Mass Stopping Power and Range of Protons in Biological Human Body Tissues (Ovary, Lung and Breast)

In this work, the mass stopping power and range of protons in biological human body tissues (ovary, lung and breast) were calculated at the energy ranging from 0.04 MeV to 200 MeV using the MATLAB Program. The data relating to the densities, average atomic number to mass number Z A and excitation energy for the present tissues were collected from ICRU Report 46. The mass stopping power was calculated by the Bethe formula. Moreover, the simple integration (continuous slowing down approximation) method was employed for calculating protons range at the tissues. The results of the mass stopping power versus energy and the range versus energy were presented gra-phically and the empirical formulae for calculating the mass stopping power and the ranges were obtained. The present results for mass stopping powers and ranges were compared with the results obtained by others. Good agreements were found between them, especially at the energy ranging from 3 to 200 MeV.


Introduction
The stopping power and energy dissipation of charged particles through matter has been a subject of great interest for 100 years, because of its wide areas of application, such as ion implantation, fundamental particle physics, nuclear physics, radiation damage, radiology [1].
In recent years, the theoretical and experimental studies of the Stopping Power (SP) and range of charged particles have been increasing, especially in radiation physics. Many theoretical, as well as experimental studies have been established on this topic quite efficiently [2]. In particular, the use of protons or heavier ions are increasing as an alternative to external photon beams in radiotherapy, the reason is attributed to preserving the target dose, ensuring a higher dose delivered to the tumor and being able to transfer energy in the form of point shot through the diseased tissue due to the Bragg curve [3].
Charged particles are often used for radiotherapy because they have a well-defined penetration in tissues, the depth is dependent on the incident energy of particles and the nature of the irradiated material. Charged particles (protons, deuterons, alpha particles) have an important effect in radiotherapy as they have the ability to deliver their energy to the target [4].
For protons, the main contribution to the total stopping power is provided by the electronic stopping power which is based on inelastic collisions with the target's electrons. Conversely, the least contributing to the total stopping power comes from nuclear stopping power which arises from elastic collisions with the target's nucleons and is only significant at very low energy. For instance, the nuclear stopping power conduces more than one percent to the total stopping power only at energy below 20 KeV for protons [5]. Hence, precise knowledge of stopping power and range of protons are needed for the exact dosimetry of proton radiation. When studying the stopping power and range of protons in biological tissues, one must determine or collect data from experimentally or ICRU reports.
Heavy charged particles traversing matter lose energy primarily through the ionization and excitation of atoms. The stopping power is defined as the mean energy loss per unit path length -dE/dx. It depends on the charge and velocity of the projectile and, of course, the target material [6] [7].
Early investigation of the energy loss of charged particles traversing matter arrives at a general stopping power formula: where: N is the target density, Z 2 is the target atomic number, m e is the electron mass, v and Z 1 are the projectile velocity and charge respectively and B is called the "stopping number'' [7].
The negative sign in Equation (1) signifies the fact that the particles lose energy as they pass through the material. For most practical purposes, the physics of the energy loss phenomena are complex, and will be not covered in detail [8].
If an ion beam penetrates through matter it loses energy due to collisions with electrons (electronic stopping) and target nuclei (nuclear stopping) [9].
Then, the total stopping power is just the sum of the stopping powers due to electronic and nuclear interactions. The nuclear component of the stopping power can also be ignored [10].

Methods
The first quantum mechanical study of stopping power was done by Bethe. Bethe theory of stopping is valid when the projectile's velocity is higher than the Bohr velocity. In Bethe theory, the target is assumed as an elemental material.
Bethe's treatment of the energy loss is based on the born approximation applied to the inelastic collisions between the heavy particle and the atomic electrons. In this theory, the projectile heavy particle is assumed to be structure less, and the target nucleus is assumed infinitely massive [11]. 1) Calculations of electronic stopping power The following Bethe mass stopping power equation has been used for energy range 0.04 -200 MeV [12]: where: β is v/c where v is the proton velocity and c is light velocity, I is the mean excitation energy and F (β) is given by: n is calculated using the following relation: where: N a is Avogadro number, ρ is the density of substances and Ζ/A is the ratio of atomic number to the mass number of substances. The basic data for human body tissues are given in Table 1. The calculated mass stopping power of protons for lung, breast and ovary are based on Bethe equation after substituting the constants from Table 1. In Table 2, the compositions of the human tissues are given.

2) Calculations of range
Range, R the range for a heavy particle is the straight distance traveled by the   The range, R for an incident proton in the CSDA method is given as: where: 0 E is the initial energy of incident charged particle in material, f E is the final energy of incident charged particle in material and

( )
MS E is the mass stopping power [1].

Results and Discussion
The results of mass stopping powers for the present tissues are given in Table 3. In Figures 1-3, the mass stopping power of tissues and their compositions are plotted and it is noted that the mass stopping power of all tissues represent the average values of its compositions. The comparison between the present calculated mass stopping powers and that of Ref. [5] are shown in Figures 4-6, and good agreements between two results are observed in energy range 3 -200 MeV.
In Table 4, the ranges of all present human tissues are given. In Figures 7-9, a comparison between present results of ranges in all tissues and that of Ref. [5] are shown in energy range 0.04 -200 MeV and good agreements are observed. In Table 5 and Table 6 the empirical formulae for calculating mass stopping powers and ranges for all present human tissues are given. Also percentage difference between the present results of mass stopping powers and ranges and that of Ref. [5] are calculated for all tissues and the range of differences of percentage errors are given in Table 7.

Conclusion
In this work, calculations of mass stopping power and range of protons incident on the three different human body tissues (lung, breast and ovary) have been            done and the following conclusions are drawn: 1) The mass stopping power of each tissue gives the average value of mass stopping power of its compositions in energy range of 0.04 -200 MeV; 2) It is also noted that the empirical formulae for both mass stopping power and range for each human tissue studied in the present work are simple and widely applicable; 3) Values for mass stopping power and ranges of protons for breast, lung and ovary tissues are in good agreement with the data proposed by other researchers. This comparison is very important in terms of reliability of present work; 4) It also observed that the maximum value of mass stopping power was at 0.1 MeV and after that, the mass stopping power started to decrease with increasing energy; 5) The empirical formulae suggested for mass stopping power and ranges of protons are simple and they covered a large range of proton energy  as well as gave important information for those interesting in proton therapy.