Photon and electron backscatter dose and energy spectrum analysis around Lipiodol using flattened and unflattened beams.

PURPOSE: The aim of the current study was to evaluate the backscatter dose and energy spectrum from the Lipiodol with flattening filter (FF) and flattening filter-free (FFF) beams. Moreover, the backscatter range, that was defined as the backscatter distance (BD) are revealed. METHODS: 6 MVX FF and FFF beams were delivered by TrueBeam. Two dose calculation methods with Monte Carlo calculation were used with a virtual phantom in which the Lipiodol (3 × 3 × 3 cm3 ) was located at a depth of 5.0 cm in a water-equivalent phantom (20 × 20 × 20 cm3 ). The first dose calculation was an analysis of the dose and energy spectrum with the complete scattering of photons and electrons, and the other was a specified dose analysis only with scattering from a specified region. The specified dose analysis was divided into a scattering of primary photons and a scattering of electrons. RESULTS: The lower-energy photons contributed to the backscatter, while the high-energy photons contributed the difference of the backscatter dose between the FF and FFF beams. Although the difference in the dose from the scattered electrons between the FF and FFF beams was within 1%, the difference of the dose from the scattered photons between the FF and FFF beams was 5.4% at a depth of 4.98 cm. CONCLUSIONS: The backscatter range from the Lipiodol was within 3 mm and depended on the Compton scatter from the primary photons. The backscatter dose from the Lipiodol can be useful in clinical applications in cases where the backscatter region is located within a tumor.

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

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.

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 Lipiodol's 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 Lipiodol's 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.

[Availability of using diaphragm matching in stereotactic body radiotherapy (SBRT) at the time in breath-holding SBRT for liver cancer].

PURPOSE: Liver image guided radiation therapy (IGRT) based on bone matching risks generating serious target positioning errors for reasons of lack of reproducibility of expiration breath hold. We therefore investigated the feasibility of 3D image matching between planning CT images and pretreatment cone-beam computed tomography (CBCT) images based on diaphragm surface matching. METHOD: 27 liver stereotactic body radiotherapy (SBRT) cases in whom trancecatheter arterial chemoembolization (TACE) had been performed in advance of radiotherapy were manually image-matched based on contrast, Lipiodol used in the TACE as the marker of the tumor, and the relative coordinates of the isocenter obtained by contrast matching, defined as the reference coordinate. The target positioning difference between diaphragm matching and bone matching were evaluated by using relative coordinates of the isocenter from the reference obtained for each matching technique. RESULTS: The target positioning error using diaphragm matching and 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, and 0.93±1.19 and 1.12±0.94 mm in the left-right (L-R) direction, respectively. The positioning error due to diaphragm matching was significantly smaller than for bone matching in the C-C direction (p<0.05). CONCLUSION: IGRT based on diaphragm matching has potential as an alternative image matching technique for the positioning of liver patients.