Resulting Dose Distributions Research Articles

SU-GG-T-98: An Exploration of the Use of Custom Compensators for Modulated Electron Radiotherapy

Purpose: To explore the use of custom compensators for modulated electron radiotherapy (MERT). Method and Materials: Several simple compensators, made of acrylic and paraffin, were placed in the 10×10 cm2 electron applicator of a Varian 2100C linear accelerator. Kodak EDR2 film was placed at various depths in a solid water phantom located at a SSD of 100 cm. Simulated treatments were delivered using either a single energy or multiple energies to demonstrate the effect of the compensator on the resulting dose distribution. Dose distributions for the compensator-shaped fields were obtained from film measurements and compared to distributions measured for open 10 × 10 cm2 fields. Results: The use of compensators with multiple electron energies resulted in complex dose distributions that cannot be formed easily with standard electron cutouts. A relatively smooth gradient was observed at locations where compensator thickness changed rapidly. It was observed that the compensator- shaped fields exhibited more gradual dose drop-off at the edges of the field. The penumbra for the compensator-shaped fields was somewhat larger than that for the open single-energy fields. Conclusion: The use of custom compensators for modulation of electron beam energy and intensity is a promising technique that may be of clinical use in MERT treatments.

SU-GG-T-63: Feasibility Study of Longitudinal Field Junctioning with Helical Tomotherapy

Purpose: To examine junctioning of longitudinally adjacent PTVs treated with helical tomotherapy (HT). Method and Materials: Cylindrical PTVs were defined in an elliptic cylindrical homogeneous phantom. Dose distributions (95% PTV to receive 2 Gy) created using 2.5 and 5.0 cm long HT fields were calculated and verified dosimetrically. Cranial — Caudal (CC) dose profiles were summed to study the junctioning of PTVs to create a single contiguous PTV. Junctioning adjacent PTVs with different inter‐PTV spacing, created by equal or different field sizes was studied for dose homogeneity. The use of dose stepped PTVs near the junction region was also examined. Here the SUP end of the INF PTV or the INF end of the SUP PTV was divided into smaller subPTVs of decreasing prescription dose. The resulting dose distributions were summed as a function of inter‐PTV spacing. Simulated dose profiles were verified by film dosimetry.Results: The most homogenous dose resulted when adjacent PTVs had the same CC dose profile (field size). Independent of the Inter‐PTV spacing, PTVs of different CC dose profiles could not produce homogeneous doses. Minimizing the volume dose excursion from prescription resulted in cold spots (−26%) and hot spots (+29%) with 8% of the PTV receiving < 95% of prescription. Dividing each PTV into four multiple contiguous subPTVs, with constantly decreasing prescribed dose (2, 1.5, 1.0, 0.5Gy) allowed PTV matching with dose homogeneity similar to junctioning PTVs of equal CC slope. 95% of the PTV received at least 101% of the prescribed dose, with dose excursions of −19% to +13% from prescription, (1% of the PTV received less than 95% of prescribed dose). Conclusion: Junctioning adjacent PTVs is possible, but PTVs created by different field widths present a challenge. Homogeneity is improved by breaking PTVs into multiple contiguous subPTVs modified to feather (broaden) the effective junctioning region.

MO-D-351-08: Analysis of the Effects of Multiple GEUD-Type Constraints On Dose Distribution for IMRT Optimization

Purpose: To analyze the effects of multiple gEUD‐type (convex) constraints on the resulting dose‐volume distribution/histogram (DVH). A key motivation for this work is to find a convex optimization alternative to (non‐convex) partial volume constraints in IMRT optimization. Method and Materials: A formal mathematical framework for analysis of the effects of multiple gEUD‐type constraints on the resulting DVH is proposed. The framework relies on interpreting DVH as a cumulative probability distribution of the underlying “random”, i.e., unknown variable, representing dose to a voxel. Consequently, the analysis of the effects of gEUD‐based constraints on DVH is rephrased in terms of the effects of moments of the random variable on its distribution, which corresponds to a well‐recognized moment problem in mathematics. Results: Given a set of gEUD‐type constraints, we demonstrate how to compute the worst —in a sense of generating the largest volume ratio receiving a fixed dose— dose‐volume distributions that satisfy these constraints. A generalization of gEUD‐based constraints, the Generalized Moment Constraints (GMC's), is proposed, with the rationale behind GMC's to provide more modeling flexibility for IMRT optimization. A potential applicability of the approach is discussed. Applicability analysis is based on proximity of a family of dose distributions, satisfying a fixed set of GMC's, to the desired “ideal” dose distribution. We use a hypothetical prostate cancer patient's rectum as an example. Conclusion: The newly proposed convex GMC‐based dose‐distribution modeling framework has a potential to serve as a viable alternative to partial volume constraints in IMRT optimization, at least for some critical structures. To name a few potential benefits of our approach, we mention global solution to IMRT optimization problem that minimizes proximity of a physically deliverable plan to the desired “ideal” physician‐prescribed plan, and better control over the resulting dose distributions. Further investigation of the approach is required and is ongoing.

MO-D-351-01: (Jack Fowler Junior Investigator Competition Winner) Optimization Tools for Modulated Electron Radiotherapy

Purpose: To devise optimization tools to plan Modulated Electron Radiotherapy (MERT) and to compare MERT plans to conventional or IMRT plans. Method and Materials: There are no commercially available treatment systems to plan or deliver MERT. Tertiary electron MLCs for MERT has been met with practical limitations. We've investigated use of the inherent photonMLCs. We optimized MERT plans for a left sided post‐mastectomy breast cancer patient. From CTimages, target organs such as chest wall, axillas, inter‐mammary nodes were contoured. The MERT plan was performed in a three‐step optimization process. Distances from the external contour‐to‐distal location of the PTV were calculated in transverse CTimages. Energies were selected for efficient target coverage and energy bins were generated. Optimization of the energy bins was accomplished using a custom MERT Planning graphic user interface (MERTgui). Monte Carlo simulations were then performed using different MLC instructions for each segment generated by the MERTgui. Dose distributions of each segment were imported to CERR. Finally we used a custom built dose superposition GUI to combine doses for each segment using different weights which yields their relative MUs. The resulting dose distribution was used to calculate DVHs using CERR and compared to the conventional plan. Results: The MERT plan resulted in good target organ coverage and less dose to the organs at risk than the conventional plan. The dose that 20% of the ipsilateral lung received was reduced from 45Gy (conventional) to 26Gy (MERT). Using MERT, 80% volume of both axilla and IM received the prescription dose without consequence of excess dosage as with the conventional plan. Contralateral breast dose decreased significantly to 1/10 the dose of the conventional plan. Conclusion: MERT is ideal for treatment of shallow targets such as post‐mastectomy chest wall. We have devised optimization tools to facilitate the MC based planning.

SU-GG-T-109: Four Dimensional Inverse Planning for Intensity Modulated Radiation Therapy

Purpose: This work develops 4D inverse planning methods and demonstrates the potential benefit of 4D IMRT. Method and Materials: Two 4D planning strategies are proposed and compared. The first one treats all respiration phases as a system and optimizes the dose delivery collectively in space and phase. The method is referred to as collective optimization of all phases (COAP). In this approach, a deformable model is employed to establish a voxel-to-voxel correspondence and the goal is to maximize the accumulative dose to the tumor target while minimizing the dose to the organ-at-risk (OARs). The second one treats each phase as an independent 3D inverse planning problem and optimizes them separately. The final dose distribution is obtained by summing the dose of each phase after a deformable image registration. This method is called separate optimization of each phase (SOEP). In both approaches, the dose is optimized with a linear programming technique. Results: The resultant dose distribution of COAP is markedly better than that of SOEP in both target dose coverage and organ-at-risk sparing. The improvement of COAP is resulted from reallocation of dose among the phases to cater for anatomical changes during the breathing process. It is found that, for a phase with favorable geometry for dose delivery, more doses are allocated by COAP, and vise versa. COAP optimally assigns dose for all the involved phases. Because of the lack of this degree of freedom, SOEP yields almost identical intensity maps and dose distributions for all the phases. Conclusion: Simultaneous spatio-temporal dose optimization in 4D inverse planning allows one to take consideration of the spatial variation of the patient anatomy caused by respiration and yields the optimal accumulative dose distribution.

SU-GG-T-15: Inverse Planning Optimization for Hybrid Brachytherapy Plans Using Multiple Seed Types

Purpose: The purpose of this work is to develop a brachytherapy inverse planning algorithm that can generate: 1) Hybrid plans with seeds of one isotope but of different activity levels; 2) Hybrid plans with seeds of two or more different isotopes. Method and Materials: A stand-alone research version of the inverse planning algorithm, IPSA, was modified to sample different isotopes at each source position at specific moments during the iteration process. The user specifies the isotope sources desired using the TG43 formalism. The algorithm probes this space and evaluates the resulting dose distribution, compares it to the best distribution yet attained, keeps or rejects it, and finally continues to the next iteration. To test the program, plans for prostate volumes (19, 28, 35, 44, and 48 cc) were generated with a single activity and with different combinations of two activities (e.g. 0.25 and 0.4 mCi). Dosimetric indices were compared. Results: We have developed an inverse planning tool that allows the user to incorporate multiple isotopes into any brachytherapy plan. The optimization time is approximately 20% longer than for the standard optimization due to a larger parameter space, but still well under one minute. For all three prostate implants considered, the target volume receiving 100% of the prescribed dose was within 2% for every plan and always above the clinically acceptable 90%. Also the urethra volume receiving 120% of the prescribed dose was always below the clinically acceptable value of 1 cc. Conclusion: An inverse planning algorithm was developed as an extension to IPSA with the ability to incorporate multiple activities or isotopes into the permanent implant optimization process. Studies of the hybrid plans using two activities are shown here; studies involving two separate isotopes are under progress to determine the clinical potential. Conflict of Interest: Work supported by Nucletron.

Quantification of interplay effects of scanned particle beams and moving targets

Scanned particle beams and target motion interfere. This interplay leads to deterioration of the dose distribution. Experiments and a treatment planning study were performed to investigate interplay. Experiments were performed with moving radiographic films for different motion parameters. Resulting dose distributions were analyzed for homogeneity and dose coverage. The treatment planning study was based on the time-resolved computed tomography (4DCT) data of five lung tumor patients. Treatment plans with margins to account for respiratory motion were optimized, and resulting dose distributions for 108 different motion parameters for each patient were calculated. Data analysis for a single fraction was based on dose–volume histograms and the volume covered with 95% of the planned dose. Interplay deteriorated dose conformity and homogeneity (1-standard deviation/mean) in the experiments as well as in the treatment-planning study. The homogeneity on radiographic films was below ≈80% for motion amplitudes of ≈15 mm. For the treatment-planning study based on patient data, the target volume receiving at least 95% of the prescribed dose was on average (standard deviation) 71.0% (14.2%). Interplay of scanned particle beams and moving targets has severe impact on the resulting dose distributions. Fractionated treatment delivery potentially mitigates at least parts of these interplay effects. However, especially for small fraction numbers, e.g. hypo-fractionation, treatment of moving targets with scanned particle beams requires motion mitigation techniques such as rescanning, gating, or tracking.

Characterisation of mega-voltage electron pencil beam dose distributions: viability of a measurement-based approach

The concept of electron pencil-beam dose distributions is central to pencil-beam algorithms used in electron beam radiotherapy treatment planning. The Hogstrom algorithm, which is a common algorithm for electron treatment planning, models large electron field dose distributions by the superposition of a series of pencil beam dose distributions. This means that the accurate characterisation of an electron pencil beam is essential for the accuracy of the dose algorithm. The aim of this study was to evaluate a measurement based approach for obtaining electron pencil-beam dose distributions. The primary incentive for the study was the accurate calculation of dose distributions for narrow fields as traditional electron algorithms are generally inaccurate for such geometries. Kodak X-Omat radiographic film was used in a solid water phantom to measure the dose distribution of circular 12 MeV beams from a Varian 21EX linear accelerator. Measurements were made for beams of diameter, 1.5, 2, 4, 8, 16 and 32 mm. A blocked-field technique was used to subtract photon contamination in the beam. The "error function" derived from Fermi-Eyges Multiple Coulomb Scattering (MCS) theory for corresponding square fields was used to fit resulting dose distributions so that extrapolation down to a pencil beam distribution could be made. The Monte Carlo codes, BEAM and EGSnrc were used to simulate the experimental arrangement. The 8 mm beam dose distribution was also measured with TLD-100 microcubes. Agreement between film, TLD and Monte Carlo simulation results were found to be consistent with the spatial resolution used. The study has shown that it is possible to extrapolate narrow electron beam dose distributions down to a pencil beam dose distribution using the error function. However, due to experimental uncertainties and measurement difficulties, Monte Carlo is recommended as the method of choice for characterising electron pencil-beam dose distributions.

Optimization of dose distribution with multi-leaf collimator using field-in-field technique for parallel opposing tangential beams of breast cancers

3 Dimensional Conformal Radiotherapy (3D-CRT) planning software helps in displaying the 3D dose distribution at different levels in the planned target volume (PTV). Physical or dynamic wedges are commonly applied to obtain homogeneous dose distribution in the PTV. Despite all these planning efforts, there are about 10% increased dose hot spots encountered in final plans. To overcome the effect of formation of hot spots, a manual forward planning method has been used. In this method, two more beams with multi-leaf collimator (MLC) of different weights are added in addition to medial and lateral wedged tangent beams. Fifteen patient treatment plans were taken up to check and compare the validity of using additional MLC fields to achieve better homogeneity in dose distributions. The resultant dose distributions with and without presence of MLC were compared objectively. The dose volume histogram (DVH) of each plan for the PTV was evaluated. The 3D dose distributions and homogeneity index (HI) values were compared. The 3D dose maximum values were reduced by 4% to 7%, and hot spots assumed point size. Optimizations of 3D-CRT plans with MLC fields improved the homogeneity and conformability of dose distribution in the PTV. This paper outlines a method of obtaining optimal 3D dose distribution within the PTV in the 3D-CRT planning of breast cases.

Optimal treatment margins for radiotherapy of prostate cancer based on interfraction imaging

Purpose. To present a methodology to estimate optimal treatment margins for radiotherapy of prostate cancer based on interfraction imaging. Materials and methods. Cone beam CT images of a prostate cancer patient undergoing fractionated radiotherapy were acquired at all treatment sessions. The clinical target volume (CTV) and organs at risk (OARs; bladder and rectum) were delineated in the images. Random sampling from the CTV-OAR library was performed in order to simulate fractionated radiotherapy including intra- and interpatient variability in setup and organ motion/deformation. For each simulated patient, four treatment fields defined by multileaf collimators were automatically generated around the planning CTV. The treatment margin (the distance from the CTV to the field border) was varied between 2.5 and 20mm. Resulting dose distributions were calculated by a convolution method. Doses to OARs were reconstructed by polynomial warping, while the CTV was assumed to be a rigid body. The equivalent uniform dose (EUD), the tumor control probability (TCP) and the normal tissue complication probability (NTCP) were used to estimate the clinical effect. Patient repositioning strategies at treatment were compared. Results. The simulations produced population based EUD histograms for the CTV and the OARs. The number of patients receiving an optimal target EUD increased with increasing margins, but at the cost of an increasing number receiving a high EUD to the OARs. Calculations of the probability of complication-free tumor control and subsequent analysis gave an optimal treatment margin of about 10mm for the simulated population, if no correction strategy was undertaken. Conclusions. The current work illustrates the principle of optimal treatment margins based on interfraction imaging. Clinically applicable margins may be obtained if a large patient image database is available.

Impact of MLC leaf position errors on simple and complex IMRT plans for head and neck cancer**Presented at the 48th Annual Meeting of the American Association of Physicists in Medicine, 30 July 2006.

The dosimetric impact of random and systematic multi-leaf collimator (MLC) leaf position errors is relatively unknown for head and neck intensity-modulated radiotherapy (IMRT) patients. In this report we studied 17 head and neck IMRT patients, including 12 treated with simple plans (<50 segments) and 5 treated with complex plans (>100 segments). Random errors (−2 to +2 mm) and systematic errors (±0.5 mm and ±1 mm) in MLC leaf positions were introduced into the clinical plans and the resultant dose distributions were analyzed based on defined endpoint doses. The dosimetric effect was insignificant for random MLC leaf position errors up to 2 mm for both simple and complex plans. However, for systematic MLC leaf position errors, we found significant dosimetric differences between the simple and complex IMRT plans. For 1 mm systematic error, the average changes in D95% were 4% in simple plans versus 8% in complex plans. The average changes in D0.1 cc of the spinal cord and brain stem were 4% in simple plans versus 12% in complex plans. The average changes in parotid glands were 9% in simple plans versus 13% for the complex plans. Overall, simple IMRT plans are less sensitive to leaf position errors than complex IMRT plans.

Comparison of direct machine parameter optimization versus fluence optimization with sequential sequencing in IMRT of hypopharyngeal carcinoma

BackgroundTo evaluate the effects of direct machine parameter optimization in the treatment planning of intensity-modulated radiation therapy (IMRT) for hypopharyngeal cancer as compared to subsequent leaf sequencing in Oncentra Masterplan v1.5.MethodsFor 10 hypopharyngeal cancer patients IMRT plans were generated in Oncentra Masterplan v1.5 (Nucletron BV, Veenendal, the Netherlands) for a Siemens Primus linear accelerator.For optimization the dose volume objectives (DVO) for the planning target volume (PTV) were set to 53 Gy minimum dose and 59 Gy maximum dose, in order to reach a dose of 56 Gy to the average of the PTV. For the parotids a median dose of 22 Gy was allowed and for the spinal cord a maximum dose of 35 Gy. The maximum DVO to the external contour of the patient was set to 59 Gy. The treatment plans were optimized with the direct machine parameter optimization ("Direct Step & Shoot", DSS, Raysearch Laboratories, Sweden) newly implemented in Masterplan v1.5 and the fluence modulation technique ("Intensity Modulation", IM) which was available in previous versions of Masterplan already. The two techniques were compared with regard to compliance to the DVO, plan quality, and number of monitor units (MU) required per fraction dose.ResultsThe plans optimized with the DSS technique met the DVO for the PTV significantly better than the plans optimized with IM (p = 0.007 for the min DVO and p < 0.0005 for the max DVO). No significant difference could be observed for compliance to the DVO for the organs at risk (OAR) (p > 0.05). Plan quality, target coverage and dose homogeneity inside the PTV were superior for the plans optimized with DSS for similar dose to the spinal cord and lower dose to the normal tissue. The mean dose to the parotids was lower for the plans optimized with IM. Treatment plan efficiency was higher for the DSS plans with (901 ± 160) MU compared to (1151 ± 157) MU for IM (p-value < 0.05).Renormalization of the IM plans to the mean of the dose to 95% of the PTV (D95) of the DSS plans, resulted in similar target coverage and dose to the parotids for both strategies, at the cost of a significantly higher dose to the normal tissue and maximum dose to the target. The relative volume of the PTV receiving 107% or more of the prescription dose V107 increased to 35.5% ± 20.0% for the IM plan as compared to a mean of 0.9% ± 0.9% for the DSS plan.ConclusionThe direct machine parameter optimization is a major improvement compared to the fluence modulation with subsequent leaf sequencing in Oncentra Masterplan v1.5. The resulting dose distribution complies better with the DVO and better plan quality is achieved for identical specification of DVO. An additional asset is the reduced number of MU as compared to IM.

Field‐size dependence of doses of therapeutic carbon beams

To estimate the physical dose at the center of spread-out Bragg peaks (SOBP) for various conditions of the irradiation system, a semiempirical approach was applied. The dose at the center of the SOBP depends on the field size because of large-angle scattering particles in the water phantom. For a small field of 5 x 5 cm2, the dose was reduced to 99.2%, 97.5%, and 96.5% of the dose used for the open field in the case of 290, 350, and 400 MeV/n carbon beams, respectively. Based on the three-Gaussian form of the lateral dose distributions of the carbon pencil beam, which has previously been shown to be effective for describing scattered carbon beams, we reconstructed the dose distributions of the SOBP beam. The reconstructed lateral dose distribution reproduced the measured lateral dose distributions very well. The field-size dependencies calculated using the reconstructed lateral dose distribution of the therapeutic carbon beam agreed with the measured dose dependency very well. The reconstructed beam was also used for irregularly shaped fields. The resultant dose distribution agreed with the measured dose distribution. The reconstructed beams were found to be applicable to the treatment-planning system.

Improved stratification algorithms for step-and-shoot MLC delivery in intensity-modulated radiation therapy

In inverse planning for intensity-modulated radiotherapy (IMRT), the fluence distribution of each treatment beam is usually calculated in an optimization process. The delivery of the resulting treatment plan using multileaf collimators (MLCs) is performed either in the step-and-shoot or sliding window technique. For step-and-shoot delivery, the arbitrary beam fluence distributions have to be transformed into an applicable sequence of subsegments. In a stratification step the complexity of the fluence maps is reduced by assigning each beamlet to discrete intensity values, followed by the sequencing step that generates the subsegments. In this work, we concentrate on the stratification for step-and-shoot delivery. Different concepts of stratification are formally introduced. In addition to already used strategies that minimize the difference between original and stratified beam intensities, we propose an original stratification principle that minimizes the error of the resulting dose distribution. It could be shown that for a comparable total number of subsegments the dose-oriented stratification results in a better approximation of the original, unsequenced plan. The presented algorithm can replace the stratification routine in existing sequencer programs and can also be applied to interpolated plans that are generated in an interactive decision making process of multicriteria inverse planning programs.

Does Intensity Modulation Improve Healthy Tissue Sparing in Stereotactic Radiosurgery of Complex Arteriovenous Malformations?

This planning study evaluates the potential of intensity modulated treatment fields and inverse planning techniques in stereotactic radiosurgery to reduce healthy tissue dose. Twenty patients previously treated with stereotactic radiosurgery for arteriovenous malformation (AVM) were replanned with each of 4 techniques: circular non-coplanar arcs, dynamic arcs, static conformal fields, and intensity modulated radiosurgery (IMRS). Patients were selected having a maximum AVM dimension at least 20 mm, or volume greater than 10 cm 3. Target volumes ranged from 2.12 cm 3 to 13.87 cm 3 with a median of 6.03 cm 3. Resulting dose distributions show a statistically significant improvement in target conformality between circular arcs and all other techniques ( p ≤ 0.001), between conformal and both dynamic arcs and IMRS ( p ≤ 0.03) and with no difference between dynamic arcs and IMRS. However, for AVMs of volume greater than 5.5 cm 3, IMRS gives better conformality than dynamic arcs ( p = 0.04). IMRS showed consistently lower dose inhomogeneity compared to both dynamic arcs and conformal fields ( p < 0.001). At low dose levels, the dynamic arc technique irradiates less healthy tissue than the other techniques ( p ≤ 0.001). Both dynamic arcs and IMRS provide increased ability to conform to the AVM, with IMRS showing greater ability to control dose at the periphery.

SU-FF-T-160: DMLC IMRT to Moving Tissues - Towards 4D Therapy

Purpose. The goal of traditional approaches to IMRTdelivery to moving tisues (gating, target outline tracking, IMRTtumor tracking) is to irradiate patient body so that effects of tisuue motion are eliminated and IMRT therapies in the presence of tissue motion are delivered so that the IMRT dose distributions for moving bodies are equivalent to the dose distribution delivered to the static body geometry. In contrast the proposed solution for IMRTdelivery to moving tissues and organs aims at delivering treatments that are dosimetrically superior relative to any IMRT therapy possible for stationary body geometry. Methods. Treatment plans (3D) are derived for each phase of the (static) body geometry. This is done with existing planning software in comercial TPS and for body geometries defined by data scorred in 4D CT. Using the freedom of multiple solutions of DMLC IMRTdelivery of given intensity map to moving target the exploitation of leaf trajectories and dose rate intensities are investigated (for each phase plan) to find the optimized treatment deliveries that are more advantageous than 3D IMRT plans derived for each phase of the static body geometry. The best deliverable plan among all derived for each of the motion phase is chosen as the one to be realized. The resulting intensity delivered by these strategies, and resulting dose distributions, are calculated and compared to best plans derived for each phase plans. Results. The example of treatment derived for particular phase of body motion geometry has been investigated. The DMLC IMRTdelivery of given dose to target that simultaneously minimizes the dose delivered to organ at risk due to interaction between motion of leaves and motion of the target is calculated that shows the dosimetric advantage over 3D IMRT therapy.

SU-FF-T-163: Dose Calculation On Cone Beam CT (CBCT)

Purpose:CBCT provides useful information for image guidance, however, uncertainty in CT number prevents it from being used for accuracy dose calculation. The purpose of this study is to quantify the accuracy of dose calculation using different CT to density mapping methods and investigate achievable accuracy when applying these methods for dose calculation based on CBCT.Method and Materials: The heterogeneous dose calculated on helical CT (HCT) is used as reference, and the resulting dose from other three methods are compared to reference. In homogeneous dose calculation (HO) method, the density value inside skin contour is overwritten as 1g/cm3. In air‐bone‐soft tissue (ABS) method, the source CTimage were replaced by three regions, air, bone, and soft tissue with density overwritten as 0.1 g/cm3, 1.4 g/cm3, and 1 g/cm3 respectively. In stepwise CT density (SWD) method, a CT‐density table representing a stepwise function is applied to CT and used in dose calculation. The three methods are applied to both HCT and CBCT on Pinnacle3 planning system. The resulting dose distributions are compared. Results: Head and neck patients are selected for study. For dose calculation on HCT, the maximum discrepancies in mean dose for HO, ABS, and SWD are within 4%, 2%, and 0.3% for critical organs respectively, and within 1%, 0.5%, and 0.5% for targets. The accuracy results for ABS can be extended to CBCT. For dose calculation on CBCT, dose distribution from ABS is used as reference. The maximum discrepancy between HO and ABS is 2% for critical organs and 0.5% for targets; the maximum discrepancy between SWD and ABS is %1 for critical organs, and 0.5% for targets. Conclusions: Both ABS and SWD can achieve 2% accuracy in dose calculation on CBCT. No contour information is required for SWD, however, normalization of CT number may be necessary.

SU‐FF‐T‐195: Effect of Respiratory Gating On the Dose Distributions of Dynamic Wedge and IMRT Treatment Fields

Purpose: To determine the magnitude of error introduced into treatment delivery due to timing effects between respiratory‐gated delivery and dynamic beam delivery (enhanced dynamic wedge (EDW) and step‐and‐shoot intensity modulated radiation therapy (IMRT)). Method and Materials: EDW and IMRT fields were delivered on a linear accelerator (Trilogy, Varian Medical Systems). Gating of the beam was achieved using a commercial respiratory monitoring system (RPM, Varian Medical Systems), and respiratory motion was simulated using a commercial respiratory motion phantom (RPM phantom, Varian Medical Systems). All fields were delivered with no gating, a 1‐mm gating window, and a 2‐mm gating window.Film and ion chamber measurements were made for both EDW and IMRT fields in a water‐equivalent IMRT QA phantom, which was stationary during all measurements. Film measurements used EDR‐2 film, while ion chamber measurements were made with a 0.04 cc volume pin‐point chamber. Leakage current was subtracted from the ion chamber readings.Wedged fields of 15 and 60 degrees were delivered at a dose rate of 600 MU/min. Results were evaluated for the full IMRT plan and for one of the individual beams, at dose rates of 400 and 600 MU/min. Results: All ion chamber measurements were analyzed as percent difference between the gated (1‐mm or 2‐mm gating window) and non‐gated delivery. All differences were less than 1%. There were no significant differences between 1‐mm and 2‐mm gated delivery. Film results, as analyzed from isodose curves and the gamma metric, showed no substantial differences between gated and non‐gated beam delivery. Conclusion: The use of gated treatment delivery of EDW and step‐and‐shoot IMRT fields does not introduce clinically significant differences into the resulting dose distributions, as quantified by film and ion chamber measurements.

WE‐C‐BRA‐02: Parameters for and Use of NTCP Models in the Clinic

Delivery of adequate tumor dose without causing excessive normal tissue complications is the driving principle of modern radiation therapy. Resulting dose distributions in normal tissues are very different from the “partial organ irradiation” distributions that characterize the simple beam arrangements of earlier days. Further, the growing popularity of hypofractionation drastically widens the range of biological effective doses within organs at risk (OAR). The clinical physicist is faced with uncertainty as to what aspects of an OAR dose distribution require special consideration in treatment plan design and evaluation.Normal tissue complication probability (NTCP) models are one way to account for the full dose distribution. For most serious toxicities, statistical models from various outcomes analyses help direct the planner toward dose‐volume limits. There are also several semi‐mechanistic models, each with a set of parameters that must be adjusted to describe existing data. But for conditions that differ greatly from those under which they were ‘commissioned’, two models for the same endpoint and starting from the same input dose distribution do not necessarily predict the same NTCP. A working group, with joint participation from AAPM, ASTRO and RTOG, is being established to, among other things, reconcile model differences and provide clinical guidelines in the near future.In this presentation, common NTCP models for several major dose‐limiting toxicities will be described, including parameters sets gleaned from literature review. Problems and pitfalls of integrating and interpreting diverse studies and applying them to an individual clinic's practice will be discussed and real‐world examples of application to clinical decisions will be presented.Educational Objectives:1. Understand the important features of the most widely‐used NTCP models.2. Understand how the model parameters affect predictions of normal tissue dose‐volume responses.3. Understand some of the complexities of implementing these models into routine clinical practice.

WE‐C‐AUD‐08: Sensitivity Analysis of a Non‐Cylindrically Symmetric Monte Carlo Beam Model of a Siemens Primus Accelerator

Purpose: To facilitate the development and commissioning of radiotherapy accelerator beam models by studying the impact of the variation of geometric and electron beam parameters on photon dose and fluence output for a Siemens Primus accelerator. In particular, the impact of lateral offsets of linac components and lateral and angular offsets of the electron beam were examined. Method and Materials: We have performed a sensitivity study examining the effect of varying beam and geometric parameters of a Monte Carlo model of a Siemens Primus accelerator. The accelerator and dose output were modeled using BEAMnrc and DOSXYZnrc, respectively. The BEAMnrc package was modified to allow for linear offsets of individual accelerator components, and lateral and angular offsets of the incident electron beam. Dose distributions were studied for 40 × 40 cm2 fields. Flatness, symmetry, OAR, and profile slope were studied. Results: The electron beam parameters having the greatest effect on the resulting dose distributions were found to be electron energy and angle of incidence, as high as 5% for a 0.25° deflection, with lateral offset of the electron beam and spot size having a smaller impact. Variations in target thickness were found to have a lesser effect than found in previous studies for electron beams. Small (∼2 mm) lateral offsets of the flattening filter altered the OAR by 5% for 6 MV and as much as 25% for 18 MV. Conclusion: Using large fields allows treatment head details to be more easily extracted. Lateral and angular offsets of beam and accelerator components have strong effects on dose distributions, and need to be included in any high‐accuracy beam model. Having a table cataloging the impact of model parameters on dose output can greatly facilitate beam model commissioning.Support from NIH R01 CA104777‐01A2.