Study of tissue inhomogeneity e ects on central axis radiation beam parameters using monte Carlo methods

Study of tissue inhomogeneity e ects on central axis radiation beam parameters using monte Carlo methods Santhosh VS.1*, Anand RK.2 DOI: https://doi.org/10.17511/ijmrr.2020.i05.03 1* Santhosh VS, Assistant Professor, Department of Radiation Physics, Government Medical College, Thiruvananthapuram, Kerala, India. 2 Anand RK, Assistant Professor, Department of Radiation Physics, Government Medical College, Thiruvananthapuram, Kerala, India.


Introduction
The outcome of radiotherapy in cancer care is heavily dependent on the accuracy of radiation dose  [3]. However in actual clinical practice the radiation beam has to transport through a human body and human body is not a uniform media comparing with the standard experimental media like water phantoms.
The human body consists of a variety of tissues and cavities with different physical and radiological properties. Most important among these, from a radiation dosimetry perspective, are tissues and cavities that are radiologically different from water, including lungs, oral cavities, teeth, nasal passages, sinuses and bones. In some instances, foreign materials, such as metallic prostheses, are also present.   Table. Material Density (gm/cc)

Results
The maximum dose is at 0.425cm depth for 60Co beams. For 6MV photons the PDD curves are given in figure 2(b). Depth of maximum dose obtained is 1.5 cm.     [37].
In the second part of simulation we conducted the Monte Carlo simulation for a modicum with tissue at first layer and then bone and then again tissue. Our results shows that in the first layer of tissue the PDD pattern is exactly same as that of homogeneous situation as mentioned above.
However at the interface between tissue and bone the PDD pattern changed. The absorbed dose at bone layer is higher than the dose value predicated in a homogeneous condition. The deviation of up to 5.4 percentage was observed in the bone layer.
The theoretical reason for this change in dose distribution is due to change in material composition and consecutive change in absorption of radiation beam and change in primary radiation beam flounce, secondary electron flounce as well as scattered radiation flounce. The photon attenuation at bone is more predominant due to high electron density.More over at the interface between the two media the reason for dose distribution is due to loss of electron equilibrium conditions [38]. Surendra N Rustigi et al used Monte Carlo methods to investigate the perturbation effects caused by high density Inhomogeneity for small field sizes and found good agreement between experimental and Monte Carlo simulations in dose reduction factors [39]. Cardosa et al studied the perturbation effects at the tissue bone interface and they observe that bone has a large effect on the central axis dose of small photon beams. The dose to the bone is increased while the dose beyond the bone is decreased.
They concluded that If the bony heterogeneity is not taken into account, differences of 7 and 4% can be found in PDD planning to 2×2 and 10×10 cm2 field sizes, respectively, at soft tissue after this heterogeneity [40]. Nisbet  PDD values at other points in the air medium show variation up to 95.3% than the homogeneous condition. This much variation due to the fact that absorbed dose in air will be much less than that in tissue due to the difference in their densities and change in electronic equilibrium conditions [38].
Interestingly it is also observed that a buildup condition occur at the second air tissue interface. In consideration when choosing the accelerator energies with the increasing use of IMRT and radiosurgery and small fields [44]. It was also reported that in many clinical situation the air cavities present in such as those found in upper respiratory passages.
This air cavities results the under dosing of lesions distal to air cavities and occurs due to the loss in lateral charged-particle equilibrium (CPE) especially for smaller field sizes, resulting in more frequent recurrence of the cancer treated. It was also reported that In addition to the loss of lateral charged particle equilibrium the

Conclusion
The present study clearly demonstrate that lowdensity materials such as air cavities as in lung tissue etc or high density mediums such as bone in the path of the radiation beam will alter the dose distribution in the tumor. For the accurate delivery of the radiation dose to the tumor, the dose perturbation caused by these inhomogeneity has to be taken care. Monte Carlo simulation is found to be an accurate method to evaluate the in homogeneity effects.

Author's Contribution
Dr. Santhosh VS was the primary investigator of the study, collected data, Drafting the manuscript. Dr. Anand RK helped to get data related to tissues and manuscript preparation. International Journal of Medical Research and Review 2020;8(5)