Anterior Beam Research Articles

SU-E-T-265: Reducing Pacemaker Doses with a Lead Sheet: A Multi-Detector Study.

Evaluate the usability of lead shielding to reduce the dose to pacemakers. The efficiency and risk of this type of skin block will be presented. A solid water phantom was used and all measurements were made at a depth of 0.5 cm for 6 MV and 23 MV photon beams. Measurements were performed with a parallel plate ion chamber and with a plastic scintillation detector (PSD) prototype. Measurements were made for fields of 10, 20 and 30 cm square. For every field, measurements were made in increment of 5 cm from the center of the field to the edge of the 60 cm long phantom for anterior and posterior beams. All measures have been made with and without a 1.6 mm of lead shielding wrapped in a thermoplastic. For all measurements, both detectors agree to 0.6%, well within the uncertainties of the detectors. With antero-posterior fields, the benefit of shielding is more important at 23MV (reduction of dose by65%) compared to 6MV (reduction of dose by 46%) when the shielding is out of the field. For distances larger than 35 cm, no benefits are measured. In the case were the lead is completely inside the fields, the dose is increased by the presence of shielding. The same observation is made for postero-anterior fields. For shielding out of field, the dose is slightly increased. The used of lead shielding with antero-posterior field is advised and provided an easy way to decrease dose to pacemaker. For a postero-anterior field, it is preferable to avoid shielding, but it could be used if it stays outside fields in the case of a multiple beams treatment. PSD has been shown to be an excellent candidate for in vivo monitoring dose to pacemakers and also site like foetus.

SU‐E‐T‐442: Assessing Vertex Beam with its Dosimetric Relevance for Intracranial Tumours: A Computational Exploration

To assess the selection of the minimal vertex beam angle for the avoidance of organs at risk (OARs) in radiotherapy planning of brain tumors. Seventy patients with intracranial tumors were studied. Three field conformal plans with co-planar or non co-planar beam arrangements with an anterior vertex beam were used. Two methods were studied for determining the vertex angle for avoidance of eye as organ at risk. In the standard technique, the beam's eye view (BEV) was used to determine need of vertex beam. For the vertex beam, the angle was approximated to avoid eyes. In the second method, the angle from the baseplate to the posterior surface of the head and the angle from the posterior surface of the head to the inferior-most extent of the head were measured from sagital view. The minimum vertex angle required was calculated as the complement of the sum of these angles. The dose volume histogram parameters were maintained. Depending on the spatial location of the planning target volume with reference to the eyes, patients were classified into 4 types: no overlap; overlap with eye anteriorly; overlap with eye posteriorly, overlap with eye anteriorly and posteriorly. No additional angulation, positive vertex and negative vertex angles were needed for type 1, 2 and 3 respectively. The angle subtended by the isocenter to the neck rest apex was the required vertex angle for type 4. Of the 36 type 2, 3 and 4 patients the planned vertex angle was more than the calculated vertex angle by over 5 degrees in 10 patients, and less by over 5 degrees in 4 patients. A simple method for deriving the minimal vertex beam angle for OAR avoidance in radiotherapy planning of intracranial tumors was described and validated.

SU-C-224-02: A Low-Cost Thin-Film Digital Detector for KV and MV Dosimetry

Objective: A prototypical low‐cost thin‐film digital detector system is developed and tested for kV/MV dosimetry. Similarly to radio‐graphic films, the detector is flexible, but its response is digitalized and transmitted to the host computer by a wireless sys‐tem in real‐time. Dosimetric properties of the detector are determined and compared to ion‐chamber dosimetry. Material and Methods: Different thin‐film detector cells in a solid water phantom were irradiated at various conditions: energies (80kVp, 130kVp, 6MV, 15MV), dose rates (different ms*mA, 100–600MU/min), total doses (0.1cGy–500cGy), depths (0.5cm–20cm), irradia‐tion angles with respect to the detector surface (0°–180°), and IMRT tests (closed MLC, sweeping gap). Device properties were characterized in terms of the aforementioned parameters. The detector is an embedded‐system consisting of an ultra low‐power microcontroller, RF transmitter, a low‐noise adjustable amplification circuit and commercial available (of the shelf) flexible thin‐film photodiodes. The signal is amplified and transmitted via a wireless system to be integrated and proc‐essed on the host computer. Results: The detector response is linear with total dose and does not depend on dose rate. Its energy response is similar to that of ionization chamber and gafchromic film. No angular dependence is observed for anterior beams, but small angular depend‐ence is observed for posterior beams. The detector can measure both MV and kV doses. Gain and frequency of sampling need to be maximized to obtain accurate dose for low instantaneous dose rate conditions such as closed MLC irradiations. Conclusion: Even in this early stage of development, the properties of the thin‐film photodiodedetector are comparable to the standard ionization chamber measurements. Due to its mechanical flexibility, the low‐cost realization of the device and the wireless data acquisition it is ideal for quality assurance and specifically for real‐time dose monitoring in challenging QA setup con‐figurations, including large‐area and 3D detection.

Effect of beam arrangement on oral cavity dose in external beam radiotherapy of nasopharyngeal carcinoma

This study compared the oral cavity dose between the routine 7-beam intensity-modulated radiotherapy (IMRT) beam arrangement and 2 other 7-beam IMRT with the conventional radiotherapy beam arrangements in the treatment of nasopharyngeal carcinoma (NPC). Ten NPC patients treated by the 7-beam routine IMRT technique (IMRT-7R) between April 2009 and June 2009 were recruited. Using the same computed tomography data, target information, and dose constraints for all the contoured structures, 2 IMRT plans with alternative beam arrangements (IMRT-7M and IMRT-7P) by avoiding the anterior facial beam and 1 conventional radiotherapy plan (CONRT) were computed using the Pinnacle treatment planning system. Dose-volume histograms were generated for the planning target volumes (PTVs) and oral cavity from which the dose parameters and the conformity index of the PTV were recorded for dosimetric comparisons among the plans with different beam arrangements. The dose distributions to the PTVs were similar among the 3 IMRT beam arrangements, whereas the differences were significant between IMRT-7R and CONRT plans. For the oral cavity dose, the 3 IMRT beam arrangements did not show significant difference. Compared with IMRT-7R, CONRT plan showed a significantly lower mean dose, V30 and V-40, whereas the V-60 was significantly higher. The 2 suggested alternative beam arrangements did not significantly reduce the oral cavity dose. The impact of varying the beam angles in IMRT of NPC did not give noticeable effect on the target and oral cavity. Compared with IMRT, the 2-D conventional radiotherapy irradiated a greater high-dose volume in the oral cavity.

Are 2D Detector Arrays Sufficient for VMAT Quality Assurance?

Are 2D Detector Arrays Sufficient for VMAT Quality Assurance?

Feasibility of Left-sided Chest Wall Irradiation with Volumetric Modulated Arc Therapy

To evaluate the merits and demerits of a Volumetric Modulated Arc Therapy (RapidArc) over a Forward Intensity Modulated Radiotherapy (F-IMRT) plan in the treatment of post mastectomy carcinoma of the left (LT) breast. In this study we compared RapidArc (RA) plan and F-IMRT plan for carcinoma of the LT Breast in ten patients who underwent mastectomy. PTV included chest wall (CW) and supraclavicular fossa (SCF). RA plans consisted of two partial arcs (310 deg-150 deg clock wise/ counter clock wise). F-IMRT was planned with a single isocenter using standard tangential beams for CW and direct anterior beam for SCF with additional field-in-field technique. The prescription dose to PTV was 50Gy/25 fractions at 98% coverage. Dose volume histograms were analyzed for the two modalities. Conformity index (CI) & Homogeneity index (HI) were compared between the two plans for each patient. For Organs at risk (OAR), 45Gy (V45), 20Gy (V20) & 5Gy (V5) volumes were compared for LT Lung, Heart and Normal tissue (Body minus PTV). Low dose volume (5Gy) was compared between the plans for RT Lung & RT Breast. The results were analyzed using paired-t-test and considered to be significant if p was < 0.05. The mean CI for RA plan was estimated to be 1.15 ±0.09 compared to F-IMRT plan of 2.14 ± 0.38 (p < 0.001). The mean HI obtained was better with RA plans (0.12 ± 0.05) in comparison with F-IMRT plans (0.14 ± 0.02) (p < 0.001). The mean percentage volume of LT Lung receiving doses of 45 Gy, 20 Gy, and 5 Gy in RA plans were 1.3 ± 0.25, 22.59 ± 3.92 and 74.15 ± 8.52 whereas in F-IMRT plan they were 13.15 ± 3.37, 30.2 ± 5.09 & 46.42 ± 5.56 respectively (p < 0.001). The mean percentage volume of Heart receiving 45Gy, 20Gy & 5Gy in RA plans was found to be 0.1 ± 0.07, 8.3 ± 6.11 and 71.32 ± 17.17 in comparison with F-IMRT plan of 6.23 ± 4.36, 12.22 ± 5.92 and 18.47 ± 7.970 respectively. The statistical difference for Heart V45 and V20 was p < 0.01 and for V5 was p < 0.001 between RA and F-IMRT plans. The maximum dose to RT Breast was found to be less in RA plan compared to F-IMRT plan, but the low dose volume (V5) showed 9% increase in RA plans. The same observation was found for RT Lung with 10% increase in 5Gy volume when compared to F-IMRT (p < 0.001). For normal tissue mean percentage volume receiving dose 45 Gy, 20 Gy, and 5 Gy in RA plans were 3.2 ± 0.89, 9.31 ± 2.6, and 24.02 ± 6.42 but for F-IMRT plans dose were 6.49 ± 1.16, 10.42 ± 2.28, and 14.47 ± 1.87, respectively (p < 0.001). RapidArc plans produced good conformity and homogeneity index with excellent dose coverage. RapidArc reduced the OAR volume receiving high doses (V45 and V20). F-IMRT scored in low dose volume (V5) which was more in RA plans due to multiple beams entries. RA plans provided reduced heart dose and improved lung sparing, at the cost of increased low dose volume in contra lateral breast & lung as compared to F-IMRT.

SU-GG-T-479: Anterior Fields Improve Rectal Sparing in Prostate Treatment by Proton Therapy

Purpose: Current proton prostate treatments use only lateral beams, relying solely on the lateral beam penumbra for rectal sparing, rather than the much sharper distal penumbra. In this work, we perform a treatment planning study to demonstrate the potential dosimetric benefits of anterior fields for patients treated with a rectal water balloon and also to explore the corresponding requirements for the accuracy of beam range verification/correction. Method and Materials: The study was conducted using the Xio treatment planning system (Elekta). Ten prostate patients who used endorectal water balloons during their treatments were selected. Each patient was first planned with bi‐lateral beams following the conventional treatment protocol. Then straight anterior and bi‐oblique (+/− 30 degree) anterior fields were planned, with the range compensators initially generated based on the water equivalent path length (WEPL) to the distal surface of prostate, and then manually adjusted to improve the dose conformality. The dose volume distributions were compared between three treatment plans, a) equally weighted bi‐lateral beams, b) a single anterior beam, and c) equally weighted bi‐oblique anterior beams. Results: The anterior proton beams require less beam energy, ∼10 cm WEPL less than that of lateral beams. The average values of V95%,V80%,V50% for anterior rectum wall are 39%,58%,74% for plan a, 8%,27%,49% for plan b, and 6%,26%,51% for plan c. The V95%,V80%,V50% for the bladder are 6%,12%,18% for plan a, 14%,31%,39% for plan b, and 8%,16%,25% for plan c. The femoral heads receive ∼40% of the prescribed dose in the bi‐lateral beams, but nearly none in the anterior beams. Conclusion: Compared with the bilateral beams, treatment of prostate with anterior and/or oblique anterior beams offers significant improvement in sparing the anterior rectal wall. The femoral heads are free of radiation at the cost of a moderate increase in bladder dose.

SU-GG-T-471: Towards Range Guided Prostate Treatment Using an AP Field

Purpose:Proton treatment of prostate by anterior fields offers potential benefit in rectal sparing. However such treatment requires an accurate control of the beam range. We have recently proposed a method for in‐vivo range determination for prostate treatment by anterior fields, based on time‐resolved dose rate measurement. The method has been successfully tested against range shifting and range mixing of protons due to tissue inhomogeneity using simple geometric configurations in a water tank. In this study, we further evaluate the effectiveness of the method using a human pelvic phantom. Methods: A human pelvic phantom was used to mimic a real prostate treatment. The phantom was first CT scanned with a full bladder and a water balloon in the rectum. A treatment planningsoftware was used to compute the water equivalent path length (WEPL) between the phantom surface and the anterior inner surface of the rectum cavity, along the direction of an anterior beam. The phantom prostate was then irradiated using an anterior proton beam. A pinpoint ionization chamber was used to measure the time‐resolved dose rate function along the wall of the rectal cavity with spatial resolution of 0.25 mm. The obtained dose rate function were used to derive the corresponding WEPL values, and then compared with those calculated by the treatment planning system. Results: The time‐dependence of the measured dose rate functions showed limited deviations from those measured in a water tank, indicating a relatively low level of range mixing. This is consistent with the relatively mild tissue inhomogeneity along the beam path, despite the presence of the pubic bones. The discrepancy between the measured and calculated WEPL values is below 2mm. Conclusions: The time resolved dose rate method can be used, with reasonable accuracy, to control the beam range in prostate patient if treated by anterior fields.

SU-GG-T-253: Need for Robust Treatment Planning and Review Tools

Purpose: This paper demonstrates the need for tools to review plan robustness (PR) of the IMRTtreatment plans (TP); a special case of head and neck IMRT is presented. Methods and Materials: Patient with squamous cell carcinoma of the base of tongue was simulated on a slant board and an IMRT plan was generated to deliver 70Gy in 2Gy fractions using Pinnacle treatment planning system (TPS). Due to lack of clearance, posterior beams were avoided and planned with anterior and lateral beams. TP was reviewed and approved by an IMRTphysicist and the attending physician. During QA plan generation on a box phantom, unacceptable hotspots were noted on the lateral aspect. This is because the lateral‐beams were not opened to cover the posterior‐aspect of the target‐volume due to field‐size limitation. Therefore, optimizer heavily weighted the anterior beams to cover posterior‐aspect of the target‐volume. Results: This plan was fixed by opening the lateral beams to cover the posterior‐aspect of the target‐volume. If it was treated in the presence of systematic lateral setup error, this clinically approved TP could have lead to serious skin inflammation. Plan review couldn't catch this error because most TPS doesn't compute dose outside skin. For those who use just planar dosimetry measurements for IMRT QA would have missed this error. This demands the need for review‐tools that can check the TP robustness in the presence of setup errors. We checked the robustness of our plan by shifting the isocenter to simulate setup‐errors and computing the dose and comparing the DVH indices. We propose using PR check for all clinical patients and this can be easily implemented using pearl scripts in Pinnacle TPS. Conclusions: As the radiation delivery techniques get complex, PR check should be part of the plan review process to avoid serious treatment errors.

SU‐GG‐T‐385: Dose Perturbation by a Steel Magnetic Injection Port in a Breast Tissue Expander in Proton Radiotherapy: A Monte Carlo Study

Introduction: The use of a tissue expander in breast reconstruction is a popular option for patients undergo mastectomy. We applied Monte Carlo simulation to analyze the dose deposition from proton beams in the presence of a tissue expander containing a steel magnetic injection port. Methods:. A steel injection port for breast tissue expander (Mentor®) was studied with a single anterior (AP) proton beam. A general‐purpose Monte Carlo particle transport code FLUKA was applied for simulation. A 10×10 cm beam was delivered to a water phantom of 50×50×50 cm with the port positioned at the surface. Data on the proton fluence and dose deposited in phantom were collected for a range of initial proton energies forming the spread out Bragg peak (SOBP) of 100 to 200 MeV. Results: Our simulation shows that low energy proton fluence is disturbed significantly by the presence of the steel injection port. Impact of the port components varies at the crossbeam plane. The needle shielding plate edge decreases fluence in the beam direction by almost at 30%. The proton range shifts about 50 % for lower energies (100 MeV) and 15% for higher energies (200 MeV). Dose changes at distances close to metal‐phantom interface are between 5 and 10%; however, there are dramatic changes (up to 100%) in dose deposition at deep areas where the SOBP suppose to form. Combining multiple beams with various energies and weights into SOBP will cause multiple spots of significant over‐ and under dose. Conclusions: The dose perturbation by a steel injection port for breast tissue expanders is significant if the implant is in a proton field. The accuracy in treatment planning with conventional algorithms may be questionable in the presence of the inhomogeneity. Avoidance of such metallic components should be considered if adjuvant proton radiotherapy is considered.

SU‐GG‐J‐128: Comparison of Dose Distributions for Small Animal Radiotherapy Using a MicroCT Scanner and a Single‐Field Irradiator

Purpose: To compare dose distributions for small animal radiotherapy performed on a microCT scanner with multiple 120kV beams and a single‐field irradiator with a 200kV beam. Materials and Methods: A microCT scanner with a maximum x‐ray tube potential of 120kV and a single‐field irradiator operating at 200kV were modeled in the EGSnrc/BEAMnrc Monte Carlo code. The models were validated using dose in air and depth dose curves in solid water for various beam sizes measured with an ionization chamber and gafchromic films. Treatment plans for a mouse with subcutaneous teratoma and a mouse with a spontaneous lung tumor were created for both the microCT scanner and the single‐field irradiator. On the microCT, the teratoma treatment was prescribed as tangential 80° and 180° beams and lung tumor treatment consisted of eighteen beams equally spaced between 90° and 270°. On the single‐field irradiator, the mice were planned with a left lateral beam and an anterior posterior beam, respectively. The 5Gy dose to the PTV and the dose to critical structures were calculated in the EGSnrc/DOSXYZnrc code. Results: In both cases, the dose distributions achieved with the microCT scanner using a 120kV beam were superior to the dose calculated for a single 200kV beam. The teratoma case showed 4Gy dose to 30% of the left lung whereas the dose to all critical structures on the microCT scanner was below 0.2Gy. In the case of the lung tumor, the dose to the spinal cord and right lung is considerably larger for the single‐field irradiator plan. Left lung and heart are completely spared in the single‐field treatment, however, the dose to these structures is below 1.5Gy in the microCT plan. Conclusions: This work demonstrates that small animal radiotherapy on a microCT scanner yields better critical organ sparing and is therefore preferred over single‐field irradiator radiotherapy.

TU‐A‐204B‐02: On the Potential of CBCT for Range Verification in Proton Therapy

Purpose: We have investigated the potential use of cone beam CT (CBCT) for beam range verifications in proton therapy treatment, in addition to its primary role in geometric targeting. Specifically, we studied the intrinsic imaging variability of a CBCT and its effect on the water equivalent path length (WEPL) calculations, in the context of daily beam range verification/correction required for a recently proposed method of treating prostate using anterior fields. The current approach uses only lateral fields due to the lack of precise range control in patient. Materials and Methods: An anthropomorphic pelvic phantom was scanned using CBCT, in eight sessions on eight different days. In each session, the phantom was scanned twice, first at a standard position as determined by the room lasers, and then with a random shift of one centimeter in lateral directions. The Xio treatment planning system was used to perform the analysis. The average Hounsfield unit (HU) numbers for the water column in the rectal balloon was used to perform a linear calibration of the stopping power ratio, independently for each scan, as supported by the planning system. A number of WEPL values vertically from the anterior skin surface to the anterior surface of the water balloon were calculated on slices covering the region of the prostate, in relevance to a prostate treatment using an anterior field. Results: The HU number in the water column varied significantly even within the same CBCT. The average value also varied from day to day for up to 20 units. However, when these average values are used to calibrate the stopping power ratio, the variations in WEPL values along the anterior beam path are mostly within 2 mm. Conclusions: In‐room CBCT can be used in proton therapy to make online verification of protons range in patients with 2mm accuracy.

SU‐HH‐BRB‐11: Range Adaptive Proton Therapy for Prostate Cancer

Purpose: The rapid distal falloff of a proton beam allows for sparing of normal tissues distal to target organs. However proton beams that aim directly towards critical structures are avoided due to concerns of range uncertainties, such as CT number conversion and anatomy variations. We propose to eliminate range uncertainty and enable prostate treatment with a single anterior beam by detecting the proton's range at the prostate‐rectal interface and adaptively adjusting the range in vivo and in real‐time. Materials and Methods: A prototype device, consisting of an endorectal liquid scintillation detector and dual‐inverted lucite wedges for range compensation, was designed to test the feasibility and accuracy of the technique. Liquid scintillation filled volume was fitted with optical fiber and placed inside the rectum of an anthropomorphic pelvic phantom. Photodiode‐generated current signal was generated as a function of proton beam depth, and the spatial resolution of this technique was calculated by relating the variance in detecting proton spills to the penetration depth. The relative water‐equivalent thickness of the wedges was measured in a water phantom and prospectively tested to determine the accuracy of range corrections. Treatment simulation studies were performed to test the potential dosimetric benefit in sparing the rectum.Results: The spatial resolution of the detector in phantom measurement was 0.5 mm. The precision of the range correction was 0.04 mm. The residual margin to ensure CTV coverage was 1.1 mm. The composite distal margin for 95% treatment confidence was 2.4 mm. Planning studies based on a previously estimated 2mm margin on 27 patients showed a rectal sparing up to 51% at 70Gy and 57% at 40Gy relative to IMRT and bilateral proton treatment. Conclusion: We demonstrated the feasibility of our design. Use of this technique allows for proton treatment using a single anterior beam, significantly reducing the rectal dose.

Optimal planning parameters for simultaneous boost IMRT treatment of prostate cancer using a Beam Modulator™

Objective To determine the optimum energy and beam arrangement for prostate intensity-modulated radiation therapy (IMRT) delivery using an Elekta Beam Modulator™ linear accelerator, in order to inform decisions when commissioning IMRT for prostate cancer. Methods CMS XiO was used to create IMRT plans for a prostate patient. Arrangements with 3, 5, 7, 9 and 11 equally spaced fields, containing both a direct anterior and a direct posterior beam were used, with both 6 MV and 10 MV photons. The effects of varying the maximum number of iterations, leaf increment, number of intensity levels and minimum segment size were investigated. Treatment plans were compared using isodose distributions, conformity indices for targets and critical structures, target dose homogeneity, body dose and plan complexity. Results Target dose conformity and homogeneity and sparing of critical structures improved with an increasing number of beams, although any improvements were small for plans containing more than five fields. Set-ups containing a direct posterior field provided superior conformality around the rectum to anterior beam arrangements. Mean non-target dose and total number of monitor units were higher with 6 MV for all beam arrangements. The dose distribution resulting from seven 6 MV beams was considered clinically equivalent to that with five 10 MV beams. Conclusion Methods have been developed to plan IMRT treatments using XiO for delivery with a Beam Modulator™ that fulfil demanding dose criteria, using many different set-ups. This study suggests that 6 MV photons can produce prostate IMRT plans that are comparable to those using 10 MV. Work is ongoing to develop a complete class solution.

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.

SU‐FF‐T‐107: Selective Extended Field IMRT Can Protect Larynx and Adequately Treat the Lower Neck Lymph Nodes in Head and Neck Cancer

Objectives: Radiation dose to the larynx has been a major concern in the treatment of head and neck cancer. The use of splitting field (SF) IMRT can improve the dose to the larynx but it may result in inadequate tumor coverage and dosimetric uncertainties at the matchline. In this study, we propose a selective extended field (SEF) method to avoid the use of splitting field while reducing the larynx dose and achieving the adequate tumor coverage. Materials and Methods: Five nasopharyngeal patients with the gross tumor volume extended to the lower neck were treated with extended field (EF)‐IMRT. SEF IMRT plans were created by extending only three anterior and anterior oblique beams to the supraclavicular region while the other fields were restricted to the superior of the vocal cord. For comparison, SF‐IMRT plans were also created and matched with the conventional AP field. All plans were compared based on the tumor volume coverage, mean dose to the larynx while meeting other planning requirement in RTOG protocol of 0225. Results: The average doses to 95% (D95) of the pGTVs and pCTV were 67.10 ± 6.56 Gy, 70.65 ± 1.62 Gy, 72.74 ± 1.76 Gy and 55.21 ± 3.41 Gy, 57.17 ± 1.51 Gy, 59.92 ± 1.01 Gy for SF, SEF, and EF‐IMRT plans, respectively. The tumor dose coverage in SF IMRT plans was the worst among three methods. Accordingly, the average mean doses to the larynx were 29.25 ± 5.61 Gy, 29.41 ± 2.99 Gy, 41.87 ± 10.61 Gy, respectively. The mean dose to the larynx in EF‐IMRT plans were the highest among the three methods. Conclusions: The selective extended field IMRT provides adequate tumor dose coverage and protects the larynx with the similar mean dose as the SF method.Research is supported by Siemens Corporation.

Poster - Thurs Eve-43: Verification of dose calculation with tissue inhomogeneity using MapCHECK.

MapCHECK (Sun Nuclear, Melbourne, FL) with 445 diode detectors has been used widely for routine IMRT quality assurance (QA)1 . However, routine IMRT QA has not included the verification of inhomogeneity effects. The objective of this study is to use MapCHECK and a phantom to verify dose calculation and IMRT delivery with tissue inhomogeneity. A phantom with tissue inhomogeneities was placed on top of MapCHECK to measure the planar dose for an anterior beam with photon energy 6 MV or 18 MV. The phantom was composed of a 3.5 cm thick block of lung equivalent material and solid water arranged side by side with a 0.5 cm slab of solid water on the top of the phantom. The phantom setup including MapCHECK was CT scanned and imported into Pinnacle 8.0d for dose calculation. Absolute dose distributions were compared with gamma criteria 3% for dose difference and 3 mm for distance-to-agreement. The results are in good agreement between the measured and calculated planar dose with 88% pass rate based on the gamma analysis. The major dose difference was at the lung-water interface. Further investigation will be performed on a custom designed inhomogeneity phantom with inserts of varying densities and effective depth to create various dose gradients at the interface for dose calculation and delivery verification. In conclusion, a phantom with tissue inhomogeneities can be used with MapCHECK for verification of dose calculation and delivery with tissue inhomogeneity.

SU‐GG‐T‐470: Improving Rectal Sparing with a Single Anterior Proton Beam: A Planning Study

Purpose: To demonstrate the potential dosimetric advantage of an anterior proton beam for treatment of prostate cancer patients utilizing the Bragg peak for rectal sparing. Methods to detect the anterior proton beam at depth are being investigated to eliminate the large distal planning margin, enabling abrupt dose falloff at the anterior rectal wall rather than the relatively shallow falloff of the lateral beam penumbra with conventional lateral beam arrangement. Method and Materials: 27 consecutive prostate cancer patients treated with proton therapy were enrolled in a planning study to compare standard IMRT and bi‐lateral proton treatments with a single anterior proton beam treatment. Dose‐population histograms were generated comparing target coverage and normal tissue sparing of all critical structures between the treatment groups. Results: The anterior beam treatment significantly spared the anterior rectal wall more than the IMRT and bilateral beam treatments at the 70 Gy (12.3% vs 19.2% and 19.2%), 60 Gy (15.7% vs 23.8% and 24.3%), 45 Gy (19.4% vs 30.1% and 29.6%), and 30 Gy dose levels (22.7% vs 39.9% and 34.2%). Bladder wall dose was greater in the anterior beam treatment than IMRT and bilateral beam treatments at the 70 Gy (14.1% vs 11.2% and 12.8%), 60 Gy (22.5% vs 13.7% and 16.2%), 45 Gy (29.0% vs 17.9% and 19.8%), and 30 Gy dose levels (32.6% vs 25.1% and 23.1%). No dose was delivered to the femoral heads with the anterior beam treatment while the IMRT and bilateral treatments delivered a mean dose of 17.0 Gy and 25.1 Gy to the femoral heads respectively. Conclusion: With the implementation of in vivo proton beam detection, anterior proton beam treatment of prostate is a possible treatment alternative, sparing dose to the rectum and femoral heads at the cost of a small increase in bladder dose.

TH‐C‐AUD C‐09: Efficacy of Virtual Reality Simulation for Noncoplanar Prostate IMRT — a Peek of Future RTP System

Purpose: To evaluate the efficacy of virtual reality simulation for noncoplanar prostate IMRT optimization process. Method and Materials: 14 prostate IMRT cases, randomly selected from current clinic, were re‐simulated using a pioneer virtual reality simulation (VRS) system for noncoplanar beam arrangement. Using 3D stereoscopic technology, the VRS system displays the virtual treatment environment on a DTI™ real 3D LCD screen without aid of gaggles. With the virtual patient loaded to the screen, one can simulate the treatment by realistically manipulating the gantry and table. An optimal beam projection is suggested when the inclusion of vital organ was reduced in the volumetric beam eye‐view. When exploring a noncoplanar beam position, the anti‐collision function effectively alerted the mechanical limitations. In comparing with original seven evenly‐spread coplanar beams, in the resultant new setting all beams were rotated more anteriorly, while two anterior oblique beams were tilted 20–30° inferiorly. All IMRT plans were calculated with similar modulated intensity level and number of segments. To standardize the endpoint for comparison, PTV D95 was normalized to 45 Gy. Results: VRS created non‐coplanar beam arrangements were proved clinically to be deliverable beam arrangement without risk of collision. Dose homogeneity in new plans were improved, indicated by 2.2% reduction in global maximal dose and 2.0% in high dose tail (D5) of PTV. Enhancement of rectal dose sparing was suggested by 5.7% and 3.7% lower values for rDmean and rD10cc respectively. Less inclusion of bladder in BEV of the two anterior noncoplanar oblique beams translated a 14% and 5.1% reduction in bladder mean dose and bD30cc respectively. Conclusion: The VRS effectively concluded deliverable noncoplanar beam arrangements for prostate IMRT with resultant improvement of dose characteristics. Further development of VRS may present future RTPS.

SU‐GG‐T‐255: Experimental Verification of Lung Tumor Dose with Radiochromic Film

Purpose: To assess the absorbed doses in and around lung tumor by performing radiochromic film measurements. Method and Materials: A phantom was made to have a water equivalent central core, a low‐density middle layer and a water equivalent outer layer to simulate tumor in the lung, lung and chest wall, respectively. GafChromic™ EBT films were placed in the anterior and posterior surfaces and middle plane of the tumor. Three plans with a single anterior beam, opposed anterior‐posterior beams and five‐field‐IMRT were delivered separately. A calibration curve was generated to convert the optical density to absorbed dose. The measured doses in the three planes were compared with the calculated doses from Eclipse™ system which uses a Pencil Beam convolution‐superposition Algorithm with modified Batho inhomogeneity correction. The single‐beam, opposed beams were delivered for both low (6 MV) and high (15MV) energy photon. The IMRT plan was delivered with 6 MV. Results: For both single‐ and opposed‐beam plans, the measured dose was very close to the calculated dose in the middle plane (&lt;2 %). In the anterior and posterior surfaces of the tumor, the measured dose was substantially lower than the calculated one. For the single‐beam plan, the difference was 10.0% (6MV) and 13.6% (15MV) for the anterior surface and was 9.8% (6MV) and 6.8% (15MV) for the posterior surface. For opposed beams, the difference was about 8% (6MV) and 11% (15MV) for both surfaces. Similar dose differences were observed for the IMRT plan (∼ 10 %). Conclusion: EBT film measurements demonstrated that the dose calculation algorithm used by a typical treatment planning system is precise (&lt;2%) for the deep locations inside the tumor, but may overestimate the surface dose about 10%.