Dose In Buildup Region Research Articles

Surface and build-up region dose analysis for clinical radiotherapy photon beams

The standard clinical approach of dose measurement using a Farmer type fixed plane parallel cylindrical ionization chamber produces erroneous results in the build-up region. We studied dose distribution in this region using a Monte Carlo simulation technique and compared our results with data measured using extrapolation, parallel plate, and cylindrical farmer type ionization chambers for 6 and 10 MV photon beams from two different accelerators. The extrapolation chamber data agreed favorably with the Monte Carlo results, suggesting that dose at the skin surface and a few mm beneath is significantly lower than conventionally accepted values.

Dosimetric properties of a flattening filter-free 6-MV photon beam: a Monte Carlo study

The dosimetric features of an unflattened 6-MV photon beam of an Elekta SL-25 linac was calculated by the Monte Carlo (MC) method. The head of the Elekta SL-25 linac was simulated using the MCNP4C MC code. The accuracy of the model was evaluated using measured dosimetric features, including depth dose values and dose profiles in a water phantom. The flattening filter was then removed, and beam dosimetric properties were calculated by the MC method and compared with those of the flattened photon beam. Our results showed a significant (twofold) increase in the dose rate for all field sizes. Also, the photon beam spectra for an unflattened beam were softer, which led to a steeper reduction in depth doses. The decrease in the out-of-field dose and increase in the contamination electrons and a buildup region dose were the other consequences of removing the flattening filter. Our study revealed that, for recent radiotherapy techniques, the use of multileaf collimators for beam shaping removing the flattening filter could offer some advantages, including an increased dose rate and decreased out-of-field dose.

SU‐FF‐T‐438: Using Diode Dosimeters to Characterize Dose in the Buildup Region of High‐Energy Photon Beams

Purpose: For external photon beams, the depth dependence of the stopping power ratio between air and water in the buildup region is difficult for accurate quantitative characterization. Qualitatively the depth dependence increases as the photon energy increases above the cobalt‐60 energy. As a result, the depth dependence is often ignored and the “uncorrected” percentage depth dose curves are being used in many clinics. However, accurate knowledge on dose in the buildup region is important for applications such as the tangential beam setup. This work is to look for a dosimeter that measures with ease the correct photon beam PDD curves for all depth in phantom including the buildup region and beyond. Method and Materials: The PDDs of a 10 MV photon beam at field size of 10×10 cm2 and 100 cm SSD was scanned in a water‐tank with a cylindrical (Scanditronix, RK) ion chamber, a parallel‐plane (NACP) ion chamber, and a diode dosimeter (Scanditronix, designed for photon field with back‐scatter absorber). The raw scan readings were corrected only for the effective point shifts as defined for depth beyond dmax. The Monte Carlo code EGS was used to generate the “true” PDD curve. Results: The ion chambers, diode and EGS show good agreement beyond the dmax region. In the buildup region, the diode matches the EGS curve within 3% while the two ion chamber curves differ from the EGS curve by 5% to 20%. Conclusion: A scanning diode dosimeter can be designed to generate true PDD curves, with a simple effective point correction, for both the buildup region and beyond for high energy photon beams. The ease of use afforded by such diode dosimeters would mean more availability of true PDD curves at clinic, including the dose buildup region.

Validation of Monte Carlo calculated surface doses for megavoltage photon beams

Recent work has shown that there is significant uncertainty in measuring build-up doses in mega-voltage photon beams especially at high energies. In this present investigation we used a phantom-embedded extrapolation chamber (PEEC) made of Solid Water to validate Monte Carlo (MC)-calculated doses in the dose build-up region for 6 and 18 MV x-ray beams. The study showed that the percentage depth ionizations (PDIs) obtained from measurements are higher than the percentage depth doses (PDDs) obtained with Monte Carlo techniques. To validate the MC-calculated PDDs, the design of the PEEC was incorporated into the simulations. While the MC-calculated and measured PDIs in the dose build-up region agree with one another for the 6 MV beam, a non-negligible difference is observed for the 18 MV x-ray beam. A number of experiments and theoretical studies of various possible effects that could be the source of this discrepancy were performed. The contribution of contaminating neutrons and protons to the build-up dose region in the 18 MV x-ray beam is negligible. Moreover, the MC calculations using the XCOM photon cross-section database and the NIST bremsstrahlung differential cross section do not explain the discrepancy between the MC calculations and measurement in the dose build-up region for the 18 MV. A simple incorporation of triplet production events into the MC dose calculation increases the calculated doses in the build-up region but does not fully account for the discrepancy between measurement and calculations for the 18 MV x-ray beam.

Contaminant Dose Variation in Buildup Region for Clinical Photon Beams

An important consideration in the use of photon beams for treatment is the estimation of contaminant dose delivered to the buildup region. It has been known that the buildup dose varies as a function of treatment field size. The major contributors of buildup region doses are the flattening filter and the air volume. We calculated the effect by contaminant dose to the buildup dose in a water phantom, and compared it to experimental data. The contribution of contaminant dose can be extrapolated from the depth dose data by using the depth kerma derived from the measured quantities and the inverse square law for the incident photon beams. The depth dose was measured by using radiation field analyzer. Also it was measured for a range of field sizes with acrylic tray, external wedge, and square Lipowizs metal blocks on a solid acrylic tray. In the analysis for the 6 MV photon beams, the maximum factor of contaminant dose varies from 2% to 16% for square field size ranging from 5 to 40 cm, respectively. Comparing with the open beam, the contaminant dose variation increases as a solid tray is used, and its magnitude increases with field size. The contaminant dose variation for the wedged field is less than that for the open field.

A comparison of 18-MV and 6-MV treatment plans using 3D dose calculation with and without heterogeneity correction

Homogeneity of the dose distribution in irradiation of the intact breast for stage I and II cancers is an important factor, particularly for larger breasts. In the present work, we have studied dose homogeneity for 6- and 18-MV treatment plans in 10 patients, typically with larger breasts. For each patient, 6 3-dimensional (3D) dose distributions were calculated using patient computed tomography data and the ADAC Pinnacle 3 treatment planning system. First, a dose distribution was calculated, assuming the patient was water, with the 6-MV beam parameters used to treat the patient. Second, the calculation was repeated using the actual patient anatomy. Comparison of these 2 distributions showed how patient heterogeneity affected dose. Third, individual beam weights were optimized, and the dose calculation was repeated. Each of these 3 dose calculations was repeated at 18 MV. Results showed that: (1) at 6 MV, the ratio of mean dose in the target volume calculated with heterogeneity considerations to that without was 1.014 ± 0.006, and the ratio of the standard deviation of dose in the target volume was 0.919 ± 0.042; (2) at 18 MV, the ratio of mean dose to the target volume calculated with heterogeneity considerations to that without was 1.001 ± 0.005, and the ratio of the standard deviation of dose in the target volume was 1.15 ± 0.09; and (3) the dose homogeneity, measured by the standard deviation of the dose distribution in the target volume, was 25% less for the 18-MV treatment plan for patients with breast volumes greater than 1600 cm 3. We conclude that: (1) 3D, heterogeneity-corrected dose calculation is necessary to fairly evaluate any advantage of 18 MV over 6 MV; (2) excluding the dose buildup region, 18 MV produces a significantly more homogeneous dose distribution for breast volumes greater than 1600 cm 3; and (3) when prescribing dose using heterogeneity-corrected dose distributions, dose prescriptions should be increased by 1.5% at 6 MV, but no increase is needed for 18 MV.

Comparison of EGS4 and MCNP Monte Carlo codes when calculating radiotherapy depth doses

The Monte Carlo codes EGS4 and MCNP have been compared when calculating radiotherapy depth doses in water. The aims of the work were to study (i) the differences between calculated depth doses in water for a range of monoenergetic photon energies and (ii) the relative efficiency of the two codes for different electron transport energy cut-offs. The depth doses from the two codes agree with each other within the statistical uncertainties of the calculations (1-2%). The relative depth doses also agree with data tabulated in the British Journal of Radiology Supplement 25. A discrepancy in the dose build-up region may by attributed to the different electron transport algorithims used by EGS4 and MCNP. This discrepancy is considerably reduced when the improved electron transport routines are used in the latest (4B) version of MCNP. Timing calculations show that EGS4 is at least 50% faster than MCNP for the geometries used in the simulations.

A Measurement and Analysis of Buildup Region Dose for Open Field Photon Beams (Cobalt-60 through 24 MV)

The central axis depth dose in the build-up region (surface to dmax) of single open field photon beams (cobalt-60 through 24MV) has been measured utilizing parallel plate and extrapolation chamber methodology. These data were used to derive, for a prescription dose of 100cGy, values of surface dose, the maximum value of dose along the central axis (Dmax) and the depth (nearest the surface) at which 90% of the prescription dose occurs (d90). For both single and parallel opposed pair (POP) open field configurations, data are presented at field sizes of 5 × 5, 15 × 15 and 25 × 25cm2 for prescription depths of 10, 15 and 20cm (midplane for POP). For the treatment machines, field sizes, and prescription depths studied, it is possible to conclude that: for single open field irradiation, surface dose values (as a percentage of the prescription dose) can be either low (<10%) or comparable to the prescription dose itself; for POP open fields, surface dose values are relatively independent of photon energy and midplane depth, and range between 30% and 70% of prescription dose, being principally dependent on field size; the depth of the initial 90cGy point for a prescription dose of 100cGy, d90, was larger for POP fields. For either single or POP open field treatments, d90 was always less than 22mm, while for 6MV or less, values of d90 were less than 4mm; Dmax values can be very large (e.g., above 300 cGy) for certain treatment situations and are reduced significantly for POP treatments; for open field POP treatments, the percent reduction in Dmax with each increment in beam energy above 10MV is reduced over that seen at 10MV or less and, possibly, this further reduction may be clinically insignificant; for open field POP treatments, changes in surface dose, d90 and Dmax with beam energy above 10MV do not suggest, with regard to these specific build-up curve parameters, any obvious advantage for treatment with beam energies greater than 10MV for the specific machines and situations studied.

Varian 社製 Clinac 2100C 18 MV photon beam の特性

Dosimetry data are presented for the 18 MV photon beam of a Varian Clinac 2100C dual energy linear accelerator. The clinical parameters of tissue-peak ratio (TPR), buildup region and surface dose, beam flatness and symmetry, isodose curves, dose rate dependence are discussed to 18 MV photon beam data. For the dose distribution of parallel opposed field with 18, 6, 10 MV related to this paper. We obtained 0.782 for the ionization ratio and 18.1 MV for the nominal energy with AAPM guidelines. Depth of the maximum dose for a 10×10cm^2 field at 100cm source-chamber distance (SCD) was 3.8cm. The relative surface dose increase with field size from 19.9% for a 5×5(cm)^2 field to 55.4% for a 40×40(cm)^2 field.

Improvement in the build-up characteristics of a 10 MV photon beam with electron filters

The combination of electron filter material, thickness and position which would minimise the surface dose for a 10 MV X-ray beam from a varian Clinac 018 was investigated. It was found that a lead filter of 0.17 g cm-2 thickness, placed below the lower collimator at a distance of 50 cm from the X-ray target, offers the greatest reduction in build-up region dose. Tin and copper filters with thicknesses of 0.19 and 0.23 g cm-2 respectively are slightly less effective in reducing the dose in the build-up region. The maximum reduction in surface dose achieved by added filtration is only 7% for a field size of 25 cm*25 cm. Smaller improvement is achieved with smaller field sizes. In cases where blocking trays are used, the filters should be placed under the trays toward the patient's skin.

The influence of bone on 45 MV photon dose distributions.

In an attempt to assess the pertubation of 45 MV depth dose distribution in a water phantom equipped with various thicknesses of bone-equivalent plastic. An increased dose was observed immediately behind the bone material and a reduced dose was found for points located greater than 5 cm behind the bone-equivalent plastic. The most significant change of the depth dose curve was produced by bone located in the dose build-up region.

Observations on late effects in mice exposed to 400 MeV neutrons.

Life-long observations on mortality and pathology at death were carried out on groups of mice irradiated with 250 kV X-rays or exposed to a 400 MeV neutron beam, both directly and after attenuation corresponding to the maximum dose build-up region, at comparable dose-rates. Doses up to 84 rad of 400 MeV neutrons and up to 200 rad of X-rays showed no effect on the longevity of the animals, which suggest an upper limit to the r.b.e. for life-shortening of approximately 2-5. Similar conclusions were drawn from the data on all types of leukemias. For all other neoplasms, the age-specific death-rate showed a similar shortening of the latency times for groups of mice irradiated with 0-84 rad of 400 MeV direct neutrons and 0-400 rad of X-rays, also suggesting an upper limit to the r.b.e. slightly higher than that previously indicated for life-shortening. No definite effect was observed after exposure to the attenuated neutron beam at the doses used in these experiments.