Yusuke Ochi

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.

Time-Adjusted Internal Target Volume: A Novel Approach Focusing on Heterogeneity of Tumor Motion Based on 4-Dimensional Computed Tomography Imaging for Radiation Therapy Planning of Lung Cancer

PURPOSE To consider nonuniform tumor motion within the internal target volume (ITV) by defining time-adjusted ITV (TTV), a volume designed to include heterogeneity of tumor existence on the basis of 4-dimensional computed tomography (4D-CT). METHODS AND MATERIALS We evaluated 30 lung cancer patients. Breath-hold CT (BH-CT) and free-breathing 4D-CT scans were acquired for each patient. The tumors were manually delineated using a lung CT window setting (window, 1600 HU; level, -300 HU). Tumor in BH-CT images was defined as gross tumor volume (GTV), and the sum of tumors in 4D-CT images was defined as ITV-4D. The TTV images were generated from the 4D-CT datasets, and the tumor existence probability within ITV-4D was calculated. We calculated the TTV80 value, which is the percentage of the volume with a tumor existence probability that exceeded 80% on ITV-4D. Several factors that affected the TTV80 value, such as the ITV-4D/GTV ratio or tumor centroid deviation, were evaluated. RESULTS Time-adjusted ITV images were acquired for all patients, and tumor respiratory motion heterogeneity was visualized. The median (range) ITV-4D/GTV ratio and median tumor centroid deviation were 1.6 (1.0-4.1) and 6.3 mm (0.1-30.3 mm), respectively. The median TTV80 value was 43.3% (2.9-98.7%). Strong correlations were observed between the TTV80 value and the ITV-4D/GTV ratio (R=-0.71) and tumor centroid deviation (R=-0.72). The TTV images revealed the tumor motion pattern features within ITV. CONCLUSIONS The TTV images reflected nonuniform tumor motion, and they revealed the tumor motion pattern features, suggesting that the TTV concept may facilitate various aspects of radiation therapy planning of lung cancer while incorporating respiratory motion in the future.

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.

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.

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-187: Feasibility Study of Stereotactic Liver Radiation Therapy Using Multiple Divided Partial Arcs in Volumetric Modulated Arc Therapy

PURPOSE To verify volumetric modulated arc therapy (VMAT) using flattening filter free (FFF) mode with jaw tracking (JT) feature for single breath hold as long as 15 s per arc in liver stereotactic body radiation therapy (SBRT) against intensity modulated radiation therapy (IMRT) FFF-JT. METHODS Ten hepatocellular carcinoma (HCC) cases were planned with 10 MV FFF using Pinnacle3 treatment planning system which delivered by TrueBeam to administer 48 Gy/ 4 fractions. Eight non-coplanar beams were assigned to IMRT using step-and-shoot technique. For VMAT, two or three non-coplanar partial arcs (up to 180 degrees) were further divided into subarcs with gantry rotation less than 80 degrees to limit delivery time within 15 s. Dose distributions were verified using OCTAVIUS II system and pass rates were evaluated using gamma analysis with criteria of 3%/3 mm at threshold of 5% to the maximum dose. The actual irradiation time was measured. RESULTS The VMAT-FFF-JT of partial-arcs with sub-divided arcs was able to produce a highly conformal plan as well as IMRT-FFF-JT. Isodose lines and DVH showed slight improvement in dosimetry when JT was employed for both IMRT and VMAT. Consequently, VMAT-FFF-JT was superior in reducing the dose to liver minus gross tumor volume. VMAT-FFF-JT has shorter total treatment time compared with 3D conformal radiation therapy (3D-CRT) FFF because the gantry was rotated simultaneously with the beam delivery in VMAT. Moreover, due to the small and regular shape of HCC, VMAT-FFF-JT offered less multileaf collimator motion, thus the interplay effect is expected to be reduced. The patient specific QA of IMRT and VMAT acquired the pass rates higher than 90%. CONCLUSION VMAT-FFF-JT could be a promising technique for liver SBRT as the sub-divided arcs method was able to accommodate a single breath hold irradiation time of less than 15 s without deterioration of the dose distribution compared with IMRT-FFF-JT.

SU-E-J-140: Availability of Using Diaphragm Matching in Stereotactic Body Radiotherapy (SBRT) at the Time in Breath-Holding SBRT for Liver Cancer.

PURPOSE IGRT based on the bone matching may produce a larger target positioning error in terms of the reproducibility of the expiration breath hold. Therefore, the feasibility of the 3D image matching between planning CT image and pretreatment CBCT image based on the diaphragm matching was investigated. METHODS In fifteen-nine liver SBRT cases, Lipiodol, uptake after TACE was outlined as the marker of the tumor. The relative coordinate of the isocenter obtained by the contrast matching was defined as the reference coordinate. The target positioning difference between diaphragm matching and bone matching were evaluated by the relative coordinate of the isocenter from the reference coordinate obtained by each matching technique. In addition, we evaluated PTV margins by van Herk setup margin formula. RESULTS The target positioning error by the diaphragm matching and the bone matching was 1.31±0.83 and 3.10±2.80 mm in the cranial-caudal(C-C) direction, 1.04±0.95 and 1.62±1.02 mm in the anterior-posterior(A-P) direction, 0.93±1.19 and 1.12±0.94 mm in the left-right(L-R) direction, respectively. The positioning error by the diaphragm matching was significantly smaller than the bone matching in the C-C direction (p<0.05). The setup margin of diaphragm matching and bone matching that we had calculated based on van Herk margin formula was 4.5mm and 6.2mm(C-C), and 3.6mm and 6.3mm(A-P), and 2.6mm and 4.5mm(L-R), respectively. CONCLUSION IGRT based on a diaphragm matching could be one alternative image matching technique for the positioning of the patients with liver tumor.