Source-skin Distance Research Articles

A Prospective Study to Investigate the Placement of Setup Skin Markings for Larger Breasted Women Undergoing External Beam Radiotherapy (RT) for Breast Cancer

Background: The technique of placing all three skin marks (reference skin marks) on a single position during CT simulation for setup of patients undergoing conventional breast radiotherapy becomes a challenge when presented with larger breasted women (bra cup size ≥ D). A new way of using skin marks in setting these patients up has been developed where three skin marks are made on the patient (one on the sternum and two lateral skin marks more inferiorly beneath the breasts) for setup as against the departmental standard of using only a single skin mark on the sternum, and employing an SSD (Source to Skin Distance) technique. This study therefore reviewed the placement of the skin markings for larger breasted women undergoing external beam radiotherapy for breast cancer by quantifying treatment field alignment errors and setup errors between the two different setup techniques. Method: 36 patients were used in this study. Out of this number, 18 were setup using three reference skin marks and the remaining 18 were setup using one reference anterior skin mark. With an acceptable patient treatment field alignment error of 2 degrees, portal images (AP and lateral) of the different skin mark techniques were analysed for field alignment errors using an Iview GT system. More so, portal images (AP and lateral) of setup for both techniques were analysed for systematic (Σ) and random (σ) errors. Results:The AP images of the single skin mark setup and the three skin mark setup yielded no significant difference as they recorded a p- value (p<0.05) of 0.089 and 0.110 respectively when compared to the treatment field alignment threshold error of 2 degrees. The lateral images of the three skin mark also yielded no significant difference as a p- value (p<0.05) of 0.091 was recorded. The lateral images of the single skin mark yielded a significant difference with a p- value (p<0.05) of 0.026. Secondly, mean comparisons of the field alignment errors between the two setup techniques yielded no significant difference in the AP images as a p- value (p<0.05) of 0.089 was detected. On the contrary, a p- value (p<0.05) of 0.026 was recorded in the field alignment errors of the lateral images. This difference is significant. Lastly, random errors were reduced in all directions (AP- anterior-posterior, SI- superior-inferior and LR- Left-right) in the three skin mark setup (4.5mm AP, 4.9mm SI and 2.4mm LR) as compared to the single skin mark setup (4.7mm AP, 5.2mm SI and 2.6mm LR). Systematic errors were also reduced in the three skin mark setup (1.7mm AP and 1.8mm SI) compared to the single skin mark setup (2.0mm AP, 2.1mm SI). Systematic errors in the LR direction on the other hand increased from 2.0mm in the single skin mark to 2.2mm in the three skin marks. Conclusion For setup of larger breasted women undergoing external beam radiotherapy for breast cancer, the three skin mark setup technique is superior to the single skin mark setup technique.

Dosimetric evaluation of electron total skin irradiation using gafchromic film and thermoluminescent dosimetry.

The aim of this study is to evaluate some dosimetry parameters such as uniformity, surface dose, and max depth dose with thermoluminescent dosimetry (TLD) and EBT3 film in total skin electron beam therapy (TSEBT). Stationary and rotary methods were set on Varian linear accelerator, Clinac 2100C. To create a radiation field large enough (168 cm × 60 cm) and uniform, the source skin distance was set 400 cm. Electron beam energy was 6 MeV. The skin dose values were obtained in 21 different points on the phantom surface. The results of dose uniformity in stationary technique were obtained as 10% and 2.6% by TLDs and 6% and 2.3% by films in longitudinal axis and transverse axis, respectively. The measurements at rotational technique by TLDs at the referred conditions showed a homogeneous total field with intensity variation of 10% in the longitudinal axis and 4% at horizontal axis. Based on the results of this study, stationary techniques are preferred for TSEBT. The main advantage of rotational techniques is reducing the time of treatment. The results also demonstrate that TLD should be routinely used in TSEBT treatment. Due to the high sensitivity of radiochromic films, this type of film was suitable for a wide therapeutic field. Comprehensive treatment to Rando phantom showed that the uniformity is better at the trunk than in the mobile parts of the body; the soles of the feet, perineum region, and scalp vertex should be treated in boost.

Interpretation of In-air Output Ratio of Wedged Fields in Different Measurement Conditions

Introduction: The head scatter factor (Sc) is one of the important parameters for monitor unit (MU) calculation. There are multiple factors that impact the Sc values, such as, head structures, back scattering in to dose monitoring chambers, wedges and so on. This study aimed to investigate the variations of SC with different build-up cap materials, wall thickness, Source to Skin Distance (SSDs), ionization chambers and variation of the SC with wedge angles in 6MV photon beam. Materials and Methods: In this study, Copper and Perspex build-up caps were designed and fabricated with two different thicknesses for each build-up cap. Measurements were performed on an Elekta Compact medical linear accelerator (6 MV) using RK with sensitive volume of 0.12 cc and Farmer-type ion chamber with sensitive volume of 0.65 cc. In all measurements, build-up caps and ionization chambers were positioned in a stand vertical to the beam central axis. It is also investigated the effect of different SSDs on SC. Results: The presence of wedge influences the SC values significantly. Variation of SC in open and wedged fields had maximum deviation of 0.9% and 6.8% and minimum of 0.4% and 2.7% for 30° and 60° angles of wedge, respectively. different SSD and type of ion chamber had no significant influence on the SC. In small field sizes, the SC values in copper build-up cap is higher than Perspex and in large field sizes, measured SC in perspex build-up cap is higher than copper. Conclusion: It was found that the presence of internal wedge has significant contribution to head scatter factor and it increases with increasing wedge angle. Therefore, it should be takes into account in manually MU calculations.

23 Vulvar cancer: Comparative study of novel 3D radiation therapy technique “ Advanced conformal technique ” and AP-PA irradiation techniques

Introduction The commonly used technique of vulvar cancer radiation therapy consists of antero-posterior “AP” and postero-anterior “PA” fields. This is the first study that reports dosimetric comparison between the AP-PA techniques and the new 3D radiation therapy technique “advanced conformal technique ACT” based on the multiplicity of treatment fields in patients with squamous cell cancer of the vulva in post-operative setting. Methods This is a comparative planning study of fifteen patients with vulvar carcinoma treated with adjuvant radiation therapy at the National Institute of Oncology in Rabat. Three treatment plans were performed, corresponding to three techniques; photons with source skin distance inguinal supplement, modified segmental boost technique, advanced conformal technique. For each plan, the dose-volume histogram was used to generate target volumes (total and inguinal PTV) and organs at risk OAR (bladder, rectum, bowel and femoral heads) parameters. Results The isodose 95% volume was significantly reduced with the advanced conformal technique (p 0.0001) without compromising the total PTV coverage (p: 0.94). This technique resulted in the best conformity and homogeneity index. The ACT decreased significantly the PTVs Dmax and Dmean (p 0.0001), and offered better homogeneity for inguinal PTV 1.07 ± 0.01 respectively (p 0.0001). The ACT decreased the rectum absorbed dose; V40, V45 and Dmax with 50.21 ± 27.21 22.81 ± 10.22 and 46.56 ± 1.11 respectively. Concerning femoral heads, the ACT decreased the Dmax and V45 in comparison to the both other techniques. Conclusions The ACT seems to be an alternative to the AP-PA irradiation techniques in post-operative setting when IMRT is not available.

OA099] Patient dose evaluation and optimization during hysterosalpingography using digital X_ray equipment

Introduction Hysterosalpingography (HSG) is the radiological examination of the uterus and fallopian tubes and is most primarily used to evaluate tubal patency in healthy patients of child bearing age with infertility. In light of ICRP (International Commission on Radiological Protection) recommendations and Italian Legislation concerning the protection of individuals against ionizing radiation risk related to medical exposure, the present work aimed at monitoring and optimizing the radiation dose imparted to patients undergoing this procedure. Materials and Methods A first survey was carried out in our radiology department, collecting data from 31 subjects during HSG, carried out using digital equipment (model Opera, Housing GMM, Italy, with over-couch tube, CsI flat panel detector Thales, model Pixium RAD 4343 with pixel size 148 μ m, Automatic Exposure Control). Data showed a wide variability, with the mean DAP (Dose Area Product) being higher than those reported by other authors [1] from other departments, suggesting need for optimization. After intervention (consisting in reducing the field size, the fluoroscopy time and the number of radiographs) a new group of 53 subjects was monitored, showing significant DAP reduction. ESAKs (Entrance Surface Air Kerma – free in air) – calculated by using normalized radiation output (previously assessed during quality controls), current*time product (recorded during HSG) and source-skin distance – were converted to dose to ovaries by using appropriate coefficients. Also ED (effective dose) was evaluated. Results Mean DAP decreased from 3348 to 634 mGy*cm2. The 75th percentile of the DAP distribution, 800 mGy*cm2, was adopted as Institutional Diagnostic Reference Level. Mean estimated dose to ovaries was about 0.8 mGy and mean effective dose was about 0.3 mSv. Conclusions Periodic review of the procedures, in light of both technological advances and clinical requests, is necessary in order to improve clinical practice with guaranteed patient safety.

DVH Analytics: A DVH database for clinicians and researchers.

In this study, we build a vendor‐agnostic software application capable of importing and analyzing non‐image‐based DICOM files for various radiation treatment modalities (i.e., DICOM RT Dose, RT Structure, and RT Plan files). Dose‐volume histogram (DVH) and planning data are imported into a SQL database, and methods are provided to manage, edit, view, and download data. Furthermore, the software provides various analytical tools for plan evaluations, plan comparisons, benchmarking, and plan outcome predictions. DVH Analytics is developed using Python, including libraries such as pydicom, dicompyler, psycopg2, SciPy, Statsmodels, and Bokeh for parsing DICOM files, computing DVHs, communicating with a PostgreSQL database, performing statistical analyses, and creating a web‐based user interface. This software is open‐source and compatible with Windows, Mac OS, and Linux. For proof‐of‐concept, a database with over 3,000 DVHs from a single physician's head & neck practice was built. From these data, differences in means, correlations, and temporal trends in dose to multiple organs‐at‐risk (OARs) were observed. Furthermore, an example of the predictive regression tool is reported, where a model was constructed to predict maximum dose to brainstem based on minimum distance from planning target volume (PTV) and treatment beam source‐to‐skin distance (SSD). With DVH Analytics, we have developed a free, open‐source software program to parse, organize, and analyze non‐image‐based DICOM data for use in a radiation oncology setting. Furthermore, this software can be used to generate statistical models for the purposes of quality control or outcome predictions and correlations.

Radiomic Feature Robustness and Reproducibility in Quantitative Bone Radiography: A Study on Radiologic Parameter Changes

The purpose of this study was to investigate the robustness of different radiography radiomic features over different radiologic parameters including kV, mAs, filtration, tube angles, and source skin distance (SSD). A tibia bone phantom was prepared and all imaging studies was conducted on this phantom. Different radiologic parameters including kV, mAs, filtration, tube angles, and SSD were studied. A region of interest was drawn on the images and many features from different feature sets including histogram, gradient, run-length matrix, co-occurrence matrix, autoregressive model, and wavelet derived parameters were extracted. All radiomic features were categorized based on coefficient of variation (COV). Bland-Altman analysis also was used to evaluate the mean, standard deviation, and upper/lower reproducibility limits for radiomic features in response to variation in each testing parameters. Results on COV in all features showed that 22%, 34%, and 45% of features were most robust (COV ≤ 5%) against kV, mAs, and SSD respectively and there was no robust features against filtration and tube angle. Also, all features (100%) and 76% of which showed large variations (COV > 20%) against filtrations and tube angle respectively. Autoregressive model feature set has no robust features against all radiologic parameters. Features including sum-average, sum-entropy, correlation, mean, and percentile (50, 90, and 99) belong to co-occurrence matrix and histogram feature sets were found as most robust features. Bland-Altman analysis showed the high reproducibity of some feature sets against radiologic parameter changes. The results presented here indicated that radiologic parameters have great impacts on radiomic feature values and caution should be taken into account when work with these features. In quantitative bone studies, robust features with low COV can be selected for clinical or research applications. Reproducible features also can be obtained using Bland-Altman analysis.

Entrance Skin Dose (Radiation) Measurement and Evaluation (for all age groups) at the State Specialist Hospital, Okitipupa, Nigeria

This study was carried out on entrance skin dose (ESD) (Radiation) measurement and evaluation (for all age groups) at the State Specialist Hospital, Okitipupa, Ondo State, Nigeria. Entrance skin doses for randomly selected patients between 0-4 years, 5-9 years, 10-17 years and above 18 years old undergoing X-ray chest (Poster Anterior) examinations were measured and evaluation of the source-to-skin distance (SSD) records for the patients during the x-ray chest examinations were carried out. The mean ESD reported for chest PA projections were 66.43 µGy, 105.10 µGy, 215.76 µGy and 291.81 µGy respectively for age range 0 - 4 years, 5 - 9 years, 10 - 17 years and above 18 years. The ESD values reported in this present study were same with the NRPB recommendations of year 2000 of 50 µGy for newborn to 1-year-old, 70 µGy for 5 years and 120 µGy for 10 years old children. The values reported for age 1 to 4 years were lower than 0.1 mGy recommended and the mean ESD for age 5 to 9 years for chest PA examination was 0.11 mGy which is lower than the recommended 0.12 mGy. However, other values reported in this present study were compared with the guidance levels set by the International regulation bodies and were found to be within safe limits and they do not pose any significant health risk to the patients of all age ranges or the health workers. The effective radiation doses (ERDs) for all the age groups were computed, and by comparisons, it was found that the radiation risk factors (RRFs) are relatively low.

Source to Skin Distance (SSD) Characteristics from Varian CX Linear Accelerator

This study aims to describe the characteristics of the source to skin distance (SSD) of Varian CX linear accelerator (LINAC) using the X-ray beam of 6 MV and 10 MV. The variation of the source to the SSD are 90, 100 and 110 cms; the depth of the water phantom used are 5, 10, 15, 20, and 25 cms, respectively. The depth of the water phantom was created for analysis of percentage depth dose (PDD) and profile dose. It can be concluded from the tests that from the measured SSD, SSD of 110 cm with the depth water phantom of 20-25 cm for energy beam of 6 MV and at all levels of depth for 10 MV energy corresponding tolerance limits to be used in clinical radiotherapy. For the SSD 90 and 100, the values beam symmetry and flatness obtained slightly beyond the limits of tolerance.

A novel Hemi-Body Irradiation technique using electron beams (HBIe−)

PurposeCertain radiation responsive skin diseases may develop symptoms on the upper or the lower half of the body. The concept of a novel Hemi-Body Electron Irradiation (HBIe−) technique, described in this work, provides a low cost, LINAC based, intermediate treatment option in between extremely localized and Total Skin irradiation techniques. Materials and methodsThe HBIe− technique, developed in our department, incorporates a custom crafted treatment chamber equipped with adjustable Pb shielding and a single electron beam in extended Source-Skin Distance (SSD) setup. The patient is positioned in ‘Stanford’ technique positions. The geometrical setup provides both optimal dose homogeneity and dose deposition up to a depth of 2 cm. To confirm this, the following characteristics were measured and evaluated: a) percentage depth dose (PDD) on the treatment plane produced by a single electron beam at perpendicular incidence for six fields at ‘Stanford’ angles, b) 2D profile of the entrance dose on the treatment plane produced by a single field and c) the total surface dose on an anthropomorphic phantom delivered by all 6 fields. ResultsThe resulting homogeneity of the surface dose in the treatment plane for an average patient was 5–6%, while surface dose homogeneity on the anthropomorphic phantom was 7% for both the upper and the lower HBIe− variants. The total PDD exhibits an almost linear decrease to a practical range of 2 g/cm2. ConclusionIn conclusion, HBIe− was proven effective in delivering the prescribed dose to the target area, while protecting the healthy skin.

17. Evaluation of the EDD-53G diodes for their utilization in vivo dosimetry control during total body irradiation

Introduction During total body irradiation (TBI), the control of the dose received by the patient during each fraction of the treatment is necessary. The aim of this study was to verify that the EDD-53G diodes could be used in the TBI conditions and afterward to set up a measurement protocol for in vivo dosimetry during the fractions of treatment. Methods The diodes used were IBA’s EDD-53G (black) diodes with an OmniPro-InViDos supports DPD-12 electrometer. All measurements were made with the patient’s treatment conditions for a TBI: gantry at 270°, collimator at 45°, irradiation fields size 40 cm by 40 cm and source-skin distance (SSD) of 303 cm, 333 cm, 373 cm or 403 cm (clinically used SSD). Diode calibration was carried out with diodes placed at the input and the output of a phantom of 20 cm composed of RW3 polystyrene plaque with a FARMER ionization chamber (IBA FC65-P) positioned at mid-thickness. During the evaluation phase of the diodes, repeatability and reproducibility studies were carried out. The diodes angulation influence on the irradiation incidence and the diodes influence on each other were evaluated. A verification of the diode’s dose linearity was also carried out. Secondly, thickness correction factors (from 4 cm to 30 cm) were established at the four SSD, taking the patient’s different thicknesses compared to the reference conditions into account. Results In the study of the diodes angulation influence, an anisotropy of the diodes according to their rotation axis was established: relative mean deviation σ average = 1.2% (max deviation of 4%) σ average = 0.3% if the diode is always placed in accordance with the same axis. Therefore, the repeatability and reproducibility studies were carried out taking into account this effect and positioning the diode always at the same angle of incidence with the beam. These two studies yielded the following results: σ average = 0.08% for repeatability (σmax = 1.55%) and σ average = 0.88% for reproducibility (σmax = 0.11%). The diode presence in the vicinity of another did not show influence on the measurement (σ average = 0.09%). The response linearity of the diodes according to the delivered dose has been demonstrated (slope coefficient = 1.000 ± 0.001). For each SSD, a correction factor curve to be applied as a function of the thickness of the phantom was performed. Conclusions The first results obtained with these diodes were satisfying. Subsequently, an anthropomorphic phantom test is considered to verify the in vivo dosimetry measurement protocol with the diodes.

A Technique to Irradiate Scar-like Line Target With Dynamic Electron Arc Radiation Therapy

For superficial target such as surgical scar, standard radiotherapy technique is the static electron beam. Its use can become cumbersome when target area is long and/or the skin has large curvature, because the combination of multiple beams often produces hot and cold spots in the junction regions and surrounding healthy tissues can be over-irradiated. In addition, manual setup of each field is required which is often impractical clinically. In this study, we investigate a new technique to irradiate any line target with the dynamic electron arc radiotherapy (DEAR) where both the couch position and Linac gantry angle are synchronized with the radiation beam. The electron applicator is kept in DEAR to preserve beam penumbra. To avoid collision between Linac and patient, couch is moved simultaneously with gantry rotation to maintain a constant source to skin distance (SSD) and an en face beam direction at all times. The DEAR plan, in the form of XML, follows the MU-position trajectory model using control point (CP) similar to IMRT and VMAT, is currently deliverable on Truebeam Linac in research mode. The line target is represented by a series of connected points whose 3D coordinates are first extracted from treatment planning system. They are used to determine the gantry angle, couch position, collimator angle, and MU at each CP. The XML plan also includes some fixed parameters: energy, applicator, cutout, and dose rate. The plan and CT are fed to the Virtual Linac Monte Carlo simulation platform for the final dose calculation and prescription normalization. Finally the plan is verified dosimetrically on phantom with radiochromic film. Various target orientations and lengths were studied on flat, cylindrical, and anthropomorphic phantoms. Either 6 or 9 MeV electron beams were chosen depending on the depth of the target. The smallest applicator (6 × 6 cm2) is used for increased clearance. Cutout shape is in square (3 × 3 cm2), rectangular (3 × 1 cm2), or circular (dia = 3 cm). Dose rate is based on the prescription dose, speed of couch motion and gantry rotation, and delivery time. Plans are generated in both forward and reverse motion directions. Any number of setup beams during the motion trajectory can be created and mode up during the dry run. Gantry angle is chosen so that beam is always perpendicular to the target. For circular cutout, the collimator angle is fixed; for others, it is parallel to the scanning direction. Finally, the MU at each CP is proportional to the distance between the current and the neighboring target points. For a 20 cm long target with 200 cGy prescription dose, the delivery can be completed within 2 minutes at dose rate of 1000 MU/min. The film measurements agree well with the Monte Carlo dose calculations. We have developed a technique to irradiate any line target using DEAR. It improves dose distributions substantially over the standard electron beam. Further investigations are in progress to expand into two dimensional targets.

Surface Dose Assessment for Different Clinical set up Parameters from High Energy Photon Beams

Introduction: The purpose of this study was to investigate surface doses of 6 and 10 MV energies of linear accelerator for different clinical setup parameters including (field size, gantry angle, SSD (source skin distance), PW (physical wedge), acrylic block tray and bolus), Comparison between measured surface dose by P.P (parallel plate) and calculated surface dose by TPS (Treatment Planning system). Comparison between surface doses measured by TLD (Thermo luminescence dosimeter) and P.P ion chamber, Comparison between the surface dose of 3DCRT and IMRT techniques. Materials and Methods: Surface dose measurements were carried out using a (PTW) Markus parallel-plate ion chamber in a plastic water plastic water phantom for various setup parameters using Primus Siemens (6, 10 MV) linear accelerator. For the normalization depth, i.e., the depth of maximum dose 1.5, 2.5 cm were chosen for 6, 10 MV photon beams, respectively. Results and Discussion: The measured skin dose values for 10 MV were lower than those of 6 MV, the skin dose increased as field size increased. The measured surface dose by P.P for 6 and 10 MV are 16%, 25%, for 10 × 10 cm2 square field size, within the first 2 millimetres of the build-up region, at field size 10 × 10 cm2 the PDD for a 6 and 10 MV photon beam increases from 24% to 62%, 16% to 44% respectively. With increasing the gantry angles (0 to 30) produces a minimal effect of dose, (40-70) gantry angles produce a significant increase. When studying the surface dose with different SSD found that the percentage of surface dose is nearly stable, the absolute surface dose (cGy) increased with decreasing SSD. The absolute skin dose for wedge fields were lower than for open fields, for field size 10 × 10 cm2, while the values of surface dose of wedge 30 are 20% and 14% for 6 and 10 MV respectively. The skin dose for a wedge field increased as field size increased. Bolus 1 cm material increased the surface dose for 10 × 10 cm2 from 24% to 96%, and from 16% to 87%, for 6 and 10 MV respectively. With the use of an acrylic block tray, the surface dose increased to the open fields for all field sizes higher than 10 cm2, but the increase was dominant for large fields, for field size 20 × 20 cm2 the surface dose increased from 29% to 41%, from 22% to 34% for 6 and 10 MV respectively. Conclusion: Agreement between skin doses calculated by multidata TPS and those measured by P.P ion chamber in water plastic water phantom was better than 20% for 96% of measurement points and this is indicate that the dose calculation in the build-up region using multidata TPS is good. For irregular tumor shape and closed OAR the IMRT is a good solution to cover the tumor and spare the critical organs without increasing the skin dose.

The dose penumbra of a custom-made shield used in hemibody skin electron irradiation.

We report our technique for hemibody skin electron irradiation with a custom‐made plywood shield. The technique is similar to our clinical total skin electron irradiation (TSEI), performed with a six‐pair dual field (Stanford technique) at an extended source‐to‐skin distance (SSD) of 377 cm, with the addition of a plywood shield placed at 50 cm from the patient. The shield is made of three layers of standard 5/8” thick plywood (total thickness of 4.75 cm) that are clamped securely on an adjustable‐height stand. Gafchromic EBT3 films were used in assessing the shield's transmission factor and the extent of the dose penumbra region for two different shield‐phantom gaps. The shield transmission factor was found to be about 10%. The width of the penumbra (80%‐to‐20% dose falloff) was measured to be 12 cm for a 50 cm shield‐phantom gap, and reduced slightly to 10 cm for a 35 cm shield‐phantom gap. In vivo dosimetry of a real case confirmed the expected shielded area dose.PACS number(s): 87.53.Bn

Electron beam output of an ELEKTA Sli-Plus linear accelerator for irregular shaped fields and for extended SSD

Introduction A dosimetric study of the electron beam output of the ELEKTA Sli-Plus linear accelerator is being presented: (a) when customized electron cutouts are used in order to shape the field, (b) when the treatment is performed in extended Source-Skin Distance. Purpose We have investigated how the machine output varies when electron cutouts of different shapes and sizes are inserted into the beam with the aim to establish a method to predict the beam output in similar circumstances. In order to determine the effective SSDs and the position of the effective point source, the Inverse Square Law (ISL) method has been used for all electron energies and applicators in order to correct the output at extended SSDs applying the ISL correctly. Materials and methods An extended set of output measurements has been performed using electron beams blocked by customized electron cutouts. Measurements have also been carried out in a water phantom at the depth of maximum dose as a function of the air gap g between the far end of the applicator and the phantom surface. Measurements have been compared to calculations with the Oncentra Masterplan treatment planning system. Results & conclusion Our results indicate that the electron beam output does not vary significantly with the insertion of an electron cutout into the beam and therefore, the monitor units determined for a given electron applicator can be used for the blocked beam as well. Monitor Units can also be safely determined by using the SSD eff in order to account for the change in output and depth-dose distribution. Disclosure The authors declare that they have no conflict of interest or any relationship that may bias their presentation.

The Comparison Study of Quadratic Infinite Beam Program on Optimization Instensity Modulated Radiation Therapy Treatment Planning (IMRTP) between Threshold and Exponential Scatter Method with CERR® In The Case of Lung Cancer

This research compares the quadratic optimization program on Intensity Modulated Radiation Therapy Treatment Planning (IMRTP) with the Computational Environment for Radiotherapy Research (CERR) software. We assumed that the number of beams used for the treatment planner was about 9 and 13 beams. The case used the energy of 6 MV with Source Skin Distance (SSD) of 100 cm from target volume. Dose calculation used Quadratic Infinite beam (QIB) from CERR. CERR was used in the comparison study between Gauss Primary threshold method and Gauss Primary exponential method. In the case of lung cancer, the threshold variation of 0.01, and 0.004 was used. The output of the dose was distributed using an analysis in the form of DVH from CERR. The maximum dose distributions obtained were on the target volume (PTV) Planning Target Volume, (CTV) Clinical Target Volume, (GTV) Gross Tumor Volume, liver, and skin. It was obtained that if the dose calculation method used exponential and the number of beam 9. When the dose calculation method used the threshold and the number of beam 13, the maximum dose distributions obtained were on the target volume PTV, GTV, heart, and skin.

SU‐F‐T‐82: Dosimetric Evaluation of a Shield Used for Hemi‐Body Skin Electron Irradiation

Purpose:We had several mycosis fungoides patients with a limited disease to about half of the skin surface. A custom‐made plywood shield was used to protect the non‐targeted skin region with our total skin electron irradiation (TSEI) technique. We report a dosimetric evaluation for our “hemi‐body” skin electron irradiation technique.Methods:The technique is similar to our clinical total skin electron irradiation (TSEI), performed with a six‐pair dual field (Stanford technique) at an extended source‐to‐skin distance (SSD) of 377 cm, with the addition of a plywood shield placed 50 cm from the patient. The shield is made of three layers of standard 5/8″ thick plywood (total thickness of 4.75 cm) that are clamped securely on an adjustable‐height stand. Gafchromic EBT3 films were used in assessing the shield's transmission factor and the extend of the dose penumbra region. To verify the dose delivered for hemi‐body skin radiation in a real patient treatment, in‐vivo dosimetry using Gafchromic EBT3 films were performed. Film pieces were taped on the patient skin to measure the dose received during the first two fractions, placed on the forehead and upper body (shielded region); and also at the level of pelvic area, left thigh, and left ankle.Results:The shield transmission factor was found to be 10%, and the width of the penumbra (80‐to‐20% dose fall‐off) was about 12 cm. In‐vivo dosimetry of a real case confirmed the expected shielded area dose.Conclusion:Hemi‐Body skin electron irradiation at an extended SSD is feasible with the addition of a plywood shield at a distance from patient skin. The penumbra dose region and the shield's transmission factor should be evaluated prior to clinical use. We have treated several hemi‐body skin patients with our custom‐made plywood shield, the current patient measurements are representative of these for other patients as well.

Surface dose measurements and comparison of unflattened and flattened photon beams.

The purpose of this study was to evaluate the central axis dose in the build-up region and the surface dose of a 6 MV and 10 MV flattened photon beam (FB) and flattening filter free (FFF) therapeutic photon beam for different square field sizes (FSs) for a Varian Truebeam linear accelerator using parallel-plate ionization chamber and Gafchromic film. Knowledge of dosimetric characteristics in the build-up region and surface dose of the FFF is essential for clinical care. The dose measurements were also obtained empirically using two different commonly used dosimeters: a p-type photon semiconductor dosimeter and a cylindrical ionization chamber. Surface dose increased linearly with FS for both FB and FFF photon beams. The surface dose values of FFF were higher than the FB FSs. The measured surface dose clearly increases with increasing FS. The FFF beams have a modestly higher surface dose in the build-up region than the FB. The dependence of source to skin distance (SSD) is less significant in FFF beams when compared to the flattened beams at extended SSDs.

Wax Tissue Compensator for Head and Neck Radiotherapy - Fabrication and Dosimetric Evaluation

For a photon field of size 20x20cm in a telecobalt machine with ‘Source Diaphragm Distance (SDD)’ of 45 cm, DWTC is designed to provide compensation at ‘source skin distance (SSD)’ of 80 cm. A jig stand with 3 acrylic perforated sheets kept together at specified gaps allow required radial divergence of the metal spokes on equally spaced grid. Dose compensator could be fabricated by a tray mounted on the top side of the jig, using molten dental wax by measuring patient anatomy created by a positive of ‘Plaster of Paris’ (POP).

A quantitative method to implement and to assess the single isocenter technique for breast cancer radiation therapy

Purpose: We propose a process of quality assurance to validate and implement the single isocenter technique for breast cancer radiotherapy. We evaluated the dosimetric and temporal gains using the single isocenter technique compared to classic source to skin distance (SSD) technique. Methods: 6 patients of breast cancer localization were studied. For each patient 2 treatment plans were generated. In plan 1 the dose was calculated using SSD technique. In plan 2 the dose was calculated using single isocenter technique. To implement the plan 2 a dosimetric analysis including monitor units (MU), isodose curves, cumulative and differential dose volume histograms cDVH, dDVH respectively, coverage index, conformity index for planning target volume were used. The measurements using a PMMA phantom consist of measuring point dose by an ionization chamber and 2D dose distributions using 2D diodes arrays. Wilcoxon signed rank and Spearman’s tests were used to calculate p -value and correlation coefficient, respectively. Results: The single isocenter technique reduced the MU by average on -30.1 ± 13.6%, ( p = 0.03). We observed an improvement with statistical significance between the two techniques for the mean dose, minimum dose and volume receiving 95% of the prescribed dose without over-dosage. The analysis for dDVH showed that the dose distribution in the target volume calculated in the single isocenter technique is more homogeneous than the SSD technique. Wilcoxon test showed that the two treatment plans had the same quality ( p > 0.05). The difference between calculated and measured dose was within 2.4 ± 3.3% for absolute point dose and the percentage of points passing gamma criteria was on average 99.8 ± 0.2%. Conclusion: This method provides a quantitative evaluation and comparison of the two irradiation techniques for breast cancer and the consequences of the technical change on dose calculation.