Cerrobend Blocks Research Articles

A novel cerrobend block in the radiation therapy

This study was carried out to look for the substitute products of lead cerrobend block and determine whether the products can give more radiation dose to the tumor in the skin and superficial tumors in radiotherapy treatment. Five kinds of PbO/Er2O3/Epoxy(E-51) samples used as the cerrobend block in the electron beam radiotherapy treatment were prepared successfully by a solidified forming approach. The central axis depth dose and off-axis dose profiles in the water phantom were calculated using BEAMnrc/DOSEXYZnrc user codes. The results revealed that there were more energy deposition events induced by low energy electron beams in the surface of the water phantom. These electron beams were attributed to Compton scattering and photoemission between photons and erbium, plumbum elements. The maximum dose (Dm) and the depth at the maximum dose (Rm) were linearly dependent on the mass fraction of erbium and plumbum element in the sample. The max dose and its depth varied depending on the combined actions of new electrons and transmission electrons.

The Dosimetric and Delivery Advantages of a New 160-leaf MLC

The purpose of this study was to conduct a measurement and treatment planning study on the dosimetric and delivery advantages of a new 160-leaf multileaf collimator (MLC). Recently, a new 160-leaf multileaf collimator (Siemens 160 MLC(TM)) was introduced. The 160-MLC is a single focused design that consists of 160-leafs (80 pairs), each 95 mm thick with a projected leaf width of 5 mm at the machine isocenter. Compared to its double focused predecessors, the 82-leaf MLC (Siemens OPTIVIEW((tm)) MLC) and 58-leaf MLC (Siemens 3-D MLC((tm))), the 160-MLC has leaf widths of half the size. The most notable difference is the new slanted leaf design that replaced the tongue and groove system and allows for complete interdigitation. A systematic study that compared the dosimetric and delivery differences among the 160-MLC, 58-MLC, and divergent Cerrobend blocks was performed. Dosimetric conformity for each collimator type was determined by conforming each to circular targets of various diameters. The effective penumbra for each collimator type was calculated by conforming each, at various collimator angles, to a square stationary target. The quality of 3D conformal radiotherapy treatment (3D-CRT) plans and the quality intensity modulated radiation treatment (IMRT) plans were respectively compared with each collimator type. The 160-MLC was found to have improved dosimetric conformity over the 58-MLC. The divergent Cerrobend block showed marginal dosimetric conformity improvement over the 160-LMC. Overall, the 160-MLC had a 45% and 29% reduction in the 20/80 and 30/90 effective penumbra over the 58-MLC, respectively, while exhibiting only a slightly larger effective penumbra over the divergent Cerrobend block. Comparing 3D-CRT plans generated for small lesions of the head and neck, the V100 for the PTV of the plans generated with the Cerrobend blocks, the 58-MLC, and the 160-MLC were 97.78%, 92.51%, and 99.18%, respectively, while with regards to the OARs, the three produced similar DVHs. IMRT plans generated with the 160-MLC were found to significantly reduce the total delivered monitor units by up to 14.7% and the number of segments by as much as 10.7% compared to the 58-MLC. The average delivery time for the direct aperture optimized (DAO) IMRT plans generated with the 160-MLC was approximately 5 minutes. Overall, compared to the 58-MLC, the new 160-MLC was found to improve dosimetric conformity and IMRT delivery efficiency.

SU-E-T-625: A Method to Estimate Lung Doses under a Block during Total Body Irradiation (TBI) Treatment

Purpose: Total Body Irradiation(TBI) treatment with photon beams has been accepted as an important radiotherapytreatment for a variety of malignant diseases. Usually lung blocks are used to minimize the lungdose and prevent radiation‐induced pneumonitis. The purpose of this paper is to estimate the mid‐plane lungdoses under block during TBI treatments. Methods: The accuracy of the dose calculations under a block from a commercial planning system was first studied. The central axis depth doses under a block were measured using a parallel plate chamber in homogeneous solid water phantom at 100SSD for both 6MV and 18MV beams. A 7.5cm thick, 15×15cm2 cerrobend block positioned on the central axis was used to attenuate the beam with a setting of 30×30cm2. The measurements were compared with calculations generated with the Pinnacle treatment planning system. In addition, measurements were performed in a heterogeneous phantom positioned in a TBI geometry. The phantom consisted of two 4cm solid water slabs sandwiched 16cm foam slabs simulating lung tissues. A parallel plate chamber, garfchromic films and optically stimulated luminescent(OSL) dosimeters were used to measure depth doses for both 6MV and 18MV beams. These data were used to model the mid‐plane lungdoses under the blocks. Results: The depth dose curves calculated by Pinnacle showed significant deviations from measurements. An estimate of the mid‐plane doses was calculated by taking the average of measured doses at depth of 2cm and 24cm for 6MV, 4cm and 24cm for 18MV. Our estimated value for the mid‐plane doses within ±7% of measurement. Conclusions: We have observed that the Pinnacle cannot accurately model the dose in the build‐up region under a central block. A method to estimate the mid‐plane lungdose under a block was proposed. More measurements and Monte Carlo simulations are proceeding to investigate the robustness of this method.

SU‐E‐T‐319: Radiochromic Film Characterization of a Small Animal Irradiator

Purpose: To characterize the output of a Cs‐137‐based Gammacell 40 Exactor Low Dose‐Rate Research Irradiator using radiochromic film measurements.Methods: Gafchromic EBT radiochromic film was used to characterize the the timer error, uniformity, penumbra, and scatter dose for a mouse irradiation device used in conjunction with a Cs‐137 irradiator with a nominal dose rate of 100.04cGy/min. Radiochromic film was chosen because of its good spatial resolution and relative insensitivity to radiation spectral changes. An Epson Expression 10000XL film scanner was used to digitize the films, and generated images were imported into Matlab for analysis. To verify the calibration curve and to calculate timer error, a second set of films from the same batch were irradiated with a 4MV photon beam. Differences between the vendor supplied dose rate and Cs‐137 extrapolated dose rate were compared with Fricke dosimetry results. A uniformity film was placed in the irradiation chamber and exposed for 6 minutes. To measure dose under shielding attributed to scatter, films were cut to fit around the perimeter of the irradiation chamber and irradiated for 100 minutes. To measure the penumbra across the edge of a cerrobend blocking device, films were cut and placed across the block edges. Results: The timer error was estimated to be approximately 2.6sec. Uniformity of better than ±5% was observed both in horizontal and vertical film profiles. A scatter dose of approximately 10% of the irradiation dose was observed. The 20%–80% penumbra distance of 11.9mm was observed, measured from the dose profile in the horizontal dose plane Conclusions: Our study provides independent verification for a number of dosimetric properties of interest when designing mouse irradiation experiments. This was used to improve the dose computation in small animal studies for correlation of dose with biological effects, and improve accuracy of partial body dose estimates.

High-Dose Spatially Fractionated GRID Radiation Therapy (SFGRT): A Comparison of Treatment Outcomes With Cerrobend vs. MLC SFGRT

Spatially fractionated GRID radiotherapy (SFGRT) using a customized Cerrobend block has been used to improve response rates in patients with bulky tumors. The clinical efficacy of our own multileaf collimator (MLC) technique is unknown. We undertook a retrospective analysis to compare clinical response rates attained using these two techniques. Seventy-nine patients with bulky tumors (median diameter, 7.6 cm; range, 4-30 cm) treated with SFGRT were reviewed. Between 2003 and late 2005, the Cerrobend block technique (n= 39) was used. Between late 2005 and 2008, SFGRT was delivered using MLC-shaped fields (n= 40). Dose was prescribed to dmax (depth of maximum dose) and was typically 15 Gy. Eighty percent of patients in both groups received external beam radiotherapy in addition to SFGRT. The two-sided Fisher-Freeman-Halton test was used to compare pain and mass effect response rates between the two groups. Sixty-one patients (77%) were treated for palliative intent and 18 (23%) for curative intent. The majority of patients had either lung or head-and-neck primaries in both groups; the most frequent site of SFGRT application was the neck. The majority of patients complained of either pain (65%) or mass effect (58%) at intake. Overall response rates for pain and mass response were no different between the Cerrobend and MLC groups: pain, 75% and 74%, respectively (p= 0.50), and mass effect, 67% and 73%, respectively (p= 0.85). The majority of toxicities were Grade 1 or 2, and only 3 patients had late Grade 3-4 toxicities. MLC-based and Cerrobend-based SFGRT have comparable and encouraging response rates when used either in the palliative or curative setting. MLC-based SGFRT should allow clinics to more easily adopt this novel treatment approach for the treatment of bulky tumors.

Use of a matchline dosimetry analysis tool (MDAT) to quantify dose homogeneity in the region between abutting tangential and supraclavicular radiation fields

In this work, we develop and test a matchline dosimetry analysis tool (MDAT) to examine the dose distribution within the abutment region of two or more adjoining radiotherapy fields that employ different blocking mechanisms and geometries in forming a match. This objective and quantitative tool uses calibrated radiographic film to measure the dose in the abutment region, and uses a frequency distribution of area versus dose (a dose‐area histogram) to visualize the spatial dose distribution. We tested the MDAT's clinical applicability and parameters by evaluating the dose between adjacent photon fields incident on a flat phantom using field‐matching techniques employing collimator‐jaw and multileaf collimator (MLC) configurations. Additionally, we evaluated the dose in the abutment regions of four different clinical tangential‐breast and supraclavicular matching techniques using various combinations of collimator and MLC matches. Using the MDAT tool, it was determined that a 1 cm abutment region width (centered about the theoretical matchline between fields) is the most appropriate width to determine dose homogeneity in a field matching region. Using the MDAT, both subtle and large differences were seen between fields that used MLCs to form the match, compared to flat edge devices such as collimators and external cerrobend blocks. We conclude that the MDAT facilitates a more precise evaluation of the distribution of dose within the region of abutment of radiotherapy fields.PACS number: 87.55.dk

A Novel Method of Island Blocking in Whole Abdominal Radiotherapy Using a Modified Electronic Tissue Compensation Technique

Traditionally, large fields requiring island blocking used external beam radiation therapy (EBRT) with Cerrobend blocks to limit dose to the critical structures. It is laborious to construct blocks and use them on a daily basis. We present a novel technique for island blocking using a modified electronic tissue compensation (MECOMP) technique. Five patients treated at our institution were selected for this study. The study compared two planning techniques: a novel MECOMP and a conventional EBRT technique. Conventional fields were defined using anterior-posterior and posterior-anterior (PA) fields. The kidneys were contoured and an aperture cut-out block was fitted to the OAR with a 1-cm margin (OAR CTV) and placed in the PA field. A dynamic multileaf collimation (DMLC) plan with ECOMP was developed using identical beam and blocking strategy; this tissue compensation–based fluence map was modified to deliver a “zero” dose to the CTV OAR from the PA field. There were no significant differences in the mean, maximum, and minimum doses to the right or left kidney between the two methods. The mean, maximum, and minimum doses to the peritoneal cavity were also not significantly different. The number of monitor units (MUs) required was increased using the MECOMP (273 vs. 1152, p < 0.01). The MECOMP is effectively able to deliver DMLC-based radiotherapy, even with island blocks present. This novel use of MECOMP for whole abdominal radiotherapy should substantially reduce the labor, daily treatment time, and treatment-related errors through the elimination of cerrobend blocks.

SU‐FF‐T‐135: Dosimetric and Delivery Advantages of a New 160‐Leaf MLC

Purpose: To conduct a measurement and treatment planning study on dosimetric and delivery advantages of a new 160 leaf MLC (160MLC, Siemens). Methods and Materials: The uses of 160MLC were compared with a commonly used 58‐leaf MLC (Siemens) and Cerrobend blocks. Radiation leakages and penumbra were measured using films and using a small‐size ionization chamber. A treatment planning system (Xio, CMS) was used to generate (1) isodose distributions for a series of single circular and square fields defined by the MLCs or blocks, and (2) 3DCRT plans for a pituitary adenoma case with a small target volume and closely adjacent organs at risks. Static IMRT plans for a variety of prostate and head and neck cases using Xio and another planning system (Panther, Prowess) were generated and delivered. Results: The 160MLC has lower radiation leakage (0.5% for 160MLC vs. 2% for 58MLC) and narrower penumbra (5–6 mm for 160MLC vs. 9–14 mm for 58MLC calculated between 80/20% isodose lines for a 10×10cm2 field with various MLC rotations), and slightly wider penumbra (within 1 mm) than the blocks. The dose conformity between using 160MLC and blocks determined for a series of circular fields is similar, and higher than that 58MLC. The 3DCRT plans for 160MLC and blocks are comparable and more conformal than that for 58MLC as assessed by DVH parameters. The use of 160MLC reduces the numbers of segments and MU by 13% versus the 58MLC. The average delivery time for the Prowess IMRT plans with 160MLC was 5 minutes. Conclusion: The 160MLC offers several dosimetric and delivery advantages including low leakage, narrow penumbra, conformal dose distributions similar to divergent blocks, reduced numbers of IMRT segments and MU, and reduced IMRT delivery time. These advantages lead to reduced normal tissue doses, without need of Cerrobend blocks, and improved treatment delivery.

SU‐FF‐T‐369: Electron Therapy with Hanging Lens Block for Orbital Lymphoma: Dosimetry Measurements Using Diode Arrays

Purpose: An effective technique to treat orbital lymphoma is to use an anterior electron beam with a hanging lens block. Traditionally the dose distributions under the lens block are measured with film, TLD or a stereotactic diode. The measurements are labor intensive and time consuming, considering that the dose distribution varies with several parameters, including distance between the block and the skin, electron energy used and the thickness of the beam scatterer. Multiple measurements are therefore required to optimize the dose distribution in each patient. We have developed an efficient technique to determine the optimal treatment parameters by utilizing commercially available diode arrays to instantly monitor the dose distributions of electron fields as physics parameters are varied; Methods: A cerrobend block of 1.8cm long, 1.0cm in diameter was made and placed at distances(gaps) of 0.5–2cm above patient's eye. Dose distributions for different physics parameters were measured interactively by using a two‐dimensional diode array (Mapcheck) with solid water build‐ups to observe the dose changes with varying electron energies and set‐up parameters. Once the energy, SSD and beam scatterer are determined, films, ion‐chambers and a one‐dimensional diode array(Profiler) were used to measure the dose distributions and the results were compared with measurements using the Mapcheck; Results: It was found that the effective thickness of the internal buildup in the Mapcheck device is 4mm less than the specified 2cm water equivalence for the electron beam used. After this thickness‐correction was applied, good agreement was achieved between different methods of measurement for both outside and under the block; Conclusions: The diode arrays have been proven to be the ideal devices in the initial stage of electron treatment planning to determine the optimal physics parameters that produce the desired dose distributions, with substantial savings in time and effort comparing with traditional dosimetry measurements.

Skin dose estimation for various beam modifiers and source-to-surface distances for 6MV photons

The purpose of this study was to learn the skin dose estimation for various beam modifiers at various source-to-surface distances (SSDs) for a 6 MV photon. Surface and buildup region doses were measured with an acrylic slab phantom and Markus 0.055 cc parallel plate (PP) ionization chamber. Measurements were carried out for open fields, motorized wedge fields, acrylic block tray fields ranging from 3 × 3 cm2 to 30 × 30 cm2. Twenty-five percent of the field was blocked with a cerrobend block and a Multileaf collimator (MLC). The effect of the blocks on the skin dose was measured for a 20 × 20 cm2 field size, at 80 cm, 100 cm and 120 cm SSD. During the use of isocentric treatments, whereby the tumor is positioned at 100 cm from the source, depending on the depth of the tumor and size of the patient, the SSD can vary from 80 cm to 100 cm. To achieve a larger field size, the SSD can also be extended up to 120 cm at times. The skin dose increased as field size increased. The skin dose for the open 10 ×10 cm2 field was 15.5%, 14.8% and 15.5% at 80 cm, 100 cm and 120 cm SSDs, respectively. The skin dose due to a motorized 60° wedge for the 10 × 10 cm2 field was 9.9%, 9.5%, and 9.5% at 80 cm, 100 cm and 120 cm SSDs. The skin dose due to acrylic block tray, of thickness 1.0 cm for a 10 × 10 cm2 field was 27.0%, 17.2% and 16.1% at 80, 100 and 120 cm SSD respectively. Due to the use of an acrylic block tray, the surface dose was increased for all field sizes at the above three SSDs and the percentage skin dose was more dominant at the lower SSD and larger field size. The skin dose for a 30 × 30 cm2 field size at 80 cm SSD was 38.3% and it was 70.4% for the open and acrylic block tray fields, respectively. The skin doses for motorized wedge fields were lower than for open fields. The effect of SSDs on the surface dose for motorized 60° wedge fields was not significant for a small field size (difference was less than 1% up to a 15 × 15 cm2 field size), but for a larger field (field size more than 15 × 15 cm2), the difference in a percentage skin dose was significant. The skin dose for the open field was more than that for the MLC blocked field and lower than that for the acrylic blocked tray field. The block was 25% of the 20 × 20 cm2 open field. Skin doses were increased as the SSD decreased and were dominant for larger field sizes. The surface dose was weakly dependent on the MLC block.

Technical Note: On Cerrobend shielding for electron beams

The purpose of this study is to investigate (1) the depth at which the measurement of the block transmission factor should be made, and (2) the level of the transmission of 18 and electron beams through conventional Cerrobend. We measured the block transmission in water phantom as ionization profiles across the beam and as ionization distributions along the central axis of the beam for 18 and electron beams, for cone sizes ranging from . In our analysis, we separated the bremsstrahlung component produced in the Cerrobend block from the component originating in the head in the transmitted dose under the standard Cerrobend block. The block transmission for both beam energies and cone sizes was maximum on the central axis of the beam at depths between 0.4 and . For the beam, the maximum transmission was 6.2% for the cone, and 7.4% for the cone. For the beam, it was 9.5% for the cone, and 11.3% for the cone. For the beam and cone, it takes 2.95 and of Cerrobend to reduce the maximum block transmission to 5% and 10%, respectively. The maximum dose under a blocked electron beam occurs on the central axis closer to the surface than it does for the open beam, and the block transmission factor should be defined at this shallower depth. To decrease the block transmission factor to the level of 5% on the central axis, electron beams with energy and greater require additional shielding.

Dosimetric characteristics of dual‐layer multileaf collimation for small‐field and intensity‐modulated radiation therapy applications

The purpose of the present work was to measure the performance characteristics in the penumbra region and on the leaf‐end of an innovative dual‐layer micro multileaf collimator (DmMLC). The DmMLC consists of two orthogonal (upper and lower) layers of leaves; a standard MLC consists of one layer. The DmMLC provides unique performance characteristics in smoothing dose undulation, reducing leaf‐end transmission, and reducing MLC field dependence of the leaf stepping angle. Two standard MLCs (80‐leaf and 120‐leaf versions: Varian Medical Systems, Palo Alto, CA), a DmMLC (AccuKnife: Initia Medical Technology, Canton, MA), and a Cerrobend (Cerro Metal Products, Bellefonte, PA) block were used in performance studies involving a triangular field, a cross leaf‐end field, and a circular field. Measurements were made with 6‐MV X‐rays and extended dose range film at a depth of 5 cm in Solid Water (Gammex rmi, Middleton, WI) at a source–axis distance of 100 cm. The field penumbra width measured between the 20% and 80% isodose lines through the MLC‐80, MLC‐120, DmMLC, and Cerrobend block were 9.0, 5.0, 3.0, and 2.0 mm respectively. The dose undulation amplitude of the 50% isodose line was measured as 5.5, 2.0, and 0.5 mm for the MLC‐80, MLC‐120, and DmMLC respectively. The planar dose difference between the MLC‐80, MLC‐120, and DmMLC against Cerrobend block was measured as ranging at ±52.5%,±35.0%, and ±20.0% respectively. The leaf‐end transmission was measured at 22.4% in maximum and 15.4% in average when closing a single layer of the DmMLC, and at 2.4% in maximum and 2.1% in average when closing both layers. The MLC dependence of the leaf stepping angle with the DmMLC ranged from 45 degrees to 90 degrees. The standard MLC leaf stepping angle ranged from 0 degrees to 90 degrees. In conclusion, the dose undulation, leaf‐end transmission, and MLC field dependence of the leaf stepping angle with the DmMLC were remarkably reduced as compared with those of the standard MLCs. And as compared with Cerrobend block, the DmMLC provided very comparable performance in field‐edge smoothing and in the shaping of complex fields.PACS numbers: 87.56.Jk, 87.56.Nk, 87.56.Nj, 87.57.Nt

SU-FF-T-323: On the Adequacy of Shielding for 15-22 MeV Electron Beams

Purpose: It is customary to define block transmission as a fraction of open field dose at the depth of maximum dose (dmax) for an electron beam. The purpose of this study is to investigate the depth at which transmission should be measured and the adequacy of shielding for 15–22MeV electron beams to reduce the transmission to the regulatory level. Method and Materials: Central axis data were collected in open and blocked beams in water phantom. Measurements were taken for a set of cones from 6×10 to 25×25cm2, 100 cm SSD and 5 cm gap between the cone and water surface. Transmission factors as a function of depth and cone size were calculated. For the 22MeV electron beam, the effect of adding thicknesses of different attenuators (cerrobend, lead, bolus) was investigated. The data were verified independently in three institutions with similar accelerator model and electron energies. Monte Carlo simulations with PENELOPE code were performed to compare with measurements. Results: Photon contamination of 15–22MeV electron beams in the open beam was in agreement with the manufacturer's specification. Maximum transmission for the blocked beam for all energies and cones occurred between 4–7 mm depth. The 15MeV electron beam was well attenuated for the standard cerrobend thickness. For 18 and 22MeV beams, maximum transmission varied from 5.9% to 7.5% and from 9.5% to 11% for the 6×10 to 25×25cm2 cones, respectively. For the 15×15cm2 cone, 30 mm thick cerrobend attenuated 22MeV beam to 5%. For 21MeV beam, Monte Carlo simulations showed maximum transmission from 10.6% to 11% for 10×10cm2 and 25×25cm2 cones, respectively. Conclusion: The dmax dose under a blocked beam occurs closer to the surface than for the open beam. In order to decrease transmission to the regulatory level of 5%, 18–22MeV electron beams require additional shielding to supplement the standard cerrobend block.

2019: Is Local Recurrence Increased With the Use of a Heart Block in Radiation Therapy for Breast Cancer?

Cardiac irradiation has long been implicated in the development of coronary artery disease. In order to limit the volume of heart in the irradiated field, we have routinely designed a custom-shaped cerrobend block to shield the cardiac silhouette when irradiating the left breast. This study examines the impact of this technique on in-breast (local) recurrence. A retrospective cohort study was performed based on medical chart review. The local recurrence rate among breast cancer patients with left-sided lesions whose radiation treatment planning included a heart block was compared to right-sided patients (treated without a heart block). Left-sided patients were treated following-breast conserving surgery with CT-based treatment planning to allow for delineation of the heart and lumpectomy site. Care was taken to place the block away from the lumpectomy site. Patients with a heart block were matched 2:1 to right-sided patients based on the date of breast cancer diagnosis in order to ensure comparable follow-up time between the two cohorts. Data were collected on 199 patients treated at the Dana-Farber Cancer Institute between 1997 and 2001 (67 left-sided patients treated with a heart block, 132 patients with right-sided tumors). Tumor and treatment characteristics were generally similar between the cohorts. Median follow-up was 5.0 years. Isolated local recurrence as a first site of failure occurred in 1 (1.5%) left-sided patient, and 2 (1.5%) right-sided patients. Other events, including distant metastasis, regional nodal recurrences, contralateral breast cancers, second primary cancer or death from other causes occurred 10.4% and 11.4% of patients, respectively. Fisher’s exact test for the comparison of local recurrence vs other events vs no events was not significant The sole left-sided patient with a local recurrence developed inflammatory disease 39 months after diagnosis. The use of a heart block did not result in a higher rate of tumor recurrence in the breast. These data support the use of a heart block (away from the area of the primary tumor) as a safe practice. A large randomized trial would be necessary to determine whether the use of a heart block decreases non-breast cancer mortality.

Po‐Thur Eve General‐06: Experimental Validation of the Eclipse AAA Algorithm

The Medical Physics departments of the Tom Baker Cancer Center (TBCC) and the Cross Cancer Institute (CCI) independently performed preliminary evaluation of the new Analytical Anisotropic Algorithm (AAA) implemented in Varian's Eclispe (v. 6.0) treatment planning system (TPS). The TPS was pre‐commissioned with “Golden Beam Data” from the vendor. We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beams; wedged square and half‐blocked beams; square fields at three SSDs; open and wedged oblique beams; irregular field defined by MLC and cerrobend blocks. All measurements were performed on Varian 2100EX linear accelerators. Measurements were made to assess the dose in heterogeneous media at both the CCI (CIRS Thorax IMRT phantom) and at the TBCC (TLDs in a Rando phantom). Profiles were evaluated in the buildup, penumbra, inner and outer beam regions as per AAPM Task Group 53.Measured and calculated profiles agreement was very good in all regions except for the inner beam region at the CCI, attributed a difference in interpolation schemes at the two institutions and the large volume ion chamber used for measurements. The AAA penumbra was also found to be steeper than measured penumbra since AAA was pre‐commissioned using diode measurements. Total scatter factors for most measurements differed by less than 2% from the calculated ones except for the hard wedges where differences up to 4% were found. Anthropomorphic phantoms measurements differed from AAA by as much as 5.6%.Funding provided by Varian.

SU‐FF‐T‐353: Parameters Study for the Severity Acute Radiation Induced Skin Reaction for the Breast Cancer Patients

Purpose: Radiation induced skin reaction is a common side effect for the breast cancer patient undergoing radiotherapy (RT). Severe acute radiation skin reaction (ARSR) not only brings distress to the patient, it may also cause an unexpected treatment delay and consequently reduce the treatment efficacy. It is important to understand the factors that cause ARSR and eventually to prevent or minimize it. Method and Materials: A retrospective study was conducted to 154 breast cancer patients who received a whole breast external photon beam treatment at source‐to‐axis distance (SAD) setup with 6MV or 6MV combined with 18MV photon energy at our hospital between 2003 and 2005. The relation between ARSR, the source‐to‐surface distance (SSD) and treatment technique was evaluated in this study. The treatment techniques included conformal three‐dimensional (3D) treatment and intensity‐modulated radiation therapy (IMRT). These techniques were further classified into: (1) 3D open field, (2) 3D with cerrobend block, (3) 3D with MLC, (4) IMRT sliding window, and (5) IMRT step‐and‐shoot. Receiver operating characteristic (ROC) curve and p‐value were applied for the statistic analysis. Results: Out of total 154 patients, 21 (13.6 %) patients developed ARSR. The results showed that ARSR was sensitive to SSD variation when the patient was treated with 3D MLC and IMRT sliding window treatment techniques (Az=0.9±0.12 with p‐value=0.014, and Az=0.8±0.08 with p‐value=0.017, respectively). And ARSR was non‐sensitive to the SSD variation when patient treated 3D open field and 3D block field treatment techniques (Az=0.37±0.16 with p‐value=0.67 and Az=0.66±0.13 with p‐value=0.19, respectively). There was not distinction in the development of ARSR observed between each of the treatment techniques (p‐value&gt;0.08). Conclusions: SSD was found to be a sensitivity factor to predict ARSR for the patient undergoing RT with 3D MLC and IMRT sliding window treatment. A larger SSD should decrease the risk of ARSR.

SU‐FF‐T‐137: Commissioning the Eclipse AAA Algorithm with Golden Beam Data

Purpose: Evaluation of the accuracy of Eclipse AAA TPS when commissioned with Golden Beam or measured data. Method and Materials: Two cancer centers independently performed preliminary evaluation of the Analytical Anisotropic Algorithm (AAA) implemented in Eclipse TPS. The AAA photon algorithm was commissioned with vendor supplied “Golden Beam Data”(GBD). We measured central and off‐axis profiles in several beam configurations including: open square, rectangular and asymmetric (half‐blocked) beam; wedged square and half‐blocked beam; square fields at three different source to surface distances; open and wedged beam at oblique incidence; beams shaped using cerrobend blocks and MLC. The measurements were performed on the Varian 2100 EX linear accelerators installed at the two institutions. After the initial tests, Eclipse was recommissioned with measured data from one of the machines. Results: The evaluation of the profiles was performed in the buildup, penumbra, inner and outer beam regions as per AAPM TG53. With the GBD the agreement of the measured and calculated profiles at the two institutions was very good in all regions except for the inner beam region on one machine. The tests that had a significant number of failures in the inner portion of the beam were mainly those cases where the TG53 tolerances are very tight. In these cases a significant number of points were just beyond the tolerance, and some of the off axis scans had 100% fail. On one particular test case where 16 profiles were measured for a particular geometry, 59% of points passed in the inner section with GBD, while 91% of points in that region passed once the beam was commissioned. Conclusion: When Eclipse is commissioned with GBD it was quite accurate, however, commissioning with measured data can improve the overall match. Conflict of Interest: Funding provided by Varian.

SU‐FF‐T‐188: Dosimetric Evaluation of Parallel Opposed GRID Radiation Therapy for Deep‐Seated Bulky Tumors

Purpose: Grid radiation therapy, using single field of megavoltage x‐ray beam has been proven to be an effective method for management of bulky (&gt;8 cm in diameter) malignant tumors. However, the effectiveness of this treatment modality for deep seated tumor is limited by the dose to the overlying normal tissue. In this investigation the use of parallel opposed beam is being evaluated for treatment of deep seated bulky tumors, using two different grid patterns. Method and Materials: Dosimetric characteristics of single field and parallel opposed radiation field were experimentally determined with film in Solid Water phantom material and using ion chamber in water. These measurements were performed with 6 and 18 MV x‐ray beams from a Varian Clinac 2100EX linear accelerator. Two different Grid block patterns, fabricated by Radiation Products Design, Inc. were utilized in these investigations. The GRID blocks molded into a cerrobend block of 7.5 cm thickness were manufactured with hole diameters of 5.9 mm and 8.5 mm. Results: Dose profiles and percentage depth doses of two GRID blocks for the parallel opposed beam were compared with a single radiation field. The results of these investigations indicated that with a 5.9 mm GRID block, dose to the overlaying normal tissue at dmax reduced from 200% for a single field to 100%, for the same tumor dose, with equally weighted parallel opposed fields. Similarly, for the 8.5 mm GRID block, dose to normal tissue at dmax were reduced from 185% to 100%. Conclusion: Parallel opposed GRID therapy resulted in a substantial decrease in dose delivered to the normal tissues for the same prescribed tumor dose. Thus, parallel opposed GRID radiotherapy is a viable option for treatment of deep‐seated bulky tumor. With this technique, the tumor dose can be increased for a higher therapeutic result.

SU‐FF‐T‐195: Evaluation of Film Calibration Procedures for Quality Assurance of Helical Tomotherapy Treatments

Purpose: To evaluate and compare film calibration procedures using a conventional 6 MV static beam and an optimized helical tomotherapy plan for the patient‐specific QA of tomotherapy treatments. Method and Materials: The conventional 6 MV static beam was delivered by a Varian 2300CD Clinac to blocks of solid water. Three different doses were delivered to each of two Kodak EDR2 films, giving a total of six dose values. Cerrobend blocks were used to cover portions of the film outside each field in order to prevent contamination from one field to the others. The optimized helical tomotherapy plan was delivered by a TomoTherapy Hi‐Art™ System to a cylindrical solid water phantom. Nine cylindrical ROIs were contoured within the phantom along the horizontal and vertical axes. This procedure produces two films with a total of nine different dose values. All films were analyzed using in house software coded in LabVIEW™. Each calibration procedure was evaluated on the basis of accuracy, efficiency and ease of implementation. Results: Both the conventional 6 MV static beam and optimized helical tomotherapy plan were able to deliver the dose accurately, with the static beam producing a steeper gradient in the vertical direction but better uniformity in the coronal plane. It took approximately the same amount of time to complete each procedure. Conclusion: The conventional 6 MV static beam is able to deliver a slightly more uniform dose to the film plane. However, the optimized helical tomotherapy plan provides sufficient accuracy, is able to add more points to the calibration curve in the same amount of time and is delivered with the same beam and phantom used to deliver the patient treatments. For this reason, the optimized helical tomotherapy plan has been implemented as the film calibration procedure for patient‐specific QA of tomotherapy treatments.

SU‐FF‐T‐203: In Vivo Dosimetry Using Disposable MOSFET Dosimeters for Total Body Irradiation

Purpose: The “OneDose” MOSFET dosimeter, manufactured by Sicel Technologies, is used for individualized patient dosimetry measurements for radiation therapy. These dosimeters, which are pre‐calibrated by the manufacturer, are designed for single use for conventional radiation therapy treatments. We tested the usefulness, reliability and applicability of these dosimeters for use in total body irradiation procedures, where the field sizes, distances, and irradiation conditions differ substantially from the standard conditions used for factory calibration Method and Materials: We have compared the response of OneDose dosimeters with that of Thermoluminescent Dosimeters (TLDs) calibrated “in‐house”. OneDose dosimeters were paired with two TLDs and were placed beneath bolus to provide adequate build‐up. The detectors with build‐up were then taped to the skin of the patient at various sites of interest, including the head, neck, umbilicus and lungs. In most cases, cerrobend lung blocks were used. Readings of the OneDose were taken immediately following irradiation. The TLDs were read 2 – 4 days later. Results: Of the four patients studied thus far, the doses measured at the head, neck, and umbilicus fell between 105 and 210 cGy. For these sites agreement with the TLDs was generally within ±5%. Measured doses for the regions of the lung showed greater variability. This may be due to placement errors or the lower doses (&lt; 20 cGy) and steeper dose gradients that occurred when the lungs were shielded with cerrobend blocks. Conclusion: The dose measured by the OneDose detector shows relatively good agreement with that measured by TLDs in total body irradiation. Our research plan includes study of another six patients, with the intent of being more precise in the placement of the dosimeters in the region of the lung. Conflict of Interest Information: ASB has a sponsored research agreement with Sicel Technologies, Inc. for the study of implantable sensors.