Assessment of skyshine photon dose rates from 9 and 18 MV medical linear accelerators

Background: skyshine describes the radiation scattered by the atmosphere above a LINAC facility to a point on the ground. The aim of this study was to measure the skyshine photon dose rates from two different [9 MV and 18 MV] medical linear accelerators

Materials and Methods: the photon beam was directed upward [180 degree gantry position], with a maximum photon field size [40 × 40 cm2] at the isocenter. Measurements were obtained around the external points selected outside the room facilities at a horizontal distance from the target by the calibrated RDS-110 survey meter at four points around the isocenter

Results: the measured values of the skyshine photon exposure rates at four points for 9 MV and 18 MV were 0.6, 0.5, 0.5, and 0.4 microSv/h, and 0.6, 0.4, 0.4, and 0.5 micro Sv/h, respectively. All the measured skyshine photon exposure rates were lower than the values recommended by NCRP 147

Conclusion: there is a poor agreement between the measured and the calculated values; therefore it seems that caution is needed while using the equations available in NCRP 147 or 151

Development and implementation of a Monte Carlo frame work for evaluation of patient specific out- of - field organ equivalent dose

Background: The aim of this study was to develop and implement a Monte Carlo framework for evaluation of patient specific out-of-field organ equivalent dose [OED]

Materials and Methods: Dose calculations were performed using a Monte Carlo-based model of Oncor linac and tomographic phantoms. Monte Carlo simulations were performed using EGSnrc user codes. Dose measurements were performed using radiochromic films. Furthermore, the applicability of this framework was examined for a 3D conformal radiotherapy of breast

Results: Commissioning of the beam model was done by comparing the measured and calculated out-of-field dose values of several points in the physical and tomographic phantoms, respectively. The maximum percentage difference was 17%, which was smaller than 30% acceptance criteria for Monte Carlo modeling. The maximum statistical uncertainty in out -of-field dose calculation was 23%. Organ equivalent doses for out of field organs in 3D conformal radiotherapy of le1 breast varied from 2.4 cGy for right kidney to 134.6 cGy for the le1 humeral head

Conclusion: The framework developed in this research is a valuable tool for calculating peripheral dose and out-of field patient specific OEDs, the quantities needed for calculating risk of secondary cancer induction as a result of radiotherapy. This code can be used as a patient specific treatment plan optimization tool in order to select a treatment plan with the lowest risk of secondary cancer induction

[Quantifying the biomechanical forces applied on the vertebral column during praying]

Vertebral column or spine is the main part of the upper torso of human skeleton. The vertebral enables body to bend and turn in all directions, and supports the body and vital structures such as nerves and spinal cord. The aim of this research is to quantify and measure the moment and force on the L4 and L5 vertebrae in different races and genders, during praying. The necessity of performing this type of research on the spine is to show the effects of religious activity on physical health of people with different anthropometry. In this study, for biomechanical analysis of vertebral column, two methods were used: modeling and analysis in CATIA [Computer Aided Three-dimensional Interactive Application] V5R20 software and torque calculations based on inverse dynamics. The amount of torque and force which are applied on the spine during praying among different races were evaluated and compared by using these methods. In addition, by application of occupational health and safety standards in software, the motor position of manikin limbs was judged. Comparison between the compressive force exerted on the L4 and L5 vertebrae and standards in software indicated the amount of force on the L4 and L5 is less than NIOSH AL, which is 3433 N. In addition, in all nationalities, compressive force and moment about L4 and L5 in woman were lower than men. Also, results showed that the relative error of the torque calculated by inverse dynamics and CATIA software was 4.6 to 20.9 percent and the maximum percentage of population who were unable to perform bowing wasbelow 1% for all nationalities. The results of this study compared with international standards such as NIOSH, indicate that applying forces on the spine during daily praying task, contrary to the statements of some articles, does not create any problems for prayer

Assessment of radiation treatment chain for cervical cancer with combined external and brachy radiation therapy

Performing radiotherapy of cervical cancer by combined external radiotherapy and brachytherapy includes several stages. Inaccuracy of each stage may cause insufficient dose delivery and produce complications in neighboring radiosensitive organs. In this study a technique was developed in order to assure the quality of treatment delivery. A solid pelvic phantom was designed and fabricated for simulation of the entire radiotherapy procedure of the cervical cancer. Treatment planning for external radiotherapy was accomplished using computed tomography images and for intracavitary brachytherapy using orthogonal radiographs. Dose measurements were performed with an intracavitary ionization chamber. External radiotherapy was done using linear accelerator. The Nucletron Selectron low dose rate [LDR] machine was used for brachytherapy. For both modalities, the software calculated dose values were compared to the values measured in the pelvic phantom. The calculated data obtained from the treatment planning system was consistent with the measured data. The comparison between measurements and calculations showed a maximum variation of +/- 2 % for external radiation therapy and +/- 3.6 % for brachytherapy. The phantom and the procedure developed in this study successfully provided a tool for comprehensive evaluation of each step in the chain of radiation therapy under the same conditions found in actual treatment. This method can be used to verify the accuracy and reproducibility of this treatment in any department and also during commissioning of the treatment planning systems

quality assurance program for an amorphous silicon electronic portal imaging device using in-house developed phantoms: a method development for dosimetry purposes

Electronic portal imaging devices [EPIDs] play an important role in radiation therapy portal imaging, geometric and dosimetric verifications. A successful utilization of EPIDs for imaging and dosimetric purposes requires a reliable quality control process routine to be carried out regularly. In this study, two in-house phantoms were developed and analyzed for implementation in a quality assurance program for dosimetry purposes. An amorphous silicon [a-Si] imager [OptiVue500] was used. A low contrast resolution phantom and an image quality phantom were constructed and implemented. Low contrast resolution of the EPID was evaluated by counting the number of holes detectable in the image of phantom using human observers and a software. The image quality phantom was used for modulation transfer function, contrast to noise ratio and noise level evaluations. This phantom contains five sets of high-contrast rectangular bar patterns of variable spatial frequencies and six uniform regions. Although the manual low contrast resolution method was observer-dependent and insensitive to artifacts, the automatic method was robust and fully objective but sensitive to artifacts. The critical frequency values for 6 and 18 MV were 0.3558 +/- 0.006 lp/mm and 0.2707 +/- 0.006 lp/mm respectively. The contrast-to-noise ratio was found to be 240% higher for 6 MV compared to 18 MV. The developed phantoms provide a convenient process for periodic performance of an EPID. These phantoms are independent of the EPID system and provide robust tools for continuous monitoring of image quality parameters as well as dosimetric parameters

Design of a generally applicable abdominal shield for reducing fetal dose during radiotherapy of common malignancies in pregnant patients

In most cancer cases, the treatment choice for a pregnant patient is radiotherapy. In these patients, the abdomen is usually not exposed; therefore fetus exposure is due to peripheral dose [PD]. The purpose of this study was to estimate the fetal dose [the maximum PD in each pregnancy stage] for modalities available and to fabricate and evaluate a generally applicable fetal shield. PD values were measured for brain, breast and mediastinum irradiation in a whole body anthropomorphic phantom using a NE 2571 ionization chamber. An external shield was then designed to reduce the fetal dose to the standard dose limit, 5 mSv. The range of PD values as a function of distance from the field's edge were as follows 1] 9.4-259 cGy for Mantel field; 2] 6.5-95 cGy for chest wall irradiation with 10 MeV electrons, 3] 8.5- 52.5 cGy for tangential field with Co-60 and 4] 4.8-7.8 cGy for brain radiotherapy with 9 MV photon. PD values for the same setups using the fetal shield were as follows: 1] 1.4-22 cGy, 2] 0.5-4 cGy, 3] 1.5-5 cGy and 4] under 1 cGy. The measured PD data sets can be used to estimate fetal dose for specific treatment setups and pregnancy stages. The use of external shield designed in this research reduced the fetal dose effectively to under the threshold [a 70-90% reduction], except for the final stages of pregnancy in Hodgkin's patients

[Dosimetric calculations of a radioactive eye plaque used in management of ocular melanoma using Monte Carlo simulations]

Brachytherapy using I-125 radioactive seeds in removable episcleral plaques [EP] is often used in treatment of ocular malignant melanoma. Some radioactive seeds are fixed in a gold bowl-shaped plaque. The plaque is sutured to the sclera surface corresponding to the base of the intraocular tumor, allowing for a localized radiation dose delivery to the tumor. Minimum target doses as high as 85Gy are directed at malignant tumor. The aim of this study was to develop a Monte Carlo simulation of an ocular plaque in order to calculate the resulting isodose distributions. The MCNP-4C Monte Carlo code is used to simulate the plan of an episcleral plaque treatment. A 20-mm Collaborative Ocular Melanoma Study [COMS] plaque with 3, I-125 seed of model 6711 was used. Resulting dose distributions, including central axis dose and off-axis dose profiles, were calculated in a water phantom with 12mm radius. The calculated dose distributions were compared to the corresponding dose measured by Knuten et al., 2001. Central axis dose calculations represent a rapid dose fall off, which is an important factor in selection of appropriate eye plaque for management of tumors with known dimension. Calculated off-axis dose profiles show decreased dose uniformity at distances close to the plaque. Increasing of distance from the plaque resulted in increasing of the dose uniformity. Monte Carlo simulation of eye plaques can be used as a useful tool in process of design, development and treatment planning of ocular radioactive plaques

Determination of absorbed dose of organs [thyroid, sternum, cervical vertebra] in thyroid cancer patients following radioiodine therapy

In patients with thyroid carcinoma, radiation absorbed doses of the thyroid and surrounding tissues is important to weigh risk and benefit considerations. In nuclear medicine, the accuracy of absorbed dose of internally distributed radionuclides is estimated by different methods such as MIRD and direct method using TLD. The aim of this study is using TLD and a phantom to determine the amount of cumulated activity in thyroid and surrounding tissues. Thermoluminescent dosimeter [TLD] measurements were performed on 27 patients on the skin over the thyroid, sternum and cervical vertebra. There were 5 TLDs for each organ which they were taken after 4, 8, 12, 20 and 24 hr. To calculate the amount of activity in the thyroid a head and neck phantom with a source of 10 mCi of [131] I was used. Several TLDs were placed putted on the surface of thyroid on phantom [similar to patients] for 24 hr and then compared the dose of phantom and patients followed by calculation of the activity in patient's thyroid. TLD measurements showed cumulated radiation absorbed doses [cGy] of 315.6, 348.1 and 361.9 for thyroid with administration of 100, 150 and 175 mCi of [131] I, respectively. For sternum the values found to be 201.5 cGy, 275.2 cGy and 242.6 cGy. For cervical vertebra results were 311.5 cGy, 184.1 cGy and 325.9 cGy. The average of measurements was 33.3 cGy using of TLDs on phantom and absorbed activity in thyroid were 94.9, 104.6 and 108.8 mCi in 24 hr for mentioned doses administration. In this work a method to obtain the absorbed activity in the thyroid and other surrounding tissues is described. By this method, the amount of [131] I needed for each patient also could be determined. The results of this work can be used in estimation of absorbed dose in thyroid and other organs using of MIRD method