G Chen

A monitor unit verification calculation in intensity modulated radiotherapy as a dosimetry quality assurance.

In standard teletherapy, a treatment plan is generated with the aid of a treatment planning system, but it is common to perform an independent monitor unit verification calculation (MUVC). In exact analogy, we propose and demonstrate that a simple and accurate MUVC in intensity modulated radiotherapy (IMRT) is possible. We introduce the concept of modified Clarkson integration (MCI). In MCI, we exploit the rotational symmetry of scattering to simplify the dose calculation. For dose calculation along a central axis (CAX), we first replace the incident IMRT fluence by an azimuthally averaged fluence. Second, the Clarkson integration is carried over annular sectors instead of over pie sectors. We wrote a computer code, implementing the MCI technique, in order to perform a MUVC for IMRT purposes. We applied the code to IMRT plans generated by CORVUS. The input to the code consists of CORVUS plan data (e.g., DMLC files, jaw settings, MU for each IMRT field, depth to isocenter for each IMRT field), and the output is dose contribution by individual IMRTs field to the isocenter. The code uses measured beam data for Sc, Sp, TPR, (D/MU)ref and includes effects from multileaf collimator transmission, and radiation field offset. On a 266 MHz desktop computer, the code takes less than 15 to calculate a dose. The doses calculated with the MCI algorithm agreed within +/-3% with the doses calculated by CORVUS, which uses a 1 cm x 1 cm pencil beam in dose calculation. In the present version of MCI, skin contour variations and inhomogeneities were neglected.

Degradation of the Bragg peak due to inhomogeneities

The rapid fall-off of dose at the end of range of heavy charged particle beams has the potential in therapeutic applications of sparing critical structures just distal to the target volume. Here we explored the effects of highly inhomogeneous regions on this desirable depth-dose characteristic. The proton depth-dose distribution behind a lucite-air interface parallel to the beam was bimodal, indicating the presence of two groups of protons with different residual ranges, creating a step-like depth-dose distribution at the end of range. The residual ranges became more spread out as the interface was angled at 3 degrees, and still more at 6 degrees, to the direction of the beam. A second experiment showed little significant effect on the distal depth-dose of protons having passed through a mosaic of teflon and lucite. Anatomic studies demonstrated significant effects of complex fine inhomogeneities on the end of range characteristics. Monoenergetic protons passing through the petrous ridges and mastoid air cells in the base of skull showed a dramatic degradation of the distal Bragg peak. In beams with spread out Bragg peaks passing through regions of the base of skull, the distal fall-off from 90 to 20% dose was increased from its nominal 6 to well over 32 mm. Heavy ions showed a corresponding degradation in their ends of range. In the worst case in the base of skull region, a monoenergetic neon beam showed a broadening of the full width at half maximum of the Bragg peak to over 15 mm (compared with 4 mm in a homogeneous unit density medium). A similar effect was found with carbon ions in the abdomen, where the full width at half maximum of the Bragg peak (nominally 5.5 mm) was found to be greater than 25 mm behind gas-soft-tissue interfaces. We address the implications of these data for dose computation with heavy charged particles.

Intensity modulated radiotherapy dose delivery error from radiation field offset inaccuracy.

In Intensity Modulated Radiotherapy (IMRT), irradiation is delivered in a number of small aperture subfields. The fluences shaped by these small apertures are highly sensitive to inaccuracies in multileaf collimator (MLC) calibration. The Radiation Field Offset (RFO) is the difference between a radiation and a light field at the Source to Axis Distance (SAD) for a MLC. An Intensity Modulated Radiotherapy (IMRT) system must incorporate a RFO by closing in all leaf openings. In IMRT, RFO inaccuracy will result in a dose error to the interior of a target volume. We analyze dosimetric consequences of incorporating a wrong RFO into the CORVUS, 1 cm x 1 cm, step and shoot, IMRT system. The following method was employed. First an IMRT plan is generated for a target volume in a phantom, which produces a set of dynamic MLC (DMLC) files with the correct RFO value. To simulate delivery with a wrong RFO value, we wrote a computer code that reads in the DMLC file with the correct RFO value and produces another DMLC with an incorrect RFO specified by a user. Finally the phantom was irradiated with the correct and the incorrect RFO valued DMLC files, and the doses were measured with an ionization chamber. The method was applied to 9 fields, 6 MV, IMRT plans. We measured Dose Error Sensitivity Factor (DESF) for each plan, which ranged from (0-8)% mm(-1). The DESF(x) is defined as a fractional dose error to a point (x) in a target volume per mm of the RFO error, i.e., DESF(x) is equivalent to ¿deltaD(x)/D(x)deltaRFO¿. Therefore, we concluded that for CORVUS, 6 MV, 1 cm x 1 cm, step and shoot IMRT, RFO must be determined within an accuracy of 0.5 mm if a fractional dose error to a target volume is to be less than 4%. We propose an analytic framework to understand the measured DESFs. From the analysis we conclude that a large DESF was associated with an DMLC file with small average leaf openings. For 1 cm x 1 cm, step and shoot IMRT, the largest possible DESF is predicted to be 20% mm(-1). In addition, we wrote computer code that can calculate a DESF of a DMLC file. The code was written in Mathematica 3.0. The code can be used to screen patient IMRT plans that are highly sensitive to a RFO error.

A method of calculating a lung clinical target volume DVH for IMRT with intrafractional motion

The motion of lung tumors from respiration has been reported in the literature to be as large as 1-2 cm. This motion requires an additional margin between the Clinical Target Volume (CTV) and the Planning Target Volume (PTV). In Intensity Modulated Radiotherapy (IMRT), while such a margin is necessary, the margin may not be sufficient to avoid unintended high and low dose regions to the interior on moving CTV. Gated treatment has been proposed to improve normal tissues sparing as well as to ensure accurate dose coverage of the tumor volume. The following questions have not been addressed in the literature: (a) what is the dose error to a target volume without a gated IMRT treatment? (b) What is an acceptable gating window for such a treatment. In this study, we address these questions by proposing a novel technique for calculating the three-dimensional (3-D) dose error that would result if a lung IMRT plan were delivered without a gated linac beam. The method is also generalized for gated treatment with an arbitrary triggering window. IMRT plans for three patients with lung tumors were studied. The treatment plans were generated with HELIOS for delivery with 6 MV on a CL2100 Varian linear accelerator with a 26 pair MLC. A CTV to PTV margin of 1 cm was used. An IMRT planning system searches for an optimized fluence map phi(x,y) for each port, which is then converted into a dynamic MLC file (DMLC). The DMLC file contains information about MLC subfield shapes and the fractional Monitor Units (MUs) to be delivered for each subfield. With a lung tumor, a CTV that executes a quasiperiodic motion z(t) does not receive phi(x,y), but rather an Effective Incident Fluence EIF(x,y). We numerically evaluate the EIF(x,y) from a given DMLC file by a coordinate transformation to the Targets Eye View (TEV). In the TEV coordinate system, the CTV itself is stationary, and the MLC is seen to execute a motion -z(t) that is superimposed on the DMLC motion. The resulting EIF(x,y) is input back into the dose calculation engine to estimate the 3-D dose to a moving CTV. In this study, we model respiratory motion as a sinusoidal function with an amplitude of 10 mm in the superior-inferior direction, a period of 5 s, and an initial phase of zero.

Four-dimensional targeting error analysis in image-guided radiotherapy

Image-guided therapy (IGT) involves acquisition and processing of biomedical images to actively guide medical interventions. The proliferation of IGT technologies has been particularly significant in image-guided radiotherapy (IGRT), as a way to increase the tumor targeting accuracy. When IGRT is applied to moving tumors, image guidance becomes challenging, as motion leads to increased uncertainty. Different strategies may be applied to mitigate the effects of motion: each technique is related to a different technological effort and complexity in treatment planning and delivery. The objective comparison of different motion mitigation strategies can be achieved by quantifying the residual uncertainties in tumor targeting, to be detected by means of IGRT technologies. Such quantification requires an extension of targeting error theory to a 4D space, where the 3D tumor trajectory as a function of time measured (4D Targeting Error, 4DTE). Accurate 4DTE analysis can be represented by a motion probability density function, describing the statistical fluctuations of tumor trajectory. We illustrate the application of 4DTE analysis through examples, including weekly variations in tumor trajectory as detected by 4DCT, respiratory gating via external surrogates and real-time tumor tracking.

High‐resolution dosimetry with stimulated phosphorescence

Thermally stimulated radiophosphorescence has been studied as a means of high-resolution dosimetry. Small grains of CaSO4:Mn phosphor, embedded in a thin Teflon tape, constitute the dosimeter. The light emitted after irradiation is measured with a photomultiplier coupled to the eyepiece of a scanning microscope. With CaSO4:Mn, the phosphorescence at room temperature is sufficient for measurement after doses in excess of 3000 rads. The spatial resolution of the technique is about 0.2 mm. The method has been tested by measuring the dose distributions from a radium needle and a beta-emitting eye applicator.

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

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

SU‐FF‐I‐22: Analysis of Residual Geometric Artifacts From 4DCT

Purpose: 4DCT has been shown to provide improved imaging of the thoracic and abdominal region, reducing the temporal artifacts observed from traditional “free‐breathing” helical CT methods. However, the reconstruction accuracy of a 4DCT exam is dependent on the reproducibility of a patients respiratory cycle (amplitude, period). Variation of this respiratory can introduce residual geometric uncertainty in the resulting 4DCT data. This work examines the geometric uncertainties introduced in phase resorted 4DCT imaging arising from the variability of patient respiration, comparing computational simulation with phantom measurements. Method and Materials: Examples of residual 4DCT artifacts were obtained by scanning a moving phantom capable of reproducing patient respiratory motion along the patient superior‐inferior and lateral axes. Motion of the phantom stage was driven by RPM signals recorded from actual patient 4DCT scans. Geometric dimensions of the target volumes scanned on the moving phantom were compared to phase reconstructed 4DCT target images. A new computational tool was developed to examine the continuous variation of patient respiration upon cine CTimage reconstruction. This tool reproduces basic 4DCT acquisition, allowing variation of patient and scan parameters such as scan start time relative to the RPM signal, multi‐slice CT dimensions, amplitude of patient respiration and target volume dimensions. Results: Variation of 4DCT target volume has been observed to be as great as 13% from measured values. Spherical phantoms have shown as much as 17% deviation from the known value when compared to 4DCT reconstructed images.Conclusion: While 4DCT provides superior reconstruction of respiratory motion, it is not completely free from artifacts. A complete understanding of residual motion artifacts from 4DCT imaging is necessary before incorporating this data into patient treatment planning, especially with respect to techniques involving mid‐phase (between exhale and inhale) images where the motion artifacts are most significant.

SU‐GG‐T‐317: Impact of Tumor Motion and Size in the Irradiation of Moving Tumors in Step‐And‐Shoot IMRT: A NCAT Based 4D Monte Carlo Simulation Study

Purpose: To quantify the dosimetricimpact of the interplay effect in step‐and‐ shoot IMRT treatment of lungtumors of varying size and motion amplitude using a 4D Monte Carlo simulation framework and to find whether a threshold exists where implementing motion mitigation strategies might become important in such treatments. Method and Materials: The Non‐Uniform Rational B‐Spline (NURBS) Cardiac and Torso (NCAT) computational phantom was used to create 10‐phase 4D CT data sets of 12 theoretical patients with tumor sizes of 1–6 cm and magnitudes of tumor motion of 1 – 5 cm. Lung density for each model was changed as a function of breathing phase. IMRT leaf sequences were generated using the CORVUS treatment planning system for each of the 12 data sets and used as input into the Dose Planning Method (DPM) Monte Carlo code. Dose at each phase was mapped back to the exhale phase using the internal NCAT deformation maps. Dose volume histograms (DVHs) of tumor and organs at risk (OARs) were used as a measure of the dosimetric effect of tumor motion and size. Results: A population of computational patient phantoms has been created. The lung density of the NCAT phantom can vary from the default 0.30 gm/cm3 by up to 34% for the largest diaphragm motion. The NCAT phantom has been implemented within the CORVUS treatment planning environment as well as within our 4D Monte Carlo simulation framework. Various tumor locations in the lung and tumor amplitudes were investigated to assess dosimetric effects in IMRT therapy of lungtumors.Conclusion: This study shows the potential of using the most advanced computational model of human anatomy for finding class solutions for motion mitigation within a controlled environment. We conclude it is a valuable tool in order to study artifact free data sets for radiation therapy.

WE‐C‐ValA‐09: A Biological Lung Phantom for IGRT Studies

Purpose: We are evaluating the feasibility of a dynamic biological lung phantom for IGRT studies, with the initial goal of developing a reliable phantom suitable for use in validation of deformable registration and volume rendering studies of the lung. The properties of an ideal lung phantom would include complex geometry, anisotropic inflation, and composition, lobar structure and internal airway architecture similar to that of human lung.Method and Materials: Preserved swine lung was obtained and compared to human lung. The prepared lung was statically inflated to different volumes using a regulated nitrogen supply, and can also be dynamically inflated using a medical ventilator. The inflated phantom was imaged on a GE Lightspeed CT scanner. Volume rendering of the CTimage data was performed to visualize and determine coordinates of airway bifurcations.Results: Preserved swine lung was determined to be comparable to human lung in terms of tissue radiological and physical properties, lobar structure, airway architecture, volume and mass. Rendered airway vs. physiologic airway dimensions are undergoing verification by dissection. Analysis of CTimages and volume rendering data demonstrates that the airway architecture may be followed to at least the 5th airway bifurcation, yielding a conservative minimum of 31 reproducible anatomic landmarks evenly distributed throughout the lung. By visual inspection, it is possible to follow the displacement vector of these landmarks in sequential images.Conclusion: Initial analysis shows that a swine lung phantom meets a number of the requirements of a reliable and functional phantom for validation of deformable registration and volume rendering methods. Reference points generated using the CT/volume rendering technique may be useful as a validation tool for both feature‐ and intensity‐based deformable registration techniques. Ongoing study will evaluate the potential of the lung phantom for use in planning, delivering, and validating 4D IGRT.