Determination of the attenuation coefficient for megavoltage photons in the water phantom

Attenuation coefficient [micro] plays an important role in calculations of treatment planning systems, as well as determination of dose distributions in external beam therapy, dosimetry, protection, phantom materials and industry. So, its exact measurement or calculation is very important. The aim of this study was to evaluate the micro in different points in the water phantom analytically as a formula, in addition to derive and parameterize it with dosimetry measurements data results. To find the attenuation coefficients at each point along the central axis of the beam in the phantom for every size of the fields, the first mathematical approach was performed for derivation of micro s from percentage depth dose [PDD] formula. Then by dosimetry for different fields in different depths of water phantom, one can parameterize the obtained formula for micro in any field and depth. By comparing the mathematical and dosimetry results, the parameters of the micro-expression were derived in terms of the dimension of square field in different depths. From this formula one can find the micro for any field in different depths for two energies of the Varian 2100CD linear accelerator, 6, 18MV with the statistical coefficient of determination of R2>0.98. The measurement of the micro in each field size and depth has some technical problems, but one can easily measure the micro for every point of central axis of the beams in any field size

effect of energy and field size on determination of virtual source of the linear accelerator varian 2100 CD

In dosimetric calculation, it is desirable to represent the actual electron beam as originating from a virtual point source in vacuum so that the inverse square law can be applied. Therefore in this research with dosimetry of different treatment fields for different energy of electron beams the position of virtual source for electron beams is determined and one can use the inverse square rule for this electrons as the photons. In this work, the source position of electron beams [s[vir]] from a Varian clinac 2100 CD for different field sizes at different energies was determined by use of elastic scattering theory and dosimetry data. With a parallel plate chamber PPC40 in a 50x50x50cm[3], phantom and Omini-pro accept software the depth dose profiles of 6x6, 10x10, 15x15, 20x20 and 25x25 cm[2] applicators in different energies [6,9,12 and 15 Mev] for treatment electron beams determined. From the dose profile of maximum depth and multiple scattering rules of electrons from the target the position of virtual source of electron beams for each applicator in different energy were obtained. The obtained results from this method showed a dependence on energy and field size for S[vir]. The position of S[vir] is a function of dimension of treatment field and energy. In larger treatment fields and higher energies this distance is longer

Analytical approach for determination of effective point of measurement for cylindrical lonization chamber

Cylindrical ionization chamber is used for measurements of patient dose in radiation therapy. By determination the effective point of measurement we can determine the dose distribution and the parameters of treatment. Therefore in this research we introduce an analytical approach for determination this point in CC13 cylindrical ionization chamber that used for dosimetry of linear accelerator. In this study a CC13 ionization chamber was used for dose measurement of 6 and 18 photon beams of Variant accelerator in different field sizes, 5*5 cm[2] up to 35*35cm[2]. Measurements were performed in blue phantom, up to 5cm depth and data fit software was used for evaluation of experimental data. with our formalism the Pdis quantity in 6MV for 5x5 thought 35x35cm2 were measured 3.088 to 2.88 mm and in 18 MV are measured 4.62 to 4.20 mm. The first critical point of each curve is caused by changing environment from air to water during measuring of ionization. In fact it can be considered critical point as effective point of measurement of ionization chamber. Investigation shows that any increasing in field size, decrease and increasing of energy increase the depth of this critical point

[Effect of ionizing radiation of cobalt 60 on radiosensitivity of HeLa cells infected with Measles virus]

Radiotherapy is one of the cancer treatment methods. Prescribed dose for each fraction is considered based on radiosensitivity of tumoral and normal tissues. Viral agents are the effective factors on tissue sensitivity. This research aimed to determine the effect of ionizing radiation of Cobal 60 on radiosensitivity of Hela cells infected with Measles virus. In this study, the radiosensitivity of Hela cells is investigated experimentally and qualitively. The cells have been cultivated in two groups [experimental and blank] and plating efficiency has been obtained. Then 100 lambda measles virus with serial dilution method was used to induce infection in different ratio for experimental group. After cell growth and passage, the two groups were irradiated with 2Gy gamma radiation of cobalt 60. Results respectively indicated cell death increases up to 5-7%, 15-20% and 50-65%, after 2Gy irradiation by Co 60 for contaminating to Measles in low, moderate and high concentrations. The result in radiotherapy of cancers showed, in prescribing dose fraction non cancer disease should be considered

Calculation of analytical expressions for measured percentage depth dose data in megavoltage photon therapy

In external radiation therapy, the percentage depth dose [PDD] is an important factor for estimating patient dose and dose distribution in target volume; therefore, its exact measurement or calculation is important. The aim of this study was to evaluate analytically the dose received by different points in water phantom and to compare it with dosimetry measurement data. To find the dose distribution throughout the tumor volume, first, the mathematical approach was performed for derivation of percentage depth dose photon beams of 6MV and 18MV Varian accelerator. Second, by dosimetry for different fields in different depths of water phantom, one can parameterize the obtained formula for percentage depth dose. By comparing the mathematical and dosimetry results, the parameters of PDD-expression were computed in terms of the dimension of equivalent square field in different depths. From this formula, one can find the PDD for any fields in different depths, surface skin dose, and depth of build-up region of dose distribution, which are in agreement with empirical results with R2 > 0.995, showing a good agreement with the experimental data. So one can measure the surface skin-dose, the depth of build-up region, and their variations in terms of square field size exactly; the measurement of these quantities have some technical problems in radiation dosimetry

[Determination of the equivalent field size of asymmetric fields in photon beam therapy analytically based on physical theories]

The aim of this study was to obtain relatively theoretical methods based on physical rules in interaction of radiation with matters which can be sensitively, rapidly and easily used in different radiotherapy centers. In this study with the aid of the exponential attenuation rules of photon beams and inverse square distance rule, in 6MV and 18MV energy of 2100C Varian accelerator X-ray and 1.25MeV energy of Cobalt-60 gamma-ray in Ahvaz Golestan Hospital and considering the scatter radiation of therapeutic field and irregular field to be equal, the side of equivalent square field were calculated. In this method, since the calculation was done taking into account the situation of the field, energy and position of the shield, a very good agreement with dose measurements showed [with error, less than 2%]. Not all of these considerations are taken into account in previous methods. In addition, the calculations can be done on a computer for each therapeutic irregular field.

[Evaluation of radiosensitivity of HeLa cells infected with polio virus irradiated by Co 60]

The main purpose of radiotherapy is exposing enough doses of radiation to tumor tissue and protecting the normal tissues around it. Tumor dose for each session in radiotherapy will be considered based on radiosensitivity of the tissues. The presence of viral diseases in tumoral area can affect the radiosensitivity of cells. This study aimed to evaluate the radiosensitivity of Hela cells infected with poliomyelitis virus irradiated by Co 60. In this study, the radiosensitivity of HeLa cells, with or without the viral infection, after gamma radiation of cobalt 60, was assessed. Results of comparison of the radisensitivity of infected and uninfected cells indicates that after 2 Gy irradiation by Co 60, polio infection in low, moderate and high virus load, increases the cell death by 20-30%, 30-40% and 70-90% respectively. Radiosensitivity of tumoral cells increase when they are infected with viral agents. Results of this study showed that non cancer diseases should be considered when prescribing dose fraction in radiotherapy of cancers