Patient doses from X-ray computed tomography examinations by a single-array detector unit: axial versus spiral mode

X-ray computed tomography [CT] examinations deliver a significant amount of radiation doses to patients comparing to conventional radiography examinations. The objective of the current study was to analyze and investigate the average patient received dose from axial and spiral CT exams in a medical imaging center. In this study, the patient imaging technique, weight and height were recorded. The patients' doses provided by CT unit in terms of CTDI[w] were also recorded. Then, other dosimetric quantities including dose-length product [DLP] and effective dose were calculated for each patient using the recorded data. The average values were obtained for all the studied dosimetric quantities. Also, their distribution in terms of examined regions and imaging mode; ie, axial and spiral CT were analyzed by SPSS software. For all patients, the mean effective dose of 4.4 mGy with the standard deviation of 9.2 was found. The CTDI[w] for axial group was two times higher than spiral ones. Conversely, the effective dose of axial group was less than spiral group. Additionally, the effective doses of 2.3 and 5.2 mSv were found for axial and spiral, receptively. For both quantities of CTDI[w] and effective dose, the observed difference between axial and spiral modes were significant [P<0.001]. Our results showed that although the patient doses in the current study was comparable with the reported values by similar studies in other countries, it was higher than the reported values of a similar study in Iran. Exposure technique's optimization and further review in routine CT examinations were recommended

Monte Carlo estimation of electron contamination in a 18 MV clinical photon beam

The electron contamination may reduce or even diminish the skin sparing property of the megavoltage beam. The detailed characteristics of contaminant electrons are presented for different field sizes and cases. The Monte Carlo code, MCNPX, has been used to simulate 18 MV photon beam from a Varian Linac-2300 accelerator. All dose measurements were carried out using a PTW-MP2 scanner with an ionization chamber [0.6 CC] at the water phantom. The maximum electron contaminant dose at the surface ranged from 6.1% for 5 x 5 cm[2]to 38.8% for 40 x 40 cm[2] and at the depth of maximum dose was 0.9% up to 5.77% for the 5 x 5 cm[2] to the 40 x 40 cm[2] field sizes, respectively. The additional contaminant electron dose at the surface for the field with tray increased 2.3% for 10 x 10 cm[2], 7.3% for 20 x 20 cm[2], and 21.4% for 40 x 40 cm[2] field size comparing to the standard field without any accessories. This increase for field with tray and shaping block was 5.3% and 13.3% for 10 x 10 and 20 x 20 cm[2], respectively, while, the electron contamination decreased for the fields with wedge, i.e. 2.2% for the 10 x 10 cm[2] field. The results have provided more comprehensive knowledge of the high-energy clinical beams and may be useful to develop the accurate treatment planning systems capable of taking the electron contamination in to account

Monte Carlo characterization of photoneutrons in the radiation therapy with high energy photons: a comparison between simplified and full Monte Carlo models

The characteristics of secondary neutrons in a high energy radiation therapy room were studied using the MCNPX Monte Carlo [MC] code. Two MC models including a model with full description of head components and a simplified model used in previous studies were implemented for MC simulations. Results showed 4-53% difference between full and with the simplified model in the neutron fluence calculation. Additionally, in full MC model, increase in the field size decreased the neutron fluence but for simplified model, increase in the field size led to increase in neutron fluence. In calculating the neutron and capture gamma ray dose equivalent, simplified model overestimated [9-47%] and [20-61%] respectively in comparison to the full simulated model. However, a close agreement was seen between two models, for field size of 10x10 cm[2]. For MC modeling of photoneutrons and capture gamma in radiotherapy rooms, the detailed modeling of linac head instead of simplified model is recommended

Shielding evaluation of a typical radiography department: a comparison between NCRP reports No.49 and 147

Designing and shielding of an appropriate radiography room has been one of the major concerns of radiation scientists since the first decade after the invention of X-rays. Recently, report no.147 of National Council on Radiation Protection and Measurements [NCRP] has been published. In this study the researchers have investigated the effect of new report recommendations on primary and secondary barriers thicknesses in comparison to NCRP 49, and 116 recommendations. To calculate the walls thickness of a conventional radiography room, the workload of a radiography room of a university hospital was determined by recording the number of exposures, mAs and kVp for each patient during six months. Three types of calculations were done: [1] Using NCRP 49 formulations and dose limits [2] Using the NCRP 49 formulations and dose NCRP 116 dose limits and [3] Using the NCRP 147 recommendations. The estimated workload was 172 mA min wk [-1] for the studied radiography room which was slightly lower than the workload recommended by NCRP 147. The results showed that using the NCRP 49 formulation and NCRP116 dose limits, the barriers thickness increases substantially. Moreover, the dose limits were lower in NCRP 147, using the third method. The primary barrier the results of the two methods [1] and [3] did not differ and remained the same. Application of NCRP 49 and NCRP 116 dose limits for radiography room shielding [second method] overestimated the primary and secondary barriers thickness, significantly. But, applying NCRP 147, not only the new dose limits were considered, but also the cost of primary barrier construction was reduced

effect of electronic disequilibrium on the received dose by lung in small fields with photon beams: Measurements and Monte Carlo study

Prediction of the absorbed dose in irradiated volume plays an important role in the outcome of radiotherapy. Application of small fields for radiotherapy of thorax makes the dose calculation process inaccurate due to the existence of electronic disequilibrium and intrinsic deficiencies in dose calculation algorithms. To study the lung absorbed dose in radiotherapy with small fields, the central axis absorbed dose in heterogeneous thorax phantom was measured by ionization chamber and calculated for small fields by Monte Carlo [MC] method. A solid slab phantom consisting of unit and low density materials was used for dose measurements. The 6 and 18 MV photon beams of Elekta SL25 linac were simulated using MCNP4C MC Code. The model was validated by comparing the calculated depth dose and beam profiles with measurements in a water phantom. The MC model was used to calculate the depth doses in unit density and low density materials resembling the soft tissue and lung, respectively. Two small field sizes including 5*5 and 2*2 cm[2] were used in this study. The measured depth dose values were in good agreement with MC results and the difference less than 2% was observed. A large dose reduction was seen in lung for field size of 2*2 cm2 due to the lateral electronic disequilibrium and it reached up to 16.2% and 33.3% for 6 and 18 MV beams, respectively. Dose build up and down at material interfaces was predicted by MC method. Our study showed that the dose reductions with small fields in lung and dose variations at interfaces was very considerable, and inaccurate prediction of absorbed dose in lung using small fields and photon beams may lead to critical consequences for patients

Experimental evaluation of midline dose calculation methods in In vivo dosimetry using anatomic thorax phantom

In vivo dosimetry is a method for estimation of overall error in the delivered dose to the patients at the end of radiotherapy process. In this research, two methods for target dose calculation were evaluated on midline and central axis of photon beams in in vivo dosimetry of thorax fields. Entrance and exit doses for anterior and lateral fields of thorax were measured in thorax phantom using diode dosimeter. Also, the doses of some points on midline and central axis were measured in thorax phantom using ionization chamber. The dose at these points was calculated using entrance and exit doses by geometric and arithmetic mean methods. The calculated doses were compared with measured doses. In all cases, arithmetic mean method showed errors from%8.8 to 19% for points on midline and central axis in comparison to measurements. The range of errors for geometric method was from%1.5 to%8 depending on distance from midline. The results showed that doses of points on midline and central axis can be calculated with acceptable accuracy from entrance and exit doses using geometric mean in thorax fields

comparative Monte Carlo study on 6MV photon beam characteristics of Varian 2 iEX and Elekta SL-25 Linacs

Monte Carlo method [MC] has played an important role in design and optimization of medical linacs head and beam modeling. The purpose of this study was to compare photon beam features of two commercial linacs, Varian 21EX and Elekta SL-25 using MCNP4C MC Code. The 6MV photon beams of Varian 21EX and Elekta Sl-25 linacs were simulated based on manufacturers provided information. Photon energy spectra and absolute absorbed dose values were calculated for field sizes of 10_10 and 20_20 cm2. Also, contamination electron spectra for field size of 20 _20 cm2 were scored for both linacs. Our results showed that the relative absorbed dose values and contamination electron spectrum were similar and comparable, but photon fluence and absolute absorbed dose values were 17% and 13% higher for Varian linac respectively for the field size of 10_10 cm2. Despite the differences in head components of two commercial linacs, their relative depth dose values were very close to each other. The absolute dose per incident electron showed some discrepancy, as well. Thus, this study suggests the use of absolute absorbed dose values as an invaluable factor when different linacs head are compared using Monte Carlo Method

Comparison of prescribed and delivered dose to patients in radiotherapy department of Tabriz Imam-Khomeini hospital using in vivo dosimetry

Accuracy of the delivered dose to the patient is one of the most important and effective factors in radiotherapy. In vivo dosimetry is used to evaluate the accuracy of delivered dose in radiotherapy. Therefore, in this study the accuracy of delivered dose to the patients was verified using in vivo dosimetry in radiotherapy department of Tabriz lmam Hospital. Entrance doses of 320 fields treated by Cobalt-60 machine and linear accelerator including head and neck, trunk, pelvis, and extremities were measured using in vivo dosimetry system. Difference between measured entrance dose and prescribed dose for each field was determined. For all fields and also for each specific group of fields, average error and standard deviation of its distribution were determined. For whole fields, the average error of 1.34% with standard deviation [SD] of 7.12%, for [60]Co fields the average error of -0.17% with SD of 5.98% and for Linac fields, the average error of -3.56% and 7.17% were seen. In head and neck fields, the average error of -0.37% with SD of 4.73%; for trunk fields the average error of%-1.88 with SD of 7.27%, and for pelvis and extremities, the average error of -2.49% with SD of 7.79% were seen. The results show an acceptable systematic error in radiotherapy department. But standard deviation of 7.12% is slightly higher than the recommended value of 5%. It is possible to lower the uncertainty in delivered dose to recommended value by optimization of Linac function, regular quality control, and workload reduction

Development a simple point source model for Elekta SL-25 linear accelerator using MCNP4C Monte Carlo code

Monte Carlo [MC] modeling of a linear accelerator is a prerequisite for Monte Carlo dose calculations in external beam radiotherapy. In this study, a simple and efficient model was developed for Elekta SL-25 linear accelerator using MCNP4C Monte Carlo code. The head of Elekta SL-25 linac was simulated for 6 and 18 MV photon beams using MCNP4C MC code. Energy spectra and fluence distribution of photons crossing the phase space plane were calculated. A simple point source model was developed based on calculated photon spectra and spatial distribution. Using this model, percent depth doses [PDDs], and beam profiles were calculated for different field sizes. The results of MC calculations were compared with measurements. There was a good agreement between MC calculations and measurement for descending part of PDD curves. However, comparing calculated PDDs with measurement showed up to 10% differences for build up region of PDD curves for both energies. For beam profiles, there was 2% difference in flat region and up to 15% difference was seen for out of field region. These results were acceptable according to the recommended criteria. Using this model, the run time was decreased 24 times in comparison to original full Monte Carlo method. Our study showed the presented model to be accurate and effective for MC calculations in radiotherapy treatment planning. Also, it substantially lowers MC runtime for radiotherapy purposes

[Comparison of different computer speeds in calculating of Co[60] depth doses by MCNP4A and MCNP4B Monte Carlo codes]

Background and Objective: MCNP is a general-purpose Monte Carlo code that is used for simulating of neutrons, photons and electrons transport in different media. Recently this code has been used for radiotherapy dosimetry and treatment planning. In recent investigations, the reasonable run-time was not acquired for clinical use of Monte Carlo method. In this research, the speeds of the computers available in Iran were compared in running a percent depth dose calculation [PPD] for CO[60] teletherapy machine

Methods: Geometry of a typical CO[60] teletherapy machine and a water phantom were simulated. Both version of MCNP code were installed on Pentium 233, 866, 1500 MHz, 700 MHz Duran and Athelon 1333MHz personal computers. Percent depth dose of CO[60] gamma rays in water phantom for 10×10 cm was calculated by each computer

Findings: The time required to computer the PDD by F6 tally was 60 times greater than the F8 tally. In all the cases, the 4A version was approximately 5% faster than 4B version. This suggests that in radiotherapy application like our test problem there is not considerable computing time difference between 4A and 4B version

Conclusion: The results recommend the use of F6 tally in radiotherapy application by CO[60] gamma rays where the point of interest are not situated in electronic disequilibrium regions and when the time of calculation is important