IMRT Phantom Research Articles

SU‐EE‐A3‐04: In Vivo Detection of the Anterior Rectal Wall Using a Scintillation Fluid‐Filled Rectal Balloon for Prostate Cancer Radiotherapy

Purpose: The prostate and the anterior rectum interface is perhaps the most important landmark but the most difficult soft tissue region for localization in image‐guided procedures. The goal of this study is to utilize the detection of rectal dose as a surrogate for anterior rectal wall (ARW) localization for daily prostate patient set‐up without using an imaging method. Methods: In this study, we designed a sweeping MLC beam to scan the anterior‐posterior (AP) prostate‐rectum interface from the lateral directions. The dose response from the inside of rectum will be measured using liquid scintillator and associated fiber optics. To validate our design, a 3mm wide lateral MLC segment was designed to move across the prostate‐rectum interface, and the mean rectal dose was calculated for each position in 8 prostate cancer patients with endorectal balloon immobilization and daily CT image sets. Then the dose‐position response curve was measured daily and fitted to the reference relationship to determine the shift which best reproduces the reference dose‐position relationship relative to the isocenter. To validate this method, the predicted alignments using the dose‐position relationship were compared with the manual AP alignment of the ARW using our in‐house CT registration software. Feasibility of the sliding segment technique was tested in an IMRT phantom fitted with a cylindrical liquid scintillator filled tube, optic fibers, and photodiode for current generation and detection. Results: The mean agreement of the dose‐calculated alignments and the manual image registration was 0.76 millimeters. The primary source of uncertainty was the variation in the insertion of the endorectal balloon. Measured signal increased monotonically with beam position within the rectum and rectal dose. Conclusion: In vivo detection of the ARW is feasible without an on‐board x‐ray imaging technique to achieve better alignment to anatomically important soft tissue structures.

WE-A-AUD C-01: IMRT Commissioning: A Multi-Center Evaluation of a Standardized Test Protocol

There is evidence that IMRT treatments may not always be as accurate as practitioners believe. In 2006, the Radiological Physics Center (RPC) reported that of the 155 institutions that had irradiated a head and neck phantom as part of an IMRT credentialing process, 54 (35%) had failed to meet accuracy criteria of 7% for dose in a low gradient region and/or 4 mm distance to agreement in a high gradient region. This experience strongly suggests that some clinics have not adequately commissioned their planning and delivery systems for IMRT. By “commissioning”, we mean beam modeling in the treatment planning system and initial verification by phantom studies that treatments can be planned, prepared, and delivered with sufficient accuracy. Task Group 119 of the American Association of Physicists in Medicine (AAPM) has developed a specific set of tests for IMRT commissioning that are representative of common clinical treatments and pose a range of optimization problems requiring simple to complex modulation patterns. The tests include mock prostate, head and neck, and peri‐spinal geometries. Members of the group have planned and delivered the treatments using their local planning and delivery systems, and then assessed the resulting doses using broadly available dosimetry tools following a specified protocol. Measurements included ion chamber point doses and film dosimetry on selected planes for all fields irradiating the phantom. Institutions also evaluated dose distributions produced by individual fields using detector arrays, film, or EPID. Eight institutions have reported their results with nine different combinations of planning systems and accelerators. All have passed the RPC IMRT phantom test. The summary of the preliminary data shows: One institution identified the need to improve the beam modeling using these tests for better agreement between planned and delivered doses for IMRT. The presentation will illustrate (1) that the percent of points passing the gamma criteria is highly dependent on the details of the implementation of the test and (2) that testing the individual fields with a gamma test can be insensitive to problems and is not sufficient for commissioning. (3) The tolerance limits based on Dose‐difference distribution, distance‐to‐agreement (DTA), and a numerical gamma index for IMRT QA are often not adequate because all these methodologies do not account for space‐specific dose uncertainty information. (4) beam modeling affects the plan and delivered dose agreement. Educational Objectives: 1. To describe the uncertainties in IMRT planning and delivery‐describe the impact of spatial and dosimetric uncertainties on the IMRTdose distribution. 2. Obtain examples of commissioning results for standardized IMRT studies that can be used for comparison with a clinic's IMRT system. 3. See how these standardized tests can be downloaded or created for local testing purposes. 4. See examples of how the gamma criterion can vary depending on details of its implementation. 5. See examples of limitation of gamma criterion testing for identifying problems with individual IMRT fields.

SU‐FF‐T‐429: Treatment Planning Modeling and Dose Delivery Advantages of Standard Linac Without Flattening Filter

Purpose: The objective of this study is to determine the accuracy with which treatment planning systems can model dose distributions generated by conventional linear accelerators without the flattening filter, and the potential benefits. Method and Materials: Data was collected on a Siemens Oncor linear accelerator, where the flattening filter has been removed and energy tuned to match a standard 6 MV x‐ray beam but with dose rates of 1000 MU/min. Data was acquired to model and commission four treatment planning systems (Philips Pinnacle, CMS Xio, Nomos Corvus, and Siemens Konrad). Comparisons included: TPS model parameters w/ and w/o flattening filter, measured and computed %dd and profiles, conventional and IMRT dose distributions both via planning and delivery measurements in phantom. Results: Beam energy spectrum characteristics show only a modest softening in the beam since beam tuning matched the filtered 6 MV. Differences are seen in the off‐axis fluence and off‐axis energy softening: the fluence is reduced linearly away from the central axis and the energy spectrum is only slightly reduced by comparison. Dose profiles and %dd were accurately modeled to within 2% and 2mm for all field sizes 2–40 cm. When applied to IMRT treatment plans, the model produced equivalent dose distributions for head‐and‐neck, CNS, and prostate plans; the number of monitor units was slightly increased (∼15%) but the number of segments was identical. Delivery times at 1000 MU/min were shorter (20% reduction), and the IMRT plans delivered had a 30–50% reduction in the number of pixels showing dose‐differences beyond 5%. Conclusion: Treatment planning systems are not only capable of modeling the dose distributions produced without flattening filters, they perform superior than with the filter present, and can reduce common dosimetric errors observed in IMRT phantom studies.Research sponsored by Siemens Medical Solutions corporation.

SU-FF-T-245: Impact of Volume Effect of Detectors in Multiple Points Absolute Dose and the Choice of Plane of Measurement in 2D Planar Dose Verification in Patient Specific IMRT QA

Purpose: To evaluate the volume effect of detector's in multiple points dose in small and large field IMRT verification and to compare the planar dose distribution between fixed‐fields coronal‐plane and multifield axial‐plane verification methods. Method and Materials: Patient‐specific dosimetric verification was conducted for 20 patients of nasopharynx's small‐field, SIB‐IMRT large‐field and prostate tumors. Eclipse‐Helios system, Clinac‐2300C/D linear accelerator, Med‐Tec IMRT phantom and ionchambers of 0.6, 0.13 and 0.015cm3 and MOSFET detectors were used. For each patient, 15 IMRT plans were generated. The multiple points doses were measured at multifield and fixed‐fields on‐axis, 4‐offsets and inhomogeneous points. The agreements between calculated and measured doses were found for small and large field IMRT verification. For relative dosimetry, EDR2 films, Virtual Water phantom for fixed‐fields coronal‐plane and IMRT Body phantom for multifield axial‐plane verifications RITsystem and Vidar scanner were used. The agreements between calculated and measured dose distributions between two methods were compared using various gamma criteria. Results: The agreements between measured and calculated doses using 0.6 and 0.13cm3 chambers at multifield and fixed‐fields on‐axis points were within 4.7% ±2.3. The dose differences of 6% ±1.7, 19.7% ±5.5, and 12.1% ±3.2 at offset1 to 3 points were found. The 0.015cm3 chamber and MOSFET detectors were showed the dose differences of upto 8.4% in large field verification. The mean differences in percentage of pixels passing the gamma criteria of 3mm/5% were 98.2% ±0.9 and 90% ±5.4 for fixed‐fields coronal‐plane and multifield axial‐plane verifications. Conclusions: This study is useful in evaluating the detector's response at high and low dose, outside the fields and inhomogeneous points dose verification. In offset points, 0.6cm3 is more accurate than 0.13cm3 chamber in both small and large field verification. The fixed‐fields coronal‐plane verification is more accurate, however, the multifield axial‐plane is clinically realistic method of verification and to be adopted.

SU‐FF‐T‐239: Gamma‐Test Equivalent Error Margins for Dosimetry Analysis

Purpose: The γ‐test evaluates the difference between two dose distributions in dose‐spatial domain. Previous methods employing the γ‐test at a point to translate spatial variation to acceptable dose difference involve a test distribution and a reference dose. This work developed a method for dose comparison using pre‐calculated acceptable error margins based on one dose distribution. Method and Materials: The expected uncertainty for a reference dose distribution is predetermined without making relevance to another distribution. The error margin at a point is calculated using the γ‐index formula over a sphere of a radius being the maximum distance‐to‐agreement (MDAT). All dose values within the sphere, evaluated using a set of evenly distributed points, are acceptable, whereas appropriate dose margins, up to a maximum dose difference (MDD), are included. Comparison with another distribution is to check against the upper and lower bound of the error margins separately, so γ‐test equivalent failure can be discerned to hot and cold regions. The degree of failures can be analyzed quantitatively for determining how an agreement between the two distributions can be improved globally. For an application, we calculated the error margin of dose measurement for IMRT QA using the distribution of volume‐averaged chamber response. Results: The error margin for IMRT phantom measurement using an IC‐10 chamber was about ±4% for MDD=3% and MDAT=3 mm in a flat region. Asymmetric error margins were found in gradient regions. Expected range for the output measurement with a 1cm×1cm field was (−18.6%, 2.5%) for MDD=2%, MDAT=2 mm, and a measurement was −13.3% off. Conclusion: Using predetermined error margins for a reference distribution is equivalent to the γ‐test in dose comparison. The error margins are not skewed by another distribution and interpretable as expected uncertainty for statistical analysis. The expected values for a reproduced distribution can be different from the reference.

TH‐C‐M100E‐02: Optically Stimulated Luminescence of Aluminum Oxide Detectors for Radiation Therapy Quality Assurance

Purpose: The purpose of this experiment was to: 1) Determine if a commercially available detector system used for monitoring personnel exposure could be adapted for use as a radiation therapy dosimetry system; and 2) Evaluate the system's performance as an in‐vivo dosimeter and its ability to measure absolute surface dose, isocenter dose, and normal tissues dose in a phantom as part of patient‐specific IMRT quality assurance. Method and Materials: The dosimeters were evaluated for: 1) Signal decay; 2) Field size dependence; 3) Energy dependence; and 4) Angular dependence using the Landauer, InLight MicroStar system. In‐Vivo dosimetry measurements were taken for 22 patients treated on a Varian 21EX. The Landauer system was also tested for its ability to measure absolute dose from helical tomotherapy treatments. Results: The variation between dosimeters was evaluated and found to be ±1.6%. The dosimeters appeared to over‐respond in the first 10 minutes, however, after 10 minutes the chips were within 1 percent of the steady‐state reading. Unlike other detectors, the Al2O3 dosimeters showed no field size, energy, or angular dependence. The agreement between the dosimeters and the calculated doses for the in‐vivo dosimetry patients was 2.2±6.1 cGy or 3.7±2.5%. The dosimeters were also tested for their ability to measure absolute dose inside an IMRT phantom. The agreement between the dosimeters and the calculated doses was 0.1±5.3 cGy or 0.7±6.7%. Conclusion: dosimeters can be a convenient, inexpensive alternative to TLDs, MOSFETS, and Diodes. The agreement between calculated and measured doses for in‐vivo dosimetry and IMRT QA is comparable to TLDs, MOSFETS, and Diodes. The dosimeters can be quickly read and analyzed after 10 minutes (to allow time for signal decay). The dosimeters do not appear to have an energy, field size, or angular dependence. In addition, the detectors can be erased and re‐used.

SU‐FF‐T‐197: Effect of the Added Buildup On P‐Type Silicon Diodes' Response in Daily in Vivo Dosimetry Measurements

The silicon semiconductor diodes used for routine in vivo dosimetry in radiation therapy are commonly constructed with added buildup to place the effective point of measurement (location of the silicon chip) at the depth of maximum dose (dmax) or beyond and to shield against low energy scattered photons. This added buildup, commonly made of high‐density metals, could however introduce increased uncertainty to the accuracy of diode measurements. Previous work in this area has shown that buildup diodes show certain imperfections when applied in small photon fields.1We have studied the effect of this buildup on in vivo dosimetry measurements, especially for small‐field dosimetry associated with IMRT treatments. We compared the response of the buildup diodes to that of no‐buildup ones, commonly referred to as skin diodes. We have used identical commercially available p‐type diodes, with and without the additional buildup, to measure the dose delivered by small static fields. The measurements with the no‐buildup diode were performed by placing tissue‐equivalent bolus on top of the diode to bring the diode measurement point to the depth of Dmax. In addition, we measured the dose to sample IMRT phantom plans as well as patient IMRT plans delivered to phantom, using both types of diodes.Our results indicate that the buildup diodes, in general, produce higher readings than no‐buildup ones for small field dose deliveries, adding to the uncertainty of diode measurements. We have also observed that using the skin (no‐buildup) diodes with added tissue‐equivalent buildup decreases the measured‐to‐expected percentage differences by about 2%.

3D dosimetry by optical-CT scanning

The need for an accurate, practical, low-cost 3D dosimetry system is becoming ever more critical as modern dose delivery techniques increase in complexity and sophistication. A recent report from the Radiological Physics Center (RPC) (1), revealed that 38% of institutions failed the head-and-neck IMRT phantom credentialing test at the first attempt. This was despite generous passing criteria (within 7% dose-difference or 4mm distance-to-agreement) evaluated at a half-dozen points and a single axial plane. The question that arises from this disturbing finding is – what percentage of institutions would have failed if a comprehensive 3D measurement had been feasible, rather than measurements restricted to the central film-plane and TLD points? This question can only be adequately answered by a comprehensive 3D-dosimetry system, which presents a compelling argument for its development as a clinically viable low cost dosimetry solution. Optical-CT dosimetry is perhaps the closest system to providing such a comprehensive solution. In this article, we review the origins and recent developments of optical-CT dosimetry systems. The principle focus is on first generation systems known to have highest accuracy but longer scan times.

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.

TU-D-224C-02: Monte Carlo Direct Aperture Optimization (MC-DAO) for IMRT

Purpose: To improve the accuracy/efficiency of IMRT planning by combining Monte Carlo (MC) dose calculation with direct aperture optimization (DAO). Method and Materials: A 6 MV beam arrangement is applied to an IMRT phantom and patient examples. A phase space is calculated below the secondary jaws of a virtual Varian 21EX linac by MC simulation (BEAMnrc code (NRC, Canada)). The phase space is subdivided into 2.5×5.0 mm2 beamlets and the dose distribution from each beamlet is calculated to organs‐of‐interest within the patient/phantom using DOSXYZnrc. This information is input into DAO inverse planning software. The DAO includes multileaf collimator transmission and leaf motion limitations as it modifies the shape/weight of the treatment apertures. The optimized leaf sequence requires no additional leaf motion calculation step. A final forward MC dose calculation is performed. The MC doses are verified with ion chamber and film measurement. MC‐DAO is applied to a difficult phantom geometry, namely a c‐shaped target with embedded organ‐at‐risk located directly adjacent to a 5.0cm‐thick air slab. Clinical sites include nasopharynx and lung.Results: The MC optimization allows for accurate modeling of the electronic disequilibrium introduced by the air cavities. For the phantom example, MC reveals that the plan optimized with a pencil beam (PB) algorithm fails to provide adequate coverage to the PTV close to the air cavity, whereas the MC‐DAO plan demonstrates adequate coverage. For the nasopharynx, the PB plan showed errors during ion chamber/film verification, probably due to the small (∼5×4 cm2) fields whereas the MC‐DAO plan showed good agreement. The reduction in monitor units for MC‐DAO plans is 20 – 40% compared to a commercial fluence‐based (PB) treatment planning system. Conclusion:MC simulation generates accurate input data for IMRT inverse treatment planning in difficult‐to‐calculate regions. The addition of DAO results in a more efficient treatment plan delivery.

SU-FF-T-40: A Novel Phantom for Use in 3-Dimensional in Vitro Cell Experiments

Purpose: To describe and characterize a novel IMRT phantom for use in 3-dimensional in vitro cell experiments. Methods and Materials: A cylindrical IMRT phantom was designed and fabricated from a commercially available system (CIRS Inc., Norfolk, VA). The phantom is composed of water-equivalent plastic and contains a rectangular bore which, in combination with a set of water-equivalent plastic inserts, can be adjusted to fit 1–3 well plates. In these initial studies, the phantom was loaded with a stack of three well plates (96 individual wells/plate), and an IMRT plan was created for two separate PTVs, each receiving a scalable uniform dose. The ratio of doses between the PTVs was fixed at 2:1. Using PTV doses of 50 and 100 cGy, measurements from TLDs placed at five locations within each PTV were compared to the expected doses. For the same irradiation technique, dose distributions were acquired from films placed above and below each well plate and compared with the treatment plan using the γ-index. Head and neck tumor cell lines were also irradiated in this phantom and cell viability was assessed using the MTT assay. Results: TLD measurements yielded doses of 100.3 +/− 4.6 cGy and 51.0 +/− 2.3 cGy for 100 cGy and 50 cGy PTVs, respectively. Calculations of γ were performed using a dose difference of 3.0 mm and distance to agreement of 3.0 %. The γ indices ranged from 0.18 +/− 0.13 to 0.51 +/− 0.35 in each of the PTVs. Cell irradiation experiments showed uniform viability within each of the PTVs. Conclusion: The consistency between measurements obtained from the TLDs and film with the calculated dose distributions, along with uniform cell viability within each irradiation region, show that the phantom is a novel tool for 3-dimensional in vitro cell experiments. Conflict of Interest: Supported by a grant from MedImmune.

SU‐FF‐T‐448: Validation of a New Photon Dose Calculation Model—Analytical Anisotropic Algorithm

Purpose: To validate a new photon dose calculation model Analytical Anisotropic Algorithm (AAA) on Eclipse™ treatment planning system (TPS). Comparison of AAA dose calculation was performed with measurements and other two conventional algorithms, Pencil Beam Convolution (PBC) algorithm on Eclipse™ and Collapsed Cone Convolution/Superposition (CCC) algorithm on Pinnacle3.0 TPS.Method and Materials: Four phantoms were CT scanned and the image set was imported into both TPS for dose computation and analysis. The four phantoms were: 1) homogenous tissue equivalent phantom, 2) tissue equivalent phantom with infinite lung heterogeneity, 3) tissue equivalent phantom with finite lung, 4) IMRT dose verification phantom. Measurements were made by exposing the phantom using Varian Linac. Point measurement and film measurements were compared with calculated results from the three algorithms. Dose responses for high and low energy photon beams were investigated for several different depths and PDD curves were compared in the phantom for various field sizes. The IMRT plans were generated by both TPS and were performed on the IMRT phantom to compare fluence maps. Results: AAA dose prediction fits the film measurements well except that there is up to ±6% discrepancy for dose profile perpendicular to the interface of tissue and lung. Point measurements support the AAA algorithm calculations. AAA also accurately predicts the decrease in PDD curves due to the lung inhomogeneity for 6MV energy. For the high energy photon beam and very small field size (2cm*2cm) in lung region, AAA prediction is up to 8% lower than the measurements. Conclusion: AAA algorithm accounts for attenuation corrections and electron transport, and models the deposited dose in the lung with greater accuracy than PBC. It is also faster than CCC algorithm. AAA algorithm can not accurately model the lateral scattering in tissue heterogeneity, but it can still give a reasonably close (within ±6%) prediction.

SU‐FF‐T‐102: Dosimetric Properties of Respiratory‐Gated Intensity Modulated Arc Treatments

Purpose: We investigated the dosimetric effects of gating on Intensity Modulated Arc Treatments. Method and Materials: One method to address respiratory motion during radiotherapy is 4DCT in conjunction with gated treatment. In the treatment of a solitary lung nodule or liver metastasis, intensity modulated arc therapy (IMAT) provides desired dose distributions. Here, we determined the dosimetric errors from delivering gated IMAT under a variety of conditions, including monitor units (MU), dose rate (MU/min), and direction of gantry rotation. We employed Varian Real Time Monitoring system for the gated delivery on a 2100C/D accelerator. To test the accuracy of the radiation delivery system, we used a static solid water phantom (IMRT phantom, MedTec). We chose a lateral 90° arc to stress test the gantry rotation delivery against gravity. Film dosimetry and point dose were performed. We delivered 30MU and 270MU treatments and used Kodak XV and EDR2 films respectively. RIT software was used to perform film dosimetry and analyses. Point doses at the isocentre were measured using a Scanditronix RK ion chamber (0.12cc). Dose distributions from gated deliveries are compared to non‐gated deliveries, normalized by the dose at isocenter. Amplitude gating was used with a 25% duty cycle. Results: With gating, we measured ≤1.5% change in dose at the isocentre. From film dosimetry, we observed beam transverse asymmetry in the gated delivery. Dose error < 3% was measured away from the isocentre using gated IMAT when the gantry is rotating at its maximum speed (295°/min). Errors less than 3% were measured when gantry rotates at or slower than 200°/min under clinical conditions. Conclusion: Gate IMAT treatments yield consistent output at the isocentre but accumulate beam symmetry errors. These errors are proportional to the gantry rotational speed which can be reduced to below 3% by lowering the delivery dose rate.

SU‐FF‐T‐142: Error Analysis of An Independent MU Calculation Program for IMRT Fields

Purpose: To evaluate the accuracy of an independent monitor unit calculation program for dose‐per‐MU verification of IMRT fields. The MU calculation program runs on a PC and is based on a scatter‐factor‐based empirical model to determine D/MU. This algorithm provides a head‐scatter model for IMRT fields. Method and Materials: The D/MU of IMRT fields at the same point (mostly the isocenter) was compared between treatment planning system and the independent MU calculation program. Here MU for an IMRT field refers to the total MU. The fractional MU of each segment of an IMRT field, defined as the MU for the segment per total MU, was assumed to be the same. The commercial treatment planning system is based on a pencil‐beam convolution algorithm with headscatter modeling. D/MU of a subset of IMRT fields, which is delivered to an IMRT phantom, was compared among the treatment planning system calculations, the independent MU calculations, and measurements. Results: Among 24 patients examined, the difference of D/MU for all IMRT fields delivered to a point agreed to within 7% for 91% (22/24) of patients, the same acceptance criteria using measurement, with the maximum error of 8.4%. This error is slightly worse then the measurement, where 100% of patients are within 7% limit. When each individual IMRT field is examined, 21% (33/160) of the fields have a discrepancy larger than 7%. Most of those fields (78.8%) lie in the valley region of an IMRT field. Conclusion: The independent MU calculation program can effectively verify the dose calculation accuracy for most IMRT fields. Only in few instances (9%), which invariably involves the calculation point to be in very low dose region, actual measurements are needed to further verify the accuracy of dose calculation for IMRT field.

SU‐FF‐T‐135: Complex IMRT Plan Verification Using a Commercial MU Calculation Package

Purpose: The general availability of IMRT treatments is constrained by the need to perform dosimetry measurements as part of the patient plan verification. While secondary IMRT MU calculators have been successfully used to validate some plans for a restricted number of sites, these calculators have suffered from unacceptably large uncertainties when applied to sites confined to the head and neck region. A strategy to reduce these calculational uncertainties has been developed which permits a greater use of IMRT MU calculators and reduces the need for dosimetric measurements, enabling a larger patient population to receive the benefits offered by IMRT treatments. Method and Materials: Segmental IMRT head and neck treatments were developed using the Pinnacle 6.2b inverse planning module. The IMRT treatment plans where then calculated on a CT image set of PMH IMRT phantom, and validation points corresponding to key target and avoidance regions where identified. The dosimetric data corresponding to these points was exported to RadCalc, a commercial IMRT MU calculator, and the calculations were compared to Pinnacle calculations and in‐phantom measurements. Results: A total of 10 clinical patient cases, each containing 7 to 9 gantry angles, were assessed. Agreement was assessed on basis of total dose delivered to the point of calculation. The measured and RadCalc calculated doses were found to agree within 3% at the high dose point and 5% at the low dose point in all cases, while the Pinnacle calculated dose was found to agree with the measured dose within 2.5% at the high dose point, with deviations as large as 13.5% observed at the low dose point. Conclusion: In‐phantom IMRT verification calculations of the total dose yields similar results as in‐phantom measurements. Consequently, a secondary MU calculator can be used to verify IMRT treatment plans and reduce the frequency of validation measurements.

SU‐FF‐T‐404: Fractionated Radiosurgery by Adaptation of Resources

Purpose: Our goal was to integrate broadly accessible, multi‐purpose tools to form a system for non‐invasive MLC‐based fractionated radiosurgery. We investigated the use of a Varian Millennium MLC (mMLC, 5mm‐leaf resolution) in conjunction with an aSi electronic portal imaging device (EPID) and Varian ZMed RadioCamera system for this purpose. Method and Materials: Pinpoint and RK chambers were used to measure data from small fields (mMLC shaped). Profiles for field sizes ranging from 4×4 to 1×1‐cm were measured in water, while output measurements were taken in a MedTec IMRT phantom. These data were used to commission an mMLC radiosurgery (fRS) beam in Theraplan Plus v3.8. Dose distributions created with this beam were compared both to measurement and circular collimator‐based SRS distributions. The RadioCamera system consists of an infrared camera and customized biteblock equipped with infrared markers. This detects motion in real time and allows the beam to be halted when a threshold is reached. The radio‐opaque markers are also used as a fiducial system for EPID. The motion sensitivity threshold and consistency of the biteblock‐mounted fiducials were both examined. Finally, the full system was tested using a film phantom outfitted with a biteblock. Results: Planning system predictions using the fRS beam match measured profiles, depth doses and output factors. Plans created with this beam were shown to approach those created using radiosurgery cones, based on DVH, homogeneity and conformity analyses. The IR camera system is sensitive to 0.2mm motion in all directions. Overall system test indicated that we can treat 1cm lesions with 0.3cm PTV and MLC margins. Conclusion: Our studies indicate that the combination of mMLC, infrared motion tracking, and EPID verification is sufficient to form a practical system for non‐invasive, fractionated radiosurgery, comparable to conventional linac‐based radiosurgery. This system is being used clinically at our institution.

Does the dose protraction inherent to IMRT decreases tumor cell killing in vitro

To determine whether protracted dose delivery inherent to IMRT treatments results in decreased tumor cell killing. IMRT aims to improve the therapeutic ratio of conventional radiotherapy by optimizing the delivery of ionizing radiation (IR) using a multileaf-collimator (MLC), tailoring radiation fluence to patient treatment fields. In comparison to 3D-CRT, IMRT treatment times are prolonged (10–40 minutes) depending on the hardware, software, and planning systems employed. Previously reported data predict a decrease in the biologically effective dose (BED) of IMRT due to such factors as decreased dose rate (5cGy - 100cGy/minute) when compared with 3D-CRT (200–800cGy/min) and partial shielding of the target volume during IMRT. Herein we describe an in vitro system that allows delivery of an IMRT head and neck plans from patients using SQ-20b human tumor cells to compare clonogenic survival after radiotherapy delivered via conventional or IMRT plans. From our data, a predictive model of tumor cell death was designed. An IMRT phantom was designed to mimic a previously treated patient with head and neck cancer. The phantom was constructed to accommodate a 150mm tissue culture flask (T-150) which was located so as to act as a surrogate for the patients gross tumor volume (GTV). 24 hours prior to the first fractionated treatment (Fx), SQ-20B tumor cells (D0 = 2.3 Gy) were plated at varying densities in T-150 flasks which were filled with media to approximately 80% capacity. 7-Field IMRT plans were designed that varied only in lead movement delay to vary treatment times from 10–40 minutes. Opposed lateral plans (OLp) with similar DVHs were designed and were delivered at 400cGy/min. Treatments to the cells were delivered in 180cGy daily Fx for a total of 5 Fx with both the IMRT and OLp for comparison purposes. TLDs verified that all plans delivered dose to within a 5% error. Seven days after the last fractionated treatment, crystal violet stained colonies >100 cells were scored as viable. At least 1200 colonies were collected per time point. Initial in-vitro survival data demonstrates that survival of tumor cells was increased 30% (p < 0.02 Mann-Whitney Rank Sum Test) with IMRT treatments of 40 minutes (41% survival) compared with opposed lateral treatment (33%) with little change when treatments were reduced to 10 minutes. The linear-quadratic equation was modified to account for repair of sublethal damage between the delivery of individual fields: S(D) = exp[-n(αd+βG(t)d2)+γT] where, α,β characterize cell radiosensitivity, γ - effective repopulation rate, T-duration of treatment, G(t) -dose protraction factor, n - # of fractions (5). Using the model, prolonged treatments (>20 minutes) decrease cell killing by 1–3% /Fx. When coupled with Fx therapy, a 50–140% (α/β dependent) decrease in cell kill is observed (Table). IMRT plans that protract treatment times beyond 10–15 minutes significantly decrease tumor cell killing in Fx therapy. Protracted dose delivery coupled with shielding of some areas of the tumor from dose during the treatment may account for these experimental observations. Current studies are attempting to optimize the gantry angles and fields utilized in order to maximize tumor cell killing.

Evaluation of IMRT for institutions participating in NCI sponsored clinical trials

Materials/Methods The phantoms consist of a plastic outer shell and internal structures to represent anatomy. A thorax phantom contains lung-equivalent regions, one of which contains a GTV. A pelvic phantom contains structures representing the prostate, rectum, and bladder. A third IMRT phantom mimics the head and neck region and contains structures representing a planning target volume (PTV) close to an organ at risk (OAR), simulating an orophanryngeal tumor and the spinal cord. The phantom contains a secondary PTV that simulates peripheral nodes. In each phantom, thermoluminescence dosimeters (TLDs) were embedded in each target volume and several of the OARs, and GafChromic® film was inserted in two or three planes through the PTVs. Institutions were instructed to fill the shell with water on-site, image the phantom, plan a treatment according to RPC guidelines and irradiate the phantom. The institutions were also asked to follow the QA procedures that would be done for a patient. Upon completion the institutions returned the phantom to the RPC. The TLD and film were evaluated by the RPC. Conclusion Failures occurred in irradiations delivered by a variety of models of linear accelerator and planned with several treatment planning systems (TPS). Somewhat consistent behavior was seen among the TPSs, although no trends were apparent among the delivery devices. The phantom was valuable for evaluating IMRT treatments at institutions preparing to participate in advanced technology clinical trials.

Investigation of the use of MOSFET for clinical IMRT dosimetric verification

(Received 22 October 2001; accepted for publication 26 March 2002; published 22 May 2002) With advanced conformal radiotherapy using intensity modulated beams, it is important to have radiation dose verification measurements prior to treatment. Metal oxide semiconductor field effect transistors (MOSFET) have the advantage of a faster and simpler reading procedure compared to thermoluminescent dosimeters (TLD), and with the commercial MOSFET system, multiple detectors can be used simultaneously. In addition, the small size of the detector could be advantageous, especially for point dose measurements in small homogeneous dose regions. To evaluate the feasibility of MOSFET for routine IMRT dosimetry, a comprehensive set of experiments has been conducted, to investigate the stability, linearity, energy, and angular dependence. For a period of two weeks, under a standard measurement setup, the measured dose standard deviation using the MOSFETs was +/- 0.015 Gy with the mean dose being 1.00 Gy. For a measured dose range of 0.3 Gy to 4.2 Gy, the MOSFETs present a linear response, with a linearity coefficient of 0.998. Under a 10 x 10 cm2 square field, the dose variations measured by the MOSFETs for every 10 degrees from 0 to 180 degrees is +/- 2.5%. The percent depth dose (PDD) measurements were used to verify the energy dependence. The measured PDD using the MOSFETs from 0.5 cm to 34 cm depth agreed to within +/- 3% when compared to that of the ionization chamber. For IMRT dose verification, two special phantoms were designed. One is a solid water slab with 81 possible MOSFET placement holes, and another is a cylindrical phantom with 48 placement holes. For each IMRT phantom verification, an ionization chamber and 3 to 5 MOSFETs were used to measure multiple point doses at different locations. Preliminary results show that the agreement between dose measured by MOSFET and that calculated by Corvus is within 5% error, while the agreement between ionization chamber measurement and the calculation is within 3% error. In conclusion, MOSFET detectors are suitable for routine IMRT dose verification.