Correction factors for water-proofing sleeves in kilovoltage x-ray beams

This paper investigates the effect of the waterproofing sleeve on the calibration of kilovoltage photon beams (50-300 kV). The sleeve effect correction factor, ps has been calculated using the Monte Carlo method as the ratios of the air kerma in an air cavity of a cylindrical chamber without the waterproofing sleeve to that with a sleeve. Three sleeve materials have been studied, PMMA, nylon and polystyrene. The calculations were carried out using the EGS4 (Electron Gamma Shower version 4) code system with the application of a correlated-sampling variance-reduction technique. The results show that the sleeve correction factor for 1-mm thick nylon and polystyrene sleeves, ps varies from 0.992 to 1.000 and from 0.981 to 1.000, respectively, for the same beam quality range. The ps factor varies with sleeve thickness, beam quality and phantom depth. No significant dependence of the ps factor on field size and source-surface distance has been found. Measurements for PMMA, nylon and polystyrene sleeves of various thicknesses have also been carried out and show excellent agreement with Monte Carlo calculations.     

Study of dosimetry consistency for kilovoltage x-ray beams

In this paper, the consistency of kilovoltage (tube potentials between 40 and 300 kV) x-ray beam dosimetry using the "in-air" method and the in-phantom measurement has been studied. The procedures for the measurement of the central-axis depth-dose curve, which serve as a link between the dose at the reference depth to the dose elsewhere in a phantom, were examined. The uncertainties on the measured dose distributions were analyzed with the emphasis on the surface dose measurement. The Monte Carlo method was used to calculate the perturbation correction factors for a photon diode and a NACP plane-parallel ionization chamber at different depths in a water phantom irradiated by 100-300 kV (2.43 mm Al-3.67 mm Cu half-value layer) x-ray beams. The depth-dose curves measured with these two detectors, after correcting for the perturbation effect (up to 15% corrections), agreed with each other to within 1.5%. Comparisons of the doses at the phantom surface and at 2 cm depth in water for photon beams of 100-300 kV tube potential obtained using the "backscatter" method and those using the "in-phantom" measurement have shown that the "in-air" method can be equally applied to this energy range if the depth-dose curve can be measured accurately. To this end, measured depth ionization curves require depth-dependent correction factors.     

Calculation of backscatter factors for diagnostic radiology using Monte Carlo methods

Backscatter factors were determined for x-ray beams relevant to diagnostic radiology using Monte Carlo methods. The phantom size considered most suitable for calibration of dosimeters is a cuboid of 30 x 30 cm2 front surface and 15 cm depth. This phantom size also provides a good approximation to adult patients. Three different media were studied: water, PMMA and ICRU tissue; the source geometry was a point source with varying field size and source-to-phantom distance. The variations of the backscatter factor with phantom medium and field geometry were examined. From the obtained data, a set of backscatter factors was selected and proposed for adoption as a standard set for the calibration of dosimeters to be used to measure diagnostic reference doses.     

Relations between scatter factor, quality index and attenuation for x-ray beams

In a human in-vitro fertilization (IVF) programme, the effect of co-culture of embryos with human fibroblasts was evaluated with respect to pregnancy rate and embryo development. Patients were included in the study after giving informed written consent. The IVF treatments were randomly assigned by stratification of both age (<36 versus > or =36 years) and previous IVF attempts (yes versus no). After fertilization was established, the zygotes were transferred to a 4-well dish with or without fibroblasts and cultured for 2 days. On the third day after ovum pick-up (OPU), cell number and quality [5 (good) to 1 (poor)] of the embryos were scored and a maximum of three embryos was transferred. Supernumerary embryos of good quality were cryopreserved. The design of this study was a group sequential trial with the objective of detecting differences between pregnancy rates following IVF with conventional incubation or incubation in co-culture with fibroblasts. This design included one evaluation at half-way data collection. In the study, 148 patients had an OPU, of whom 77 were allocated to the co-culture group. There was no statistically significant difference in pregnancy rate, cell number and embryo quality between the two groups. The ongoing pregnancy rate per embryo transfer was 27% in co-culture and 30% in the conventional culture group. The implantation rates per transferred embryo were 17 and 18% respectively. Using a multivariate logistic regression model for the probability of ongoing pregnancies, the odds ratio of co-culture, adjusted for age and previous IVF attempts, was not statistically significant. In conclusion, co-culture with human fibroblasts does not contribute to an improvement of embryo quality nor to a higher pregnancy rate after IVF in an unselected group of patients.     

Polynomial and power law of central axis backscatter factors for external and gamma-ray sources

A third-degree polynomial and power-law analysis with the method of least-squares fit are computed for backscatter factor with a continuous variation of field size and different beam quality of radiation from 1.0-mm Al external x ray to 60Co gamma ray. Three coefficients are required for a third-degree polynomial and two coefficients for power law method of each beam quality.     

Field margin reduction using intensity-modulated x-ray beams formed with a multileaf collimator

PURPOSE: In axial, coplanar treatments with multiple fields, the superior and inferior ends of a planning target volume (PTV) are at risk to get underdosed due to the overlapping penumbras of all treatment fields. We have investigated a technique using intensity modulated x-ray beams that allows the use of small margins for definition of the superior and inferior field borders while still reaching a minimum PTV-dose of 95% of the isocenter dose. METHODS AND MATERIALS: The applied intensity modulated beams, generated with a multileaf collimator, include narrow (1.1-1.6 cm) boost fields to increase the dose in the superior and inferior ends of the PTV. The benefits of this technique have been assessed using 3D treatment plans for 10 prostate cancer patients. Treatment planning was performed with the Cadplan 3D planning system (Varian-Dosetek). Dose calculations for the narrow boost fields have been compared with measurements. The application of the boost fields has been tested on the MM50 Racetrack Microtron (Scanditronix Medical AB), which allows fully computer-controlled setup of all involved treatment fields. RESULTS: Compared to our standard technique, the superior-inferior field length can be reduced by 1.6 cm, generally yielding smaller volumes of rectum and bladder in the high dose region. For the narrow boost fields, calculated relative dose distributions agree within 2% or 0.2 cm with measured dose distributions. For accurate monitor unit calculations, the phantom scatter table used in the Cadplan system had to be modified using measured data for square fields smaller than 4 x 4 cm2. The extra time needed at the MM50 for the setup and delivery of the boost fields is usually about 1 min. CONCLUSION: The proposed use of intensity modulated beams yields improved conformal dose distributions for treatment of prostate cancer patients with a superior-inferior field size reduction of 1.6 cm. Treatments of other tumor sites can also benefit from the application of the boost fields.     

Characteristics of secondary electrons produced by 6, 10 and 24 MV x-ray beams

Megavoltage x-ray beams generated by linear accelerators (linacs) deliver their maximum dose a few centimetres below the treatment or phantom surface. This skin-sparing effect is degraded by the generation of secondary electrons as the x-ray beam passes to the patient or phantom. This work measures the characteristics of these electrons. A light-weight electromagnet was constructed that could be mounted in the block-tray position, 58 cm from the x-ray source of a Varian Clinac 2100C or 2500 linac. A field strength as high as 0.1500 T was generated, which was strong enough to sweep secondary electrons out of a 10 cm x 10 cm field. For 6, 10 and 24 MV x-ray beams, secondary contamination electrons produced 18, 38 and 65% of the surface dose, corresponding to 3, 5 and 12% of the maximum dose, respectively. A parameterized depth-dose curve for the contamination electrons was produced and was valid for all the x-ray energies studied.     

Clinical implementation of a two-component x-ray source model for calculation of head-scatter factors

A calculation method is described, which presumes that linac output has two components. One component is direct x rays from the target and the remaining component is an extra-focal, distributed source of radiation that is scattered from the flattening filter, primary collimator, and the jaws of the moveable collimator. This calculation method gives values for output factors that differ from measured values by no more than 0.5% for field widths from 4 to 40 cm, rectangular fields with long to short axis ratios as great as 10, symmetric and asymmetric fields, and source-to-axis distances from 65 to 360 cm. While this calculation method has great accuracy and flexibility, a minimal amount of input data is required: (1) measured output factors at a source-to-axis distance of 100 cm for square fields; (2) positions of the collimator with respect to the x-ray source target; and (3) output factors measured at various source-to-axis distances for a 10 cm x 10 cm field.     

A dual source photon beam model used in convolution/superposition dose calculations for clinical megavoltage x-ray beams

A realistic photon beam model based on Monte Carlo simulation of clinical linear accelerators was implemented in a convolution/superposition dose calculation algorithm. A primary and an extra-focal sources were used in this beam model to represent the direct photons from the target and the scattered photons from other head structures, respectively. The effect of the finite size of the extra-focal source was modeled by a convolution of the source fluence distribution with the collimator aperture function. Relative photon output in air (Sc) and in phantom (Scp) were computed using the convolution method with this new photon beam model. Our results showed that in a 10 MV photon beam, the Sc, Sp (phantom scatter factor), and Scp factors increased by 11%, 10%, and 22%, respectively, as the field size changed from 3 x 3 cm2 to 40 x 40 cm2. The variation of the Sc factor was contributed mostly by an increase of the extra-focal radiation with field size. The radiation backscattered into the monitor chamber inside the accelerator head affected the Sc by about 2% in the same field range. The output factors in elongated fields, asymmetric fields, and blocked fields were also investigated in this study. Our results showed that if the effect of the backscattered radiation was taken into account, output factors in these treatment fields can be predicted accurately by our convolution algorithm using the dual source photon beam model.     

Feasibility of a large area x-ray sensitive vidicon for medical fluoroscopy: signal and noise factors

A large area x-ray-sensitive vidicon utilizing amorphous selenium (a-Se) is being investigated as an alternative to the x-ray image intensifier and television camera combination (XRII/TV) for medical fluoroscopy. The x-ray vidicon is, to a first approximation, a scaled up version of the 1" (2.54 cm) light-sensitive vidicon. The modulation transfer function (MTF) of an x-ray vidicon is mainly determined by the number of lines used, the beam spot profile, and the electronic bandwidth. The x-ray vidicon is expected to have roughly the same MTF as the 1" vidicon from a XRII/TV system referenced to the x-ray input and as such will be superior to the MTF of the complete XRII/TV system especially in high resolution applications such as cardiac cine. Beam discharge lag is expected to be approximately the same as in the 1" vidicon since they have comparable photoconductor layer capacitance. Photoconductive lag is also expected to be low since a-Se is known to have a low trap density.