Full Scatter Conditions Research Articles

SU-FF-T-11: Measurements of Dose Discrepancies Due to Inhomogenieties and Radiographic Contrast in Balloon Catheter Brachytherpay

Purpose: Recently, a device called MammoSite consisting of balloon and catheter was developed to perform partial breast irradiation using high‐dose‐rate (HDR) brachytherapy unit with ease and reproducibility. However, the actual dose to the skin does not agree well with the calculated dose by the treatment planning system because of the difference between the calculation condition and the real treatment condition (i.e., homogeneous water and full scatter condition vs. contrast existing and lack of full scatter condition). In this study, we experimentally estimated dose discrepancies due to contrast and lack of full scatter in breast HDR brachytherapy with MammoSite. Method and Materials: Using MOSFET detectors and a breast simulating phantom, the dose discrepancies between calculation and treatment condition were measured according to contrast concentration (10 and 20% volume ratio), balloon size (35 cc and 60 cc), and source to detector distance (SDD) ranging from 25 to 50 mm. The source was Ir‐192 isotope from Nucletron HDR unit. To correct the energy dependency of MOSFET detectors, the mean energy of each measured condition was obtained using Monte Carlo simulation with simplified geometry. For uncertainty analysis, the 80% confidence level of measurements was assessed by Student's t distribution. Results: The dose discrepancies from calculation condition due to both contrast and lack of full scatter combined ranged from about −1.4% to −18.2% in studied cases. Conclusion: In all cases, the effect of lack of full scatter was dominant to that of contrast. Significant dose discrepancies existed between the calculation and the real treatment condition due to contrast and lack of full scatter. Therefore, actual skindose is expected to be less than calculated.

SU-GG-I-157: Quantification of Radiographic Image Contrast Enhancement Using Gold Nanoparticles

Purpose: To quantify radiologic imagecontrast enhancement using goldnanoparticles compared to iodinated contrast media (CM) over the entire diagnostic range of x‐ray energies. Method and Materials: A Perspex phantom with 4mm cylindrical wells was used to simulate small portions of vasculature. Each well was loaded with either goldnanoparticle solution or iodinated CM at equal concentration (0.5077 M radiopaque element). The phantom was imaged under full scatter conditions in computed radiography(CR) (40–80 kVp) and computed tomography(CT) (80–140 kVp). Images obtained at low energies (≈ 40 kVp) were validated using diagnostic type gafchromic film (Gafchromic® XRQA). CdTedetector with MCA was used to obtain transmission spectra after x‐ray beam at 130 kVp passed through solutions of goldnanoparticles or iodinated CM. Results:CT and CRimages were evaluated for contrast enhancement by contrast‐to‐noise ratio (CNR). Low energy results support previous findings, with gold exhibiting a 60% greater CNR than iodine. Goldnanoparticles also displayed excellent imagecontrast in CT, producing over two times greater signal than iodinated CM at 140 kVp. Over the x‐ray energy range of 70–100 kVp, however, both samples displayed similar contrast values. CdTe attenuation spectra are in accordance with image results where goldnanoparticles show a greater probability of attenuation than iodine for photons below approximately 35 keV and above 80 keV. Conclusion: Data indicates that a solution bearing goldnanoparticles would be an effective alternative to iodinated CM diagnostic radiology particularly at lower and higher ends of x‐ray energies used in radiology, such as mammography and CT.Conflict of Interest (only if applicable): Funding provided by NanoVic (Nanotechnology Victoria, Ltc.)

TH‐C‐AUD A‐07: Evaluation of the Correction Factor Due to the Lack of Full Scatter Conditions in Cs‐137 and Ir‐192 Brachytherapy Dosimetric Studies

Purpose: Use of a finite phantom to derive dose rate distributions around brachytherapy sources implies a lack of backscattering material near the phantom periphery. Conventional planning algorithms and newly‐developed 3D correction algorithms are based on physics data under full scatter conditions. Presently, most published Monte Carlo dosimetric studies have been obtained using either a spherical phantom (15cm in radius) or a cylinder phantom (40×40cm2). The study objective was to derive a simple relationship to correlate the radial dose function, g(r), obtained for each one of these phantoms to that obtained for an unbounded phantom. Method and Materials: Assuming bare point sources of and , kerma was calculated using Monte Carlo GEANT4 code for 1) a spherical phantom of 40 cm in radius, R, which is assumed an unbounded phantom for r ⩽ 20cm, and 2) spherical phantoms of R=15cm and R=21cm. The later size mimics the scatter conditions of a 40×40cm2 cylindrical phantom for both radionuclides. From the ratio of the dose rate distributions for unbounded/bounded phantoms we derived the relationship between g(r) for both phantoms. Results: Phantom size correction results to g(r) were obtained and fit to 3rd order polynomials (R2 > 0.999) valid for r ⩽ 10cm, which is the clinical range of interest. To validate the method, published dose‐rate distributions for two and sources in bounded/unbounded phantoms were compared with the fits of this study. Agreement was typically within 0.2% over all distances studied. Conclusion: In order to compare the dose rate distributions published for different phantom sizes, a simple expression based on fits of the dose distribution ratios for bounded/unbounded phantoms was developed for and . Using these relations, it was possible to correlate g(r) between bounded and unbounded phantoms for improved accuracy and consistency of clinical dosimetry.

Effect of scatter material on diode detector performance for in vivo dosimetry

The accuracy of Scanditronix EDE, EDP10 and EDP20 diodes for entrance dosimetry of complex fields has been assessed using static and dynamic multileaf collimator test fields on phantoms. Specifically, surrounding scatter material size and composition have been investigated. The EDP10 and EDP20 diodes incorporate steel build-up caps. The effect of varying the diameter of the wax scatter discs on the dosimeter response showed a systematic detector under-response for the smaller discs with errors up to 4.1% relative to full scatter conditions. In static fields, all diodes over-respond at a field size of 1 × 1 cm2. Diodes with non-water-equivalent build-up material exhibit over-response of up to 10.8%. In dynamic fields, diodes over-respond when there is an increased contribution from phantom scatter and under-respond in shielded regions due to low dose rate and beam hardening. For high dose regions, all diodes over-respond with the greatest over-response of 3.8% observed with a 6 mm sliding window field. The EDE diode with a 6 cm scatter disc correlated best with the reference dosimeter. A diode design with minimal non-water-equivalent components and the addition of a 6 cm diameter water-equivalent disc for scatter material is recommended for the in vivo dosimetry of 6 MV complex fields.

Verification of nose irradiation using orthovoltage x-ray beams

Determination of dose distributions from superficial and orthovoltage irradiations of basal cell carcinoma of the nose has been performed using a nose shaped phantom constructed from paraffin wax. EBT type radiochromic film was used for dose measurements. A 2 cm diameter 50 kVp anterior field was used to irradiate the nose phantom, with sheets of film placed at 7 mm, 14 mm and 23 mm physical depth. The percentage depth doses at these points were measured to be 84% +/- 1.6%, 66% +/- 2.7% and 50% +/- 1.2% respectively, compared to the expected percentage depth doses of 72%, 52% and 34%, measured in full scatter conditions. This discrepancy is believed to be due to the steep drop off at the sides of the nose phantom, resulting in reduced attenuation at the edges of the beam, which in turn results in an increase in the scatter contribution to the dose at depth on the central axis. Measured dose profiles from this technique showed a reasonably uniform distribution. A second technique using a 250 kVp tangent-like field to irradiate the tip of the nose was also tested. Radiochromic film was placed against the edges of the phantom for dose measurement. The dose at the surface was measured to be 27% +/- 1.5% less than the expected dose. It is believed that this discrepancy is due to a combination of the lack of backscatter from the phantom, and a small offset between the phantom and the treatment cone. Dose measurements and profiles showed that this technique results in a variation in dose across the treated volume of 7%. However, the difficulty in predicting the delivered dose prohibited it from clinical use.

2859

2859

Dosimetric and treatment planning considerations for radiotherapy of the chest wall

Radiotherapy treatment planning calculations of the chest wall are complex due to missing tissue, the thin chest wall and the presence of lung. The accuracy of calculated dose is dependent on the type of algorithm employed. This work evaluates a collapsed cone (CC) and a pencil beam (PB) convolution model for radiotherapy planning of the chest wall. Various irradiation geometries simulating the chest wall have been examined and calculations were compared with measurements with an ionization chamber in epoxy resin water substitute and in low-density lung substitute blocks. A retrospective treatment planning study comprising 6 patients was carried out to evaluate the differences in the dose distributions and monitor units predicted by the two algorithms. The calculated dose in unit density medium was within +/-1% for the CC model and up to +/-2% for PB. In low density medium and under full scatter conditions, CC overestimated the dose by 1% whereas PB overestimated the dose by 9%. In the tangential irradiation geometry with water and lung media, the PB overestimated dose to the isocentre by up to 10%, whereas the dose from CC was within 3%. From the treatment planning study calculated monitor units (MU) and doses were consistent with the experimental findings. The CC model is more accurate for radiotherapy treatment planning of the chest wall and especially when there is significant involvement of lung tissue.

Monte Carlo dosimetric study of Best Industries and Alpha Omega Ir‐192 brachytherapy seeds

Ir-192 seeds are widely used in the USA for low dose rate interstitial brachytherapy. There are two commercially available models: those manufactured by Best Industries filtered with stainless steel, and those manufactured by Alpha-Omega seeds filtered with Pt. Newly developed 3D correction algorithms for brachytherapy are based on dosimetry data obtained on unbounded phantom size, allowing corrections for heterogeneities and actual tissue boundaries. Published dosimetric datasets for both seeds have been obtained under bounded conditions. The aim of the present study is to obtain dosimetric datasets for these seeds under full scatter conditions. The Monte Carlo GEANT4 code has been used to estimate air-kerma strength and dose rate in water around the Ir-192 seeds. Functions and parameters following the TG43 formalism are obtained and presented in tabular forms: the dose rate constant, the radial dose function, and the anisotropy function. Tables for the anisotropy factor have been obtained in order to apply punctual approximation. Differences between dose rate distributions for both seeds show that specific dataset must be used for each type of seed in clinical dosimetry. The data in the present study improve on published data in the following aspects: (i) dosimetric data were obtained under full scatter conditions, which affect dose values at distances greater than 4-5 cm from the source; (ii) the dose rate tables are given at greater distances from the source; and (iii) the spatial resolution in high dose gradient areas, such as those near the longitudinal source axis, has been improved.

Experimental verification of convolution/superposition photon dose calculations for radiotherapy treatment planning

This work describes an experimental verification of the two-photon dose calculation engines available on the Helax-TMS (version 6.1) commercial radiotherapy treatment planning system. The performance of the pencil beam convolution and the collapsed cone superposition algorithms was examined for 4, 6, 15 MV beams, under a range of clinically relevant irradiation geometries. Comparisons against measurements were carried out in terms of absolute dose, thus assessment of the accuracy of monitor unit (MU) calculations was also carried out. Results show that both algorithms agree with measurement to acceptable tolerance levels in most cases in homogeneous water-equivalent media irradiated under full scatter conditions. The collapsed cone algorithm slightly overestimates the penumbra width and this is mainly due to discretization effects of the fluence matrix. The accuracy of this algorithm strongly depends on the resolution of the patient density matrix. It is recommended that the density matrix voxel size used for dose calculations is less than 5 × 5 × 5 mm3. The dose in media irradiated under missing tissue geometry, or in the presence of low or high-density heterogeneities, is modelled best with the collapsed cone algorithm. This is of particular clinical interest in treatment planning of the breast and of the thorax. For these treatment sites, a retrospective study of treatment plans indicated in certain cases significant overestimation of the dose to the planning target volume when using the pencil beam convolution model.

Methods for beam data acquisition offered by a mini-phantom

Mini-phantoms are an important tool for measurement of basic head scatter parameters in high-energy photon beams, and recently they have also been used for beam quality specification. Therefore the feasibility and reliability of basic beam parameter acquisition using only a mini-phantom is checked in 6, 18 and 25 MV photon beams. These parameters include head scatter correction factors, phantom scatter correction factors, total scatter correction factors, wedge factors, off-axis ratios, as well as beam attenuation coefficients and beam hardening coefficients. In order to specify beam quality variations and beam quality modifications by a wedge, two different methods are compared: the first method uses a constant source to chamber distance of 1 m, the second method refers to narrow beam geometry. values derived with two different beam quality specification methods show a systematic deviation. However, relative variations of the attenuation coefficient within the beam and the associated beam quality modifications observed with the two methods show good agreement in open and wedged beams. Phantom scatter correction factors are calculated from measured head scatter correction factors and total scatter correction factors as well as from attenuation coefficients. Measured and calculated phantom scatter correction factors agree within 1% with the values given in literature. For 18 and 25 MV photon beam, wedge factors measured in water or in the mini-phantom agree within 0.5%, but maximum deviations of % are observed at 6 MV for the largest field sizes. It is demonstrated that the determination of several beam data related to full scatter conditions does not necessarily require the availability of a full scatter phantom. The mini-phantom is a reliable but very cheap and simple tool. It offers versatile possibilities to measure, check and verify basic beam parameters in high-energy photon beams.

Calculated energy response correction factors for LiF thermoluminescent dosemeters employed in the seventh EULEP dosimetry intercomparison

Several dosimetry intercomparisons for whole body irradiation of mice have been organized by the European Late Effects Project Group (EULEP). These studies were performed employing a mouse phantom loaded with LiF thermoluminescent dosemeters (TLDs). In-phantom, the energy response of the LiF TLDs differs from free-in-air, due to spectral differences caused by attenuation and scatter of x-rays. From previous studies, energy response correction factors in-phantom relative to free-in-air were available for full scatter conditions. In the more recent intercomparisons, however, full scatter conditions were not always employed by the participants. Therefore, Monte Carlo calculations of radiation transport were performed to verify the LiF TLD energy response correction factors in-phantom relative to free-in-air for full scatter conditions and to obtain energy response correction factors for geometries where full scatter conditions are not met. For incident x-rays with HVLs in the 1 to 3.5 mm Cu range, the energy response correction factor in-phantom deviates by 2 to 4 per cent from that measured free-in-air. This is in reasonable agreement with previously published results. The energy response correction factors obtained from the present study refer to a calibration in terms of muscle tissue dose in-phantom using gamma rays. For geometries where full scatter conditions are not fulfilled, the energy response correction factors are different by up to about 3 per cent at maximum from that at full scatter conditions. The dependence of the energy response correction factor as a function of the position in-phantom is small, i.e. about 1 per cent at maximum between central and top or bottom positions.

Advances in treatment planning dosimetry for a high energy neutron beam

Before starting the clinical programme, a series of physical measurements were made using the Clatterbridge high energy neutron facility. These consisted of measurements made under standard conditions, e.g. with square fields, full scatter conditions and perpendicular beam incidence. Further measurements have now been performed to include various clinically relevant non-standard conditions. These included the effects of field shape, backscatter, oblique incidence, bolus and beam blocking on dose distributions.