Radiological Depth Research Articles

Pretreatment verification of dose calculation and delivery by means of measurements with PLEXITOM™ phantom

AimTo validate a pretreatment verification method of dose calculation and dose delivery based on measurements with Metaplex PTW phantom. BackgroundThe dose-response relationships for local tumor control and radiosensitive tissue complications are strong. It is widely accepted that an accuracy of dose delivery of about 3.5% (one standard deviation) is required in modern radiotherapy. This goal is difficult to achieve. This paper describes our experience with the control of dose delivery and calculations at the ICRU reference point. Materials and methodsThe calculations of dose at the ICRU reference point performed with the treatment planning system CMS XiO were checked by measurements carried out in the PLEXITOM™ phantom.All measurements were performed with the ion chamber positioned in the phantom, at the central axis of the beam, at depth equivalent to the radiological depth (at gantry zero position). The source-to-phantom surface distance was always set to keep the source-to-detector distance equal to the reference point depth defined in the ICRU Report 50 (generally, 100cm). The dose was measured according to IAEA TRS 398 report for measurements in solid phantoms. The measurement results were corrected with the actual accelerator's output factor and for the non-full scatter conditions. Measurements were made for 111 patients and 327 fields. ResultsThe average differences between measurements and calculations were 0.03% (SD=1.4%), 0.3% (SD=1.0%), 0.1% (SD=1.1%), 0.6% (SD=1.8%), 0.3% (SD=1.5%) for all measurements, for total dose, for pelvis, thorax and H&N patients, respectively. Only in 15 cases (4.6%), the difference between the measured and the calculated dose was greater than 3%. For these fields, a detailed analysis was undertaken. ConclusionThe verification method provides an instantaneous verification of dose calculations before the beginning of a patient's treatment. It allows to detect differences smaller than 3.5%.

Total body irradiation dose optimization based on radiological depth

We have previously demonstrated the use of Eclipse fluence optimization to define aperture sizes for a novel aperture modulated translating bed total body irradiation (TBI) technique. The purposes of the present study were to identify, characterize, and correct for sources of error inherent in our previous fluence optimization technique, and to develop a clinically viable fluence optimization module for the translating bed TBI technique. Aperture modulated TBI is delivered by translating the patient at constant speed on a custom bed under a modulated radiation beam. The patient is then turned from supine to prone and the process repeated, resulting in an AP‐PA treatment. Radiological depths were calculated along divergent ray lines through individual CT slices of a RANDO phantom. Beam apertures, defined using a dynamic multileaf collimator (DMLC), were generated using calculated radiological depths and calibration factors that relate fluence to aperture size in a dynamic environment. These apertures were defined every 9 mm along the phantom superior‐inferior axis. The calculated beam apertures were further modified to account for scatter within the patient. For dose calculation purposes the individual MLC files were imported into Eclipse. For treatment delivery, dynamic MLC files for both AP and PA beams were generated and delivered dynamically. Dose homogeneity in the head and neck region of the RANDO phantom was within ± 4% of the prescribed dose with this novel technique compared to −5% to +7% with our previous aperture modulated technique based on Eclipse fluence optimization. Fluence optimization and beam aperture calculation using the new technique offers a ten‐fold reduction in planning time and significantly reduces the likelihood of user error during the planning process. In conclusion, a clinically viable aperture modulated translating bed TBI technique that employs dynamically shaped MLC‐defined beam apertures based on radiological depth calculations, has been developed.PACS numbers: 87.55.‐x, 87.55.D‐

Effect of soft tissue thickness over the posterior border of the vertebral body and disc space on cage placement during posterior lumbar interbody fusion: A cadaveric study

In the present study, we investigated whether there is a difference between visual depth (VD) and radiological image depth (RD) of cages (i.e., structural interbody support devices) placed in disc spaces during posterior lumbar interbody fusion and whether soft tissues covering the posterior border of the vertebral body and associated disc space are the cause of any observed differences. Using digital calipers, cages were inserted at a depth of 5 mm from the soft tissues covering the posterior border of the vertebral body and disc space under direct vision; this depth was defined as VD. After insertion, RD was measured in triplicate. The reliability of RD measurements was evaluated using an intraclass coefficient test. To identify the cause of differences between VD and RD, the thicknesses of soft tissues were measured microscopically. A total of 40 lumbar intervertebral disc spaces with cages were evaluated. The mean RD of cages was 3.12 mm, while the mean difference between the VD and RD of cages (DVRD) was 1.91 mm. On histological examination, the mean thickness of the soft tissue was 2.02 mm. Comparative analysis between histological values and DVRD showed no statistical difference (P = 1.14, 1.55, 0.06). There was a significant difference between VD and RD during cage placement, and soft tissue structure appeared to be responsible for the DVRD of inserted cages. Therefore, cages should be inserted deeper to account for differences between visual and radiological image depths.

SU-E-T-730: Aperture Modulated Total Body Irradiation Dose Optimization Based on the Calculation of Radiological Depth

Purpose: Total body irradiation (TBI) techniques aim to deliver a uniform and accurate radiation dose to the entire anatomy of the patient. Given irregular patient shapes and internal inhomogeneities achieving a dose homogeneity within ±10 % of the prescribed dose represents a non trivial challenge. We propose a novel TBI technique which is capable of delivering a more uniform dose to the entire patient than conventional TBI techniques. Methods: Radiological depth calculations are performed along divergent ray lines through transverse CT slices of an anthropomorphic Rando® phantom. The radiological depths take into account both body contour variation and internal inhomogeneities. Based on the calculated radiological depths, multiple beam apertures can be generated using dynamic multileaf collimation (DMLC). To account for the scatter contribution, scatter kernels are derived using the concept of the scatter maximum ratios (SMR). Based on the scatter calculation the calculated beam apertures are modified. For dose calculation purposes the individual MLC files are imported into the Eclipse™ treatment planning system. However for treatment delivery, a single MLC file with multiple control points is generated and delivered while the phantom is translated on a motorized bed close to the floor through a stationary radiation beam with 0° gantry angle. Results: In comparison to fixed open beam translating bed TBI the dose deviation along the midline was reduced from ±8% to less than ±4% of the prescribed dose with this new technique. Lungdose was reduced by more than 15%. Agreement between calculated and measured doses was better than 3% in Rando®. Conclusions: A novel, aperture modulated translating bed TBI technique that employs dynamically shaped MLC defined beams has been developed for improving dose uniformity in three dimensions. In comparison with the open fixed beam TBI technique, the dose distribution homogeneity is greatly improved.

Generalized equivalent field size for nonuniform fluence maps in IMRT dose calculation

The equivalent field size (EFS) method is widely used to estimate dose of nonstandard fields, such as elongated or arbitrary shaped fields, for both central axis and off axis points. However, its application is limited to fluence maps with uniform intensity. In this work, we propose a generalized EFS (GEFS) for nonuniform fluence maps and present a formula for GEFS-based dose calculation. A parallel-beam dose table (PDT) consisting of central axis dose of circular fields of various diameters at various depths is used to define scatter contributions, based on which we calculate GEFS of any given fluence map. Such obtained GEFS, together with the radiological depth and PDT, is used to determine the dose at the point of interest. We tested GEFS-based dose calculation on a water phantom for both uniform and nonuniform fluence maps and compared the results with those by the collapsed cone convolution/superposition (CCCS) method. For all test cases, the gamma index is less than 1 based on the 3%/1 mm criteria for more than 96% of the calculated points. Larger discrepancies mainly occur along the field edges in the buildup region. A generalized equivalent field size for nonuniform fluence maps was proposed and its application in calculating dose at any point was presented and verified through comparison with the CCCS method.

SU-GG-T-46: An Equivalent Path Length TG-43(U) Model for Heterogeneity Corrections of HAM Applicator Treatments Using Low Energy Electronic Brachytherapy

Introduction: Low energy x‐rays, such as those produced by a 50 kV Xoft ™ electronic brachytherapy (eBx) source are influenced by heterogeneities. Brachytherapy source modeling using TG‐43(u) assumes that the source is surrounded by a water medium. We investigate an equivalent path length (EPL) correction of TG‐43(u) to predict the eBx dose in a mixture of air, water and non‐water equivalent media in a slab‐like planar geometry applicator. Methods: EPL is the radiological depth between a source and calculation point, as determined by the linear attenuation (μ) of each material in the path. EPL is used to obtain the radial dose and anisotropy but not the geometrical component of TG‐43(u). A Matlab model was computed using Matlab, for a slab applicator with fixed dwell times and locations composed of various materials. Slab applicator measurements for the Xoft system were made with GafChromic EB2 film exposed at surface and 5 mm depth. Three types of slab material were investigated; Silicon, Noryl, and Santoprene. The linear attenuation (μ) of each was determined by taking measurements in water, with and without the material in the beam path. Results: At this energy, μ(Silicon)/μ(H2O) is 2.6, μ(Noryl) /μ(H2O) is 0.87, and μ(Santoprene) /μ(H2O) is 1.11. The flatness and symmetry of the model were comparable to the measurements at both depths. At 5 mm depth, the modeled dose was within 1% of the measurement. At the surface, the model overestimates dose by 10% for silicone and 2–3% for less dense materials. Conclusion: The EPL‐TG‐43(u) model can be used to predict the effects of air and non‐water materials within 1% at depth and 10 % or better at the surface. This simple model provides a tool to investigate first order effects of heterogeneous media for low energy eBx treatments.

Evaluation of a biplanar diode array dosimeter for quality assurance of step‐and‐shoot IMRT

In this paper, we describe and characterize a novel biplanar diode array, and demonstrate its applicability to dosimetric QA of step‐and‐shoot IMRT. It is the first commercially available device of its kind specifically designed for performing measurements at varying gantry angles. The detector consists of a cylindrical PMMA phantom with two orthogonal detector boards. There are a total of 1069 p‐type 1 mm wide diode detectors covering the measurement area of 20×20cm2 in each of the measurement planes. The orthogonal detector arrays ensure that the dose modulation information is not lost regardless of the beam incidence angle. For absolute calibration, the dose to the reference detector is calculated at the appropriate SSD and radiological depth by the treatment planning system and is scaled by the measured accelerator output. The directly measured rotational response on the central axis shows the maximum variation of approximately ± 3% in the narrow ±1º angular intervals centered on the detector boards. This variation is reduced to less than ± 2% outside of the four similarly centered ± 5% angular intervals. For all detectors, the difference between the measured and the calculated dose for a plan with 12 equally spaced beams is −0.2±0.9%. Of eleven IMRT plans, ten passed the γ (3%,3 mm) criterion at or above 95%, while one passed at 92%. The biplanar diode array is a useful tool for IMRT QA, allowing for essentially instantaneous online analysis of absolute dose errors in 3D.PACS number: 87.55Qr

4D in‐beam positron emission tomography for verification of motion‐compensated ion beam therapy

Clinically safe and effective treatment of intrafractionally moving targets with scanned ion beams requires dedicated delivery techniques such as beam tracking. Apart from treatment delivery, also appropriate methods for validation of the actual tumor irradiation are highly desirable, In this contribution the feasibility of four-dimensionally (space and time) resolved, motion-compensated in-beam positron emission tomography (4DibPET) was addressed in experimental studies with scanned carbon ion beams. A polymethyl methracrylate block sinusoidally moving left-right in beam's eye view was used as target. Radiological depth changes were introduced by placing a stationary ramp-shaped absorber proximal of the moving target. Treatment delivery was compensated for motion by beam tracking. Time-resolved, motion-correlated in-beam PET data acquisition was performed during beam delivery with tracking the moving target and prolonged after beam delivery first with the activated target still in motion and, finally, with the target at rest. Motion-compensated 4DibPET imaging was implemented and the results were compared to a stationary reference irradiation of the same treatment field. Data were used to determine feasibility of 4DibPET but also to evaluate offline in comparison to in-beam PET acquisition. 4D in-beam as well as offline PET imaging was found to be feasible and offers the possibility to verify the correct functioning of beam tracking. Motion compensation of the imaged beta(+)-activity distribution allows recovery of the volumetric extension of the delivered field for direct comparison with the reference stationary condition. Observed differences in terms of lateral field extension and penumbra in the direction of motion were typically less than 1 mm for both imaging strategies in comparison to the corresponding reference distributions. However, in-beam imaging retained a better spatial correlation of the measured activity with the delivered dose. 4DibPET is a feasible and promising method to validate treatment delivery of scanned ion beams to moving targets. Further investigations will focus on more complex geometries and treatment planning studies with clinical data.

SU‐FF‐T‐82: Rapid Dose Calculation for IMRT Fields in Heterogeneous Medium

Purpose: To develop a fast algorithm to calculate patient specific 3D dose distribution for megavoltage photon beams. Method and Materials: In our PC‐based convolution algorithm, the ratio of scatter‐to‐primary dose, Ds/Dp, for a 2‐D non‐uniform field is calculated as a 3D convolution: urn:x-wiley:00942405:media:mp1555:mp1555-math-0001 with and is the 2D intensity profile within the IMRT field, pi is 1 inside field and 0 outside field collimation for segment field i of the IMRT field. A(z) is the attenuation function. This convolution is calculated using a Fast Fourier Transform. The parameters a, w, and d0 characterize the phantom scatter properties and can be determined from measured PDD and Sp. In a CT phantom, fast ray‐tracing is used to calculate radiological depth for the attenuation function. Results: SPR calculated using MC simulation for 6 and 15 MV Mohan Spectrum is compared with the FFT convolution algorithm. They agree very well for depths beyond electron contamination. Agreement is also compared between FFT convolution and MC simulation in a heterogeneous phantom. They agree to within 1%, relative to the primary dose. Conclusion: This algorithm is ideally suitable for calculating patient‐specific MU for quality assurance of patient specific IMRT fields for photon beams. The major attraction is the fast speed, which allows potential real‐time applications.

Independent calculation of dose from a helical TomoTherapy unit

A new calculation algorithm has been developed for independently verifying doses calculated by the TomoTherapy® Hi·Art® treatment planning system (TPS). The algorithm is designed to confi rm the dose to a point in a high dose, low dose‐gradient region. Patient data used by the algorithm include the radiological depth to the point for each projection angle and the treatment sinogram file controlling the leaf opening time for each projection. The algorithm uses common dosimetric functions [tissue phantom ratio (TPR) and output factor (Scp)] for the central axis combined with lateral and longitudinal beam profile data to quantify the off‐axis dose dependence. Machine data for the dosimetric functions were measured on the Hi·Art machine and simulated using the TPS. Point dose calculations were made for several test phantoms and for 97 patient treatment plans using the simulated machine data. Comparisons with TPS‐predicted point doses for the phantom treatment plans demonstrated agreement within 2% for both on‐axis and off‐axis planning target volumes (PTVs). Comparisons with TPS‐predicted point doses for the patient treatment plans also showed good agreement. For calculations at sites other than lung and superficial PTVs, agreement between the calculations was within 2% for 94% of the patient calculations (64 of 68). Calculations within lung and superficial PTVs overestimated the dose by an average of 3.1% (σ=2.4%) and 3.2% (σ=2.2%), respectively. Systematic errors within lung are probably due to the weakness of the algorithm in correcting for missing tissue and/or tissue density heterogeneities. Errors encountered within superficial PTVs probably result from the algorithm overestimating the scatter dose within the patient. Our results demonstrate that for the majority of cases, the algorithm could be used without further refinement to independently verify patient treatment plans.PACS number(s): 87.53.Bn, 87.53.Dq, 87.53.Xd

SU-GG-T-194: Experimental Determination of Correction Factors for Miniphantoms with Different Z for Measurement of In-Air Dosimetry Quantities at Off-Axis Points

Purpose: The purpose of this study was to experimentally determine the correction factors for miniphantoms of different Z for off‐axis measurements of in‐air quantities. Method and Materials: Measurements were performed for two megavoltage photon beams (6x and 15x). Different amount of miniphantom materials were used with radiological buildup depths ranging from 1.8 to 25 g/cm2. We used four mini‐phantom materials: Lucite, Graphite,Copper and Lead. In‐air quantities include: in‐air off‐axis ratio, in‐air wedge factor and output factor. Results: The mini‐phantom material has an effect on the measured in‐air dosimetric quantities. The correction factor increases with increasing Z of material, build‐up and energy. There is a maximum 1% headscatter correction factor. This error increases with increasing beam energy and increasing Z material. Off‐axis ratio correction factor also increases with increasing Z material and with increasing energy. A maximum error of 6% was observed. Conclusion: We concluded that the mini‐phantom material has an effect on the measured in‐air dosimetric quantities. The correction factor increases with increasing of Z material, build‐up and energy. There is a maximum 1% headscatter correction factor, which is the highest for lead. An off‐axis correction factor of up to 6% was determined.

A potential method for in vivo range verification in proton therapy treatment

Proton therapy potentially offers excellent dose conformality and reduction in integral dose. The superior dose distribution is, however, much more sensitive to the radiological depth along the beam path than for photon fields. Variations in this depth due to inaccurate planning calculation, setup uncertainty, respiration-induced organ motion, etc, could result in either an ‘undershooting’, missing the distal portion of the target volume entirely, or an ‘overshooting’, delivering the full prescription dose to the normal tissue beyond the target volume. In vivo dose verification plays an important role in the quality assurance of the treatments. The point dose measurements widely used for photon treatments are, however, insufficient for protons due to the particular characteristics of the proton depth–dose distribution. In this work, we explore a method for in vivo range verifications in proton treatments using range-modulated passive scattering fields. The method utilizes the time dependence of the dose distribution delivered by these fields. By measuring the time dependence of the dose rate at any point in the target volume, the residual range of the beam at this point can be obtained with millimeter accuracy.

Clinical evaluation of monitor unit software and the application of action levels

Purpose The aim of this study was the clinical evaluation of an independent dose and monitor unit verification (MUV) software which is based on sophisticated semi-analytical modelling. The software was developed within the framework of an ESTRO project. Finally, consistent handling of dose calculation deviations applying individual action levels is discussed. Materials and methods A Matlab-based software (“MUV”) was distributed to five well-established treatment centres in Europe (Vienna, Graz, Basel, Copenhagen, and Umeå) and evaluated as a quality assurance (QA) tool in clinical routine. Results were acquired for 226 individual treatment plans including a total of 815 radiation fields. About 150 beam verification measurements were performed for a portion of the individual treatment plans, mainly with time variable fluence patterns. The deviations between dose calculations performed with a treatment planning system (TPS) and the MUV software were scored with respect to treatment area, treatment technique, geometrical depth, radiological depth, etc. Results In general good agreement was found between calculations performed with the different TPSs and MUV, with a mean deviation per field of 0.2 ± 3.5% (1 SD) and mean deviations of 0.2 ± 2.2% for composite treatment plans. For pelvic treatments less than 10% of all fields showed deviations larger than 3%. In general, when using the radiological depth for verification calculations the results and the spread in the results improved significantly, especially for head-and-neck and for thorax treatments. For IMRT head-and-neck beams, mean deviations between MUV and the local TPS were −1.0 ± 7.3% for dynamic, and −1.3 ± 3.2% for step-and-shoot IMRT delivery. For dynamic IMRT beams in the pelvis good agreement was obtained between MUV and the local TPS (mean: −1.6 ± 1.5%). Treatment site and treatment technique dependent action levels between ±3% and ±5% seem to be clinically realistic if a radiological depth correction is performed, even for dynamic wedges and IMRT. Conclusion The software MUV is well suited for patient specific treatment plan QA applications and can handle all currently available treatment techniques that can be applied with standard linear accelerators. The highly sophisticated dose calculation model implemented in MUV allows investigation of systematic TPS deviations by performing calculations in homogeneous conditions.

A respiratory‐gated treatment system for proton therapy

Proton therapy offers the potential for excellent dose conformity and reduction in integral dose. The superior dose distribution is, however, much more sensitive to changes in radiological depths along the beam path than for photon fields. Respiratory motion can cause such changes for treatments sites like lung, liver, and mediastinum and thus affect the proton dose distribution significantly. We have implemented and commissioned a respiratory-gated system for range-modulated treatment fields. The gating system was designed to ensure that each gate always contains complete modulation cycles so that for any beam segment the delivered dose has the planned depth-dose distribution. Measurements have been made to estimate the time delays for the various components of the system. The total delay between the actual motion and the beam on/off control is in the range of 65-195 ms. Time-resolved dose measurements and film tests were also conducted to examine the overall gating effect.

SU‐FF‐T‐261: Independent Point Dose Verification Using TomoTherapy Quality Assurance Phantom

Purpose: To detect TomoTherapy planning errors by relating a point dose in the patient treatment plan to the dose calculated at the corresponding point in the quality assurance phantom. Methods and Materials: Standard TomoTherapy patient‐specific quality assurance applies the patient treatment plan to a cylindrical QA phantom and then compares measured phantom dose to calculated phantom dose. However, there is no check of dose in the phantom to dose in the patient. We applied a ratio of TMRs to a point dose in the patient treatment plan in order to estimate the dose to the corresponding point in the phantom. Rotational delivery of TomoTherapy was approximated using either a 360° arc, 8 equally‐spaced angles, or 4 equally‐space angles. For each approximation, calculations were done using average physical depth and average radiological depth. The method was tested using data from 87 TomoTherapy patients. TMR ratio method doses were compared to treatment planning system doses. Agreement within ±5% was considered clinically acceptable. Results: Using average radiological depth, the pass rates were 70.6%, 69.4%, and 65.9% for the 360°, 8−, and 4‐;angle approximations, respectively. Using average physical depth, the pass rates were 63.5%, 56.5%, and 58.8% for the 360°, 8‐, and 4‐angle approximations, respectively. The best results were obtained for centrally‐located tumors such as prostate (83.9% pass rate for radiological depth and 360° arc approximation). The pass rates were worst for superficial tumors (50.0% pass rate for radiological depth and 360° arc approximation). Conclusions: The TMR ratio method was used to relate dose in the QA phantom to dose in the patient. For each beam approximation, radiological depths gave better results than physical depths. The site‐specific pass rates could be used to determine action levels for implementing this method in the clinic.Supported in part by a research agreement with TomoTherapy, Inc.

Assessing Residual Motion for Gated Proton-Beam Radiotherapy

Gated radiation therapy is a promising method for improving the dose conformality of treatments to moving targets and reducing the total volume of irradiated tissue. Target motion is of particular concern in proton beam radiotherapy, due to the finite range of proton dose deposition in tissue. Gating allows one to reduce the extent of variation, due to respiration, of the radiological depth to target during treatment delivery. However, respiratory surrogates typically used for gating do not always accurately reflect the position of the internal target. For instance, a phase delay often exists between the internal motion and the motion of the surrogate. Another phenomenon, baseline drifting refers to a gradual change in the exhale position over time, which generally affects the external and internal markers differently. This study examines the influence of these two physiological phenomena on gated radiotherapy using an external surrogate.

TU‐C‐224A‐09: Implementation and Comparison of Several Proton IMRT Algorithms

Purpose: To computationally compare the plan quality provided by 3 different intensity modulation proton therapy (IMPT) techniques: 3D modulation; 2.5D modulation; and distal edge tracking (DET) with optimization. Plan quality, problem size, and efficiency were assessed. Method and Materials: The dose calculation was based a finite sized beamlet model, which was 1×1 cm2 at isocenter. The dose from each beamlet was the superposition/convolution of infinitesimal pencil beams falling within the beamlet. Ray tracing to each voxel was performed for each beamlet to find out the radiological depth to each voxel. In the 3D modulation algorithm, a stack of Bragg peaks were placed between the maximal and minimal radiological depths of targets passed by a beamlet with 2.5 mm spacing. The fluence of each Bragg peak was modulated independently by the optimizer. The placement of the Bragg peaks in the 2.5D algorithm was similar to the 3D algorithm, but the Bragg peaks belong to a beamlet were pre‐optimized to make a spread out Bragg (SOBP) whose dose level was equal to the prescription dose of the target it passed. The weights of SOBPs were optimization variables. In the DET algorithm, the Bragg peaks were set at the distal edge of the targets. Results: The 3D algorithm could produce the best plan with least beams, but the data size was large. The DET was most efficient in dose calculation and fluence map optimization, its plan quality was fair. The advantage of the 2.5D algorithm was in its small data size and efficiency in fluency map optimization. Conclusion: An appropriate algorithm should be selected to meet the trade‐off of plan quality versus computational and delivery efficiency.This work was supported in part by Florida DOH Grant 04‐NIR03.

SU-FF-T-201: Effect of Mini-Phantom Material On In-Air Dosimetry Quantities

Purpose: Recent studies suggest that miniphantom material and thickness affect the measured values of in‐air output ratio. These results prompt us to investigate the effect of mini‐phantom depth and material on several in‐air quantities. Method and Materials: Beam data for two photon beams (6x and 15x) were compared for several in‐air quantities: off‐axis ratio (OAR); central axis wedge factor (CAX WF); distance factor (DF) dependence and off‐axis wedge factor (OAX WF). Several miniphantom materials (Lucite, copper,lead and graphite) were used with a radiological depth ranging from 2 to 25g/cm2. Results: OAR decreases with increasing miniphantom thickness and off‐ axis distance x, for the same mini‐phantom material. For open field, the maximum difference is 5% and 8% for 6x and 15x, respectively. CAX for WF were measured for 15°, 45° and 60° wedge angles, for both virtual and solid wedges. WF increases up to 11% with mini‐phantom thickness, when solid wedges are used. Virtual wedges have an effect of less than 1% on WF. Maximum errors were found for 450 solid wedge. DF dependence was measured for several SAD's for 6 and 15x, open and 60° virtual and solid wedges. DF agreed within 3% regardless of beam energy, material and SAD. OAX WF were measured at a few off‐axis points along both wedge and non‐wedge gradient directions for 6x. WF varies by up to 2.6% in the toe direction, by up to 1.1% in the heel direction and by up to 3.3% in the non‐wedge direction. Conclusion: Mini‐phantom material has an effect on measured in‐air dosimetric quantities. Ideally, a water‐equivalent miniphantom should be used. When the miniphantom thickness increases (even for water equivalent material), the value of WF, OAR, DF increase by 11%, 8%, 3%, respectively. The error also increases with Z value of mini‐phantom and photon energy.

SU‐FF‐T‐284: Independent Calculation of Dose for a Helical TomoTherapy Treatment Plan

Purpose: To develop a new calculation algorithm for independently verifying doses from helical TomoTherapy treatment plans. Method and Materials: The calculation algorithm confirms dose to a point in a high‐dose, low‐gradient region where modulation is expected to be minimal. Inputs to the algorithm are the coordinates of the point, the radiological depth for each projection angle in the axial plane of the point, and the treatment sinogram. The algorithm uses common dosimetric functions (e.g., TPR, Scp), which were measured on the TomoTherapy treatment unit. Measured lateral and longitudinal profile data are used to quantify the off‐axis dose dependence. Test comparisons were made using a 7‐cm diameter PTV centered in a 20‐cm diameter cylindrical phantom. The phantom was positioned in the center of the treatment bore for one test and 10 cm off‐axis for another. Comparisons were also made for five prostate cancer treatment plans. In all cases, the point of calculation was the geometric center of the PTV. Results: Measurements of both TPR and Scp on the TomoTherapy unit demonstrated small variation with jaw width (field length) and the number of open leaves (field width). Therefore, average values of TPR(d) and Scp were used in the algorithm. Comparisons with the cylindrical phantom treatment plans demonstrated good agreement, with the calculated doses agreeing within 2% at the PTV center for both the on‐axis and off‐axis treatment plans. Results for the prostate patients also showed good agreement within 4%. Conclusion: A new calculation algorithm that uses standard dosimetric functions has been developed for verifying doses from helical TomoTherapy treatment plans. Supported in part by a research agreement with TomoTherapy, Inc.

Clinical and Physiologic Outcomes After Transvaginal Rectocele Repair

This study was designed to evaluate the clinical and physiologic outcomes after transvaginal rectocele repair. Between June 2000 and January 2003, 30 females (mean age, 62 (range, 45-78) years) with a symptomatic large rectocele (>3 cm) underwent transvaginal rectocele repair (anterior levatorplasty). Six months after surgery, a physiologic evaluation was performed by using defecography (depth of rectocele) and anorectal manometry (maximum resting pressure, maximum squeeze pressure, rectal threshold, and maximum tolerable volume). Using a questionnaire, a clinical evaluation was performed one year after surgery to analyze symptoms, including difficult evacuation, digital support, sexual discomfort, as well as patient satisfaction. Follow-up of all patients was conducted during a median duration of 38 (range, 23-54) months. There were no operative complications, such as hematoma, wound infection, or rectovaginal fistula. Difficult evacuation improved in 27 of 30 patients (90 percent) and completely disappeared in 9 patients. Postoperatively, digital support was no longer necessary during evacuation in 15 of 21 patients (71 percent). Overall patient satisfaction reached 25 of 30 (83 percent). Although mild sexual discomfort was observed in nine patients, it disappeared gradually and only one patient complained of persistent symptoms. No patient reported symptomatic recurrences at the end of the follow-up. The radiologic mean depth of the rectocele was significantly reduced: preoperative, 3.9 cm; postoperative, 0.5 cm. None of the physiologic parameters significantly changed after surgery. Transvaginal rectocele repair can provide excellent long-term symptomatic relief and a high rate of patient satisfaction without any alteration in anorectal physiologic function.