energy (0–0.3 MeV) electrons than the FF beams, and the FF beams contained more high‐energy (>0.3 MeV) electrons than the FFF beams in Lipiodol. The difference between FFF and FF beam DEFs was larger for 6× than for 10×. This occurred because the 10× beams contained more high‐energy electrons. The PEGs for photoelectric absorption and Compton scattering for the FFF beams were higher than those for the FF beams. The PEG for the photoelectric absorption was higher than that for Compton scattering. Conclusions FFF beam contained more low‐energy photons and it contributed to the dose enhancement. Energy spectra and PEGs are useful for analyzing the mechanisms of dose enhancement.

Split-VMAT technique to control the expiratory breath-hold time in liver stereotactic body radiation therapy

PURPOSE In this study, we demonstrate the feasibility of using split-arcs in volumetric modulated arc therapy (VMAT), tailored for expiratory breath-hold in stereotactic body radiation therapy (SBRT) for liver tumors. We compare it with three-dimensional conformal radiation therapy (3D-CRT) and continuous-VMAT, for ten randomly selected hepatocellular carcinoma cases. METHODS Four coplanar and four non-coplanar beams were used for the 3D-CRT plans. A pair of partial arcs, chosen using a back-and-forth rotating motion, were used for the continuous-VMAT plans. Split-VMAT plans were created using the same arc range as the continuous-VMAT plans, but were split into smaller arcs (<90°), to simulate an expiratory breath hold of <15s. The dose distribution, treatment delivery efficiency, and patient specific quality assurance of the split-VMAT, were verified to ensure that the outcomes were equal, or better than, those for 3D-CRT and continuous-VMAT. The prescription was 48Gy/4 fractions, to 95% of the PTV, using 10MV FFF X-ray beams. RESULTS The mean dose of the liver-GTV was lower in the split-VMAT compared with that of 3D-CRT. Split-VMAT was more conformal compared with 3D-CRT. The total treatment time for split-VMAT was shorter than that of 3D-CRT. Similar dosimetric indices were observed for split-VMAT and continuous-VMAT. All VMAT plans passed the gamma acceptance test. CONCLUSIONS Split-VMAT designed to accommodate an expiratory breath-hold period of 15s is a feasible and efficient use of liver SBRT, because it does not compromise the quality of the plan, when compared with 3D-CRT or continuous-VMAT.

Accuracy of the raw-data-based effective atomic numbers and monochromatic CT numbers for contrast medium with a dual-energy CT technique

OBJECTIVE To evaluate the accuracy of raw-data-based effective atomic number (Zeff) values and monochromatic CT numbers for contrast material of varying iodine concentrations, obtained using dual-energy CT. METHODS We used a tissue characterization phantom and varying concentrations of iodinated contrast medium. A comparison between the theoretical values of Zeff and that provided by the manufacturer was performed. The measured and theoretical monochromatic CT numbers at 40-130 keV were compared. RESULTS The average difference between the Zeff values of lung (inhale) inserts in the tissue characterization phantom was 81.3% and the average Zeff difference was within 8.4%. The average difference between the Zeff values of the varying concentrations of iodinated contrast medium was within 11.2%. For the varying concentrations of iodinated contrast medium, the differences between the measured and theoretical monochromatic CT values increased with decreasing monochromatic energy. The Zeff and monochromatic CT numbers in the tissue characterization phantom were reasonably accurate. CONCLUSION The accuracy of the raw-data-based Zeff values was higher than that of image-based Zeff values in the tissue-equivalent phantom. The accuracy of Zeff values in the contrast medium was in good agreement within the maximum SD found in the iodine concentration range of clinical dynamic CT imaging. Moreover, the optimum monochromatic energy for human tissue and iodinated contrast medium was found to be 70 keV. Advances in knowledge: The accuracy of the Zeff values and monochromatic CT numbers of the contrast medium created by raw-data-based, dual-energy CT could be sufficient in clinical conditions.