ESTRO 37 Abstract book

S1082

ESTRO 37

Section- Department of Clinical Support, Hiroshima, Japan

a fixed α/β ratio. The TCP distributions are calculated using a Monte-Carlo method. In case when only α varies over the population, an analytical formula can also be derived describing the TCP probability density:

Purpose or Objective Previously, we demonstrated Lipiodol-related dose enhancement using 10 MVX flattening filter free (FFF) beams for Liver stereotactic body radiotherapy (SBRT). The objective of this study was to reveal the relative biological effectiveness (RBE) and relative dose-rate effectiveness (RDRE) values by the Lipiodol using 6 and 10 MVX FFF beams with Monte Carlo calculation. Material and Methods 6 and 10 MVX FFF beams were delivered by TrueBeam linear accelerator (Varian Medical Systems, Palo, Alto, USA). The Lipiodol (3 × 3 × 3 cm 3 ) was located at the depth of 5 cm in water. The dose enhancement factor (DEF) and the dose-mean lineal energy D values were calculated from Monte Carlo calculation with Particle and Heavy Ion Transport code System (PHITS). According to the microdosimetric kinetic model (MK model), the cell killing of the human liver hepatocellular cells (HepG2) were calculated from the D values. The RBE MK was determined as the ratio of the dose at 10% survival fraction (D 10% ) of 6 and 10 MVX to 200 kVX. The RBE DEF was determined as the ratio of D 10% without to with Lipiodol. In addition, the RDRE was defined as the ratios of D 10% with 0.1-24 Gy/min to 2 Gy/min.

where all the tumor clonogens receive identical total dose D=nd . Results A TCP distribution when parameter α is normally distributed and α/β is constant is shown in the Figure; the parameter values employed in the calculations are also shown. The black dots represent values calculated using the above analytical expression, showing excellent agreement with the Monte-Carlo simulated distribution.

Conclusion The rather unexpected form of the TCP distribution is due to the steepness of the individual TCP as a function of α and the particular values of α, σα and D chosen for this study. The dependence of the form of the TCP distribution on the values of these parameters will be further investigated. EP-1988 Impact of natural tumor cell death on TCP N. Stavreva 1 , P. Stavrev 1 , D. Penev 1 1 Sofia University “St. Kliment Ohridski”, Faculty of Physics, Sofia, Bulgaria Purpose or Objective To investigate the effect of including natural tumor cell death in TCP modeling and to derive a corresponding analytical TCP expression based on Zaider-Minerbo model [1] for the case of different time intervals between fractions and dose per fraction. Material and Methods The corresponding analytical TCP, an upgrade of a TCP expression previously derived in [2], is obtained and is as follows:

Results The RBE MK

in the Lipiodol was larger than that in the water for 6 and 10 MVX FFF beams. The average RBE exp was 1.20 for 6 MVX FFF beams and 0.95 for 10 MVX FFF beams. The RDRE was 0.99-1.33 Gy for 6 MVX and 0.98- 1.32 Gy for 10 MVX beams. The RDRE was higher with low-dose rate for 6 and 10MVX. The deviation of RDRE in the water and the Lipiodol were small within 1%. Conclusion The RBE and RDRE were calculated using MK model. The RBE enhancement were confirmed in presence of the Lipiodol, but the dose-rate effect was not affected from energy and materials. EP-1987 TCP and Gaussian Radiosensitivities P. Stavrev 1 , N. Stavreva 1 , A. Nahum 2 , D. Pressyanov 1 1 Sofia University “St. Kliment Ohridski”, FACULTY OF PHYSICS, Sofia, Bulgaria 2 Liverpool University, Physics Department, Liverpool, United Kingdom Purpose or Objective To investigate the form of a population TCP distribution presuming that the radiosensitivity parameters of the LQ model of cell kill are normally distributed among a population. Material and Methods The case of spread only in parameter α is investigated. Parameter β is presumed related to parameter α through

Here b is the cell birth rate, d is the natural cell death rate, N is the initial number of tumor cells, n is the number of fractions, T k-1 is the time until after the k th fraction, p s ( T k-1 ) is the cell survival probability after the k th fraction, T n-1 is the total treatment time, p s ( T n-1 ) is the final cell survival probability.