D Lovelock

A Monte Carlo model of photon beams used in radiation therapy

A generic Monte Carlo model of a photon therapy machine is described. The model, known as McRad, is based on EGS4 and has been in use since 1991. Its primary function has been the characterization of the incident photon fluence for use by dose calculation algorithms. The accuracy of McRad is examined by comparing the dose distributions in a water phantom generated using only the Monte Carlo data with measured dose distributions for two machines in our clinic; a 6 MV Varian Clinac 600C and the 15 MV beam from a Clinac 2100C. The Monte Carlo generated dose distributions are computed using a dose calculation algorithm based on the use of differential pencil beam kernels. It was found that the match to measured data could be improved if the model is tuned by adjusting the energy of the electron beam incident on the target. The beam profiles were found to be more sensitive indicators of the electron beam energy than the depth dose curves. Beyond the depths reached by contaminant electrons, the computed and measured depth dose curves agree to better than 1%. The comparison of beam profiles indicate that in regions up to within 1 cm of the field edge, the measured and computed doses generally agree to within 2%-3%.

Preliminary Results of High-Dose Single-Fraction Radiotherapy for the Management of Chordomas of the Spine and Sacrum

BACKGROUND En bloc wide-margin excision significantly decreases the risk of chordoma recurrence. However, a wide surgical margin cannot be obtained in many chordomas because they arise primarily in the sacrum, clivus, and mobile spine. Furthermore, these tumors have shown resistance to fractionated photon radiation at conventional doses and numerous chemotherapies. OBJECTIVE To analyze the outcomes of single-fraction stereotactic radiosurgery (SRS) in the treatment of chordomas of the mobile spine and sacrum. METHODS Twenty-four patients with chordoma of the sacrum and mobile spine were treated with high-dose single-fraction SRS (median dose, 2400 cGy). Twenty-one primary and 3 metastatic tumors were treated. Seven patients were treated for postoperative tumor recurrence. In 7 patients, SRS was administered as planned adjuvant therapy, and in 13 patients, SRS was administered as neoadjuvant therapy. All patients had serial magnetic resonance imaging follow-up. RESULTS The overall median follow-up was 24 months. Of the 24 patients, 23 (95%) demonstrated stable or reduced tumor burden based on serial magnetic resonance imaging. One patient had radiographic progression of tumor 11 months after SRS. Only 6 of 13 patients who underwent neoadjuvant SRS proceeded to surgery. This decision was based on the lack of radiographic progression and the patients preference. Complications were limited to 1 patient in whom sciatic neuropathy developed and 1 with vocal cord paralysis. CONCLUSION High-dose single-fraction SRS provides good tumor control with low treatment-related morbidity. Additional follow-up is required to determine the long-term recurrence risk.

Accurate positioning for head and neck cancer patients using 2D and 3D image guidance

Our goal is to determine an optimized image‐guided setup by comparing setup errors determined by two‐dimensional (2D) and three‐dimensional (3D) image guidance for head and neck cancer (HNC) patients immobilized by customized thermoplastic masks. Nine patients received weekly imaging sessions, for a total of 54, throughout treatment. Patients were first set up by matching lasers to surface marks (initial) and then translationally corrected using manual registration of orthogonal kilovoltage (kV) radiographs with DRRs (2D‐2D) on bony anatomy. A kV cone beam CT (kVCBCT) was acquired and manually registered to the simulation CT using only translations (3D‐3D) on the same bony anatomy to determine further translational corrections. After treatment, a second set of kVCBCT was acquired to assess intrafractional motion. Averaged over all sessions, 2D‐2D registration led to translational corrections from initial setup of 3.5±2.2 (range 0–8) mm. The addition of 3D‐3D registration resulted in only small incremental adjustment (0.8±1.5mm). We retrospectively calculated patient setup rotation errors using an automatic rigid‐body algorithm with 6 degrees of freedom (DoF) on regions of interest (ROI) of in‐field bony anatomy (mainly the C2 vertebral body). Small rotations were determined for most of the imaging sessions; however, occasionally rotations >3° were observed. The calculated intrafractional motion with automatic registration was <3.5 mm for eight patients, and <2° for all patients. We conclude that daily manual 2D‐2D registration on radiographs reduces positioning errors for mask‐immobilized HNC patients in most cases, and is easily implemented. 3D‐3D registration adds little improvement over 2D‐2D registration without correcting rotational errors. We also conclude that thermoplastic masks are effective for patient immobilization. PACS number: 87.53.Kn

Analysis of the photon beam treatment planning data for a scanning beam machine

The characteristics of photon beams from the Scanditronix MM50 radiation therapy machine that are necessary for treatment planning are described. The MM50 uses a scanning beam instead of a conventional flattening filter to achieve flat dose distributions. At each beam energy, a scan pattern is chosen, depending on the field size; the small scan pattern (S) is used for field sizes up to 10 x 10 cm, the medium scan pattern (M) is used for field sizes up to 20 x 20 cm, and the large scan pattern (L) is used for the larger field sizes. The dose distributions of the beams associated with the 10 MV S, M, and L scan patterns, the 25 MV S, M, and L patterns, and the 50 MV S and M patterns are described. The data reported includes central axis data, beam profiles, and output factors. In addition to the measured data, our dose calculation model requires a pencil beam kernel for each beam. The kernel is constructed using the average photon energy spectrum, which is generated using a Monte Carlo simulation of the MM50. The simulation, based on EGS4, is also used to generate the radial variation of fluence and energy fluence, which is required by a new dose calculation model that does not require the measurement of beam profiles. The Monte Carlo generated data; the photon energy spectrum, the fluence, and the energy fluence are presented.

Measuring uncertainty in dose delivered to the cochlea due to setup error during external beam treatment of patients with cancer of the head and neck

PURPOSE To use Cone Beam CT scans obtained just prior to treatments of head and neck cancer patients to measure the setup error and cumulative dose uncertainty of the cochlea. METHODS Data from 10 head and neck patients with 10 planning CTs and 52 Cone Beam CTs taken at time of treatment were used in this study. Patients were treated with conventional fractionation using an IMRT dose painting technique, most with 33 fractions. Weekly radiographic imaging was used to correct the patient setup. The authors used rigid registration of the planning CT and Cone Beam CT scans to find the translational and rotational setup errors, and the spatial setup errors of the cochlea. The planning CT was rotated and translated such that the cochlea positions match those seen in the cone beam scans, cochlea doses were recalculated and fractional doses accumulated. Uncertainties in the positions and cumulative doses of the cochlea were calculated with and without setup adjustments from radiographic imaging. RESULTS The mean setup error of the cochlea was 0.04 ± 0.33 or 0.06 ± 0.43 cm for RL, 0.09 ± 0.27 or 0.07 ± 0.48 cm for AP, and 0.00 ± 0.21 or -0.24 ± 0.45 cm for SI with and without radiographic imaging, respectively. Setup with radiographic imaging reduced the standard deviation of the setup error by roughly 1-2 mm. The uncertainty of the cochlea dose depends on the treatment plan and the relative positions of the cochlea and target volumes. Combining results for the left and right cochlea, the authors found the accumulated uncertainty of the cochlea dose per fraction was 4.82 (0.39-16.8) cGy, or 10.1 (0.8-32.4) cGy, with and without radiographic imaging, respectively; the percentage uncertainties relative to the planned doses were 4.32% (0.28%-9.06%) and 10.2% (0.7%-63.6%), respectively. CONCLUSIONS Patient setup error introduces uncertainty in the position of the cochlea during radiation treatment. With the assistance of radiographic imaging during setup, the standard deviation of setup error reduced by 31%, 42%, and 54% in RL, AP, and SI direction, respectively, and consequently, the uncertainty of the mean dose to cochlea reduced more than 50%. The authors estimate that the effects of these uncertainties on the probability of hearing loss for an individual patient could be as large as 10%.

SU‐FF‐J‐107: Extraction of Internal and External Marker 3D‐Motion in Liver Patients with Compression Belt Using KV Cone‐Beam Radiographic Projections

Purpose: To study correlation of internal implanted vs. external skin markers for tracking respiratory motion in liver patients using radiographic projections from on‐board kV cone‐beam scans. Material and Method: Cone‐beam projections were analyzed to extract 3D‐motion of internal and external markers for five liver patients receiving hypofractionated radiotherapy. Patients were immobilized in a stereotactic body frame and an abdominal compression belt was used to constrain respiratory motion. Marker motion was derived using a tracking algorithm and analysis of a sequence of 650 cone‐beam projections acquired during a 1 minute scan. Corrections were made for imager rotation and sag. Results: External and internal markers had the same frequency of respiratory motion, however, the amplitude of external marker motion is smaller. Internal and external marker motion was also out‐of‐phase in some patients. Internal marker motion is greater in the superior‐inferior direction than in anterior‐posterior and right‐left directions, which is due to compression belt constraining of respiratory motion in these direction. Two patients showed small or no motion of the external marker, whereas, internal marker motion was as large as 1.0 cm, which may be due to the proximity of the external marker to the compression belt. Conclusions: Although, the motions of internal and external markers are usually correlated and have similar motion frequency, the amplitude of marker motion may differ significantly and in some patients markers may move our‐of‐phase. The abdominal compression belt suppresses respiratory motion strongly normal to patient skin and may contribute to phase differences. The external markers motion for monitoring internal changes of respiration should be used with caution. Marker 3D‐motion from cone‐beam projections provides real time tumor trajectory that can be used to determine accuracy of PTV margins with no extra dose other than that used in CBCTimaging.Conflict of Interest: Supported by NCI Grant P01‐CA59017.

SU-FF-I-18: Quantifying the Geometric Accuracy of the On Board Imager Over a One-Year Period

Purpose: To quantify the geometric accuracy of the On Board Imager in both the kV radiographic and cone beam imaging modes. Method and Materials: The Winston‐Lutz test was performed to localize a 5mm tungsten sphere placed within +/− 0.25 mm of the radiation isocenter. The sphere was imaged with half fan cone beam scans, and kV radiographs at the 4 principal gantry angles. The displacement of the sphere from the ‘imaging isocenter’ (the actual position of a point object that the imaging system would find to be at isocenter) was determined for each imaging mode. This test has been repeated 18 times over a period of one year. Results: The average displacement of the sphere from the imaging isocenter using a half fan technique was found to be 0.9 mm Right, 0.9 mm Anterior, and 1.1 mm Inferior, assuming a head first supine orientation. These offsets are incorporated in image‐guided patient setup procedures. Small systematic errors as a function of gantry angle were also measured for the radiographs. A point at the radiation isocenter will appear about 1mm higher in a right lateral image than in a left lateral image. A similar left / right discrepancy exits for anterior and posterior images.Conclusion: The systematic geometric errors of the kV imaging equipment and associated techniques need to be measured and incorporated into the procedure of on‐line image‐guidedpatient treatment. For the On Board Imager, a geometric accuracy of better than 1mm can be achieved.

TU‐FF‐A3‐04: An In Vivo Comparative Study of the MV and KV Cone Beam Computed Tomography Image Quality of a Lung Patient

Purpose: To compare image quality, reconstruction artifacts and tumorvisibility for kV and MV cone‐beam computed tomography(CBCT) scans reconstructed with the same algorithm. Method and Materials: A protocol lung‐cancer patient was set up in the identical treatment position for kV and MVCBCT using a Varian On‐Board ImagerCBCT and an inhouse MVCBCT imaging system. For both scans the gantry made a 1‐minute, 360° continuous rotation. For the MVCBCT, ∼460 projection images were acquired at 6MV for ∼13 MU; for kVCBCT ∼600 projections were acquired using 125 kVp, 80 mA and 25‐ms exposure time per projection, resulting in ∼2cGy at isocenter. Reconstruction was performed using the Feldkamp back projection algorithm. Both scans were registered to the treatment plan CT. The visibility of three selected regions (bronchus, vertebrae, heart) is compared using the corresponding signal‐to‐noise ratio (SNR). The contrast ratio (CR) and contrast‐to‐noise ratio (CNR) at the tumor are also compared for ease of tumor identification. Results: The SNR of bronchus, vertebrae and heart are 25, 34 and 33 respectively for MVCBCT while the corresponding values in kV scan are 17, 33 and 42. For tumor identifiability, CNR and CR are 11 and 2 respectively for MV scan, and 10 and 2 for kV scan. The CNR of the vertebrae in MV and kV cases are 2 and 6. Time to register the kV image is approximately 50% less than MV image. Similar breathing artifacts are present in both scans. Conclusions: Both kV and MV scans deliver usable images. The tumor can be discriminated from the lung background. Higher bone contrast in kV scan helps to reduce time required to register the scan with the planning CT.Conflict of Interest: Research sponsored by NCI Grant P01‐CA59017 and Varian Medical Systems; Research agreement with Varian Medical Systems.

SU-GG-T-04: Proof-of-Principle Investigation of Computer-Controlled Couch Adjustments for Correcting Drift in Target Position during Radiotherapy

Purpose: Tracking and correcting for respiratory motion during external beam therapy is technically challenging, owing to rapid and irregular variations. This study investigates the feasibility of correcting for respiration‐averaged drift in target position by means of couch adjustments on an accelerator equipped with such capability. This is particularly applicable to hypo‐fractionated radiation therapy where high precision is required subject to longer treatment times. Method and Materials: We consider a treatment scenario consisting of repeated 10s sequences of dose delivery alternating with couch adjustments for a respiration‐monitored patient. The motion period is computed at 1s intervals; averaging the motion signal over 3 periods yields a baseline representing target drift. Application of a Kaiman filter, tuned from a set of 100 patient RPM signals (3–5 minutes), allows prediction of the baseline position 5s in advance and computation of couch corrections every 10s. A motion phantom is programmed to move according to a previously recorded patient respiration signal (Varian RPM). The phantom is monitored by an optical system whose signal is assumed to represent internal target motion. The couch corrections are programmed into the accelerator and synchronized with the phantom motion to evaluate the systems ability to correct for drift. Results: For 6 longer patient respiration signals (21–65 minutes) the Kalman‐predicted correction reduced RMS baseline drift from 2.1mm without correction to 0.6mm. When the motion phantom was programmed with a breathing trace with 10mm drift over 140s, the drift was reduced to 3mm with couch corrections. Conclusion: A proof‐of‐principle is demonstrated for correcting drift in target position using periodic couch adjustments. Future machine capabilities will permit near‐real‐time couch correction computation and application. Preliminary results suggest Kalman‐filter‐based corrections at 10s intervals based on a 5s prediction window are effective at reducing drift. Research sponsored by Varian Medical Systems.

MO‐FF‐A2‐04: An Accurate Mechanical Quality Assurance Procedure for a New High Performance Linac

Purpose T o establish QA procedures for the mechanical systems of a new linac that has been developed to deliver radiation, using image‐guidance, to the target with improved spatial accuracy. The procedures are required to be able to detect mechanical errors of much less than 1 mm, be independent of the linacs own readouts and calibration procedures, and be fast enough for the physicist to perform on a monthly or more frequent schedule. Method and Materials In image guided delivery, mechanical properties that will affect the spatial accuracy with which dose is delivered include: • radiation isocenter ‐imaging origin displacement, • position errors of the jaws and MLC leaves, • accuracy with which the patient support couch can respond to a change in position request. To measure these quantities, we use the machines kV, MV, and infra‐red imaging systems. We report on the techniques used, and the estimates of their accuracy. Results The preliminary estimate of the measurement uncertainty of the radiation isocenter — imaging origin offset is ± 0.3 mm. The observed offset is within the measurement error. The accuracy of the field size seen in the MV images is ± 0.2mm. Couch accuracy for shifts of up to 2 cm, the magnitudes expected using image guidance, was found to be within the measurement error. The ability to control the machine using scripts allows gantry and collimator positioning, couch positioning, beam delivery and imaging in all modes, to be sequenced and performed automatically. Thus the time required for a complete mechanical QA procedure is greatly shortened. Conclusion The imaging components of a new linac can be positioned with sufficient reproducibility and accuracy to allow their use in a mechanical QA program that can achieve the sub‐mm accuracy needed for this machine. Research supported by Varian Medical Systems