Energy spectrum and dose enhancement due to the depth of the Lipiodol position using flattened and unflattened beams

Biological dose-enhancement analysis with Monte Carlo simulation for Lipiodol for photon beams

Background

Previously, the physical dose-enhancement factor (D_physEF) enhancement was introduced. However, the dose enhancement considering the biological effectiveness was not shown.

Purpose

The aim of the current study was to evaluate the biological dose-enhancement factor (D_bioEF) by the dose rate and to compare the D_physEF and the D_bioEF in Lipiodol for liver Stereotactic Body Radiation Therapy (SBRT).

Materials and methods

Flattening-filter-free (FFF) 6-MV (6MVX) and 10MVX beams were delivered by TrueBeam. A virtual inhomogeneity phantom and a liver SBRT patient-treatment plan were used. The D_physEF and lineal energy distribution ( y ) distribution was calculated from Monte Carlo simulations. Using a microdosimetric-kinetic (MK) model that is estimated based on the linear-quadratic formula for Lipiodol using human liver hepatocellular cells (HepG2), the biological dose and biological dose enhancement factor (D_bioEF) were calculated. The dose rate in the simulation was changed from 0.1 to 24 Gy/min.

Results

The D_bioEF (DR:2Gy/min) and D_physEF with 10MVX FFF beam were 23.2% and 19.1% at maximum and 12.8% and 11.1% on average in the Lipiodol. In the comparison of the D_bioEF between 0.1-24 Gy/min, the D_bioEF was 21.2% and 11.1% with 0.1 Gy/min for 6MVX and 10 MVX, respectively. The D_bioEF was larger than DEF for the 6MVX and 10MVX FFF beams. In clinical cases with the 10MVX FFF beam, the D_bioEF and D_physEF in the Lipiodol region can increase the in-tumor dose by approximately 11% and 10%, respectively, without increasing the dose to normal tissue.

Conclusions

The lower-energy and higher-dose-rate beams were contributed to the biological dose. The Lipiodol caused the enhancement of the physical dose and biological effectiveness.

Advances in knowledge

The biological dose enhancement (D_bioEF) should be considered in the high-density material such as the Lipiodol.

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.