C Chuang

Description and dosimetric verification of the PEREGRINE Monte Carlo dose calculation system for photon beams incident on a water phantom.

PEREGRINE is a three-dimensional Monte Carlo dose calculation system written specifically for radiotherapy. This paper describes the implementation and overall dosimetric accuracy of PEREGRINE physics algorithms, beam model, and beam commissioning procedure. Particle-interaction data, tracking geometries, scoring, variance reduction, and statistical analysis are described. The BEAM code system is used to model the treatment-independent accelerator head, resulting in the identification of primary and scattered photon sources and an electron contaminant source. The magnitude of the electron source is increased to improve agreement with measurements in the buildup region in the largest fields. Published measurements provide an estimate of backscatter on monitor chamber response. Commissioning consists of selecting the electron beam energy, determining the scale factor that defines dose per monitor unit, and describing treatment-dependent beam modifiers. We compare calculations with measurements in a water phantom for open fields, wedges, blocks, and a multileaf collimator for 6 and 18 MV Varian Clinac 2100C photon beams. All calculations are reported as dose per monitor unit. Aside from backscatter estimates, no additional, field-specific normalization is included in comparisons with measurements. Maximum discrepancies were less than either 2% of the maximum dose or 1.2 mm in isodose position for all field sizes and beam modifiers.

Investigation of the use of MOSFET for clinical IMRT dosimetric verification

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

Report of AAPM TG 135: Quality assurance for robotic radiosurgery

The task group (TG) for quality assurance for robotic radiosurgery was formed by the American Association of Physicists in Medicines Science Council under the direction of the Radiation Therapy Committee and the Quality Assurance (QA) Subcommittee. The task group (TG-135) had three main charges: (1) To make recommendations on a code of practice for Robotic Radiosurgery QA; (2) To make recommendations on quality assurance and dosimetric verification techniques, especially in regard to real-time respiratory motion tracking software; (3) To make recommendations on issues which require further research and development. This report provides a general functional overview of the only clinically implemented robotic radiosurgery device, the CyberKnife. This report includes sections on device components and their individual component QA recommendations, followed by a section on the QA requirements for integrated systems. Examples of checklists for daily, monthly, annual, and upgrade QA are given as guidance for medical physicists. Areas in which QA procedures are still under development are discussed.

Calibration of an amorphous-silicon flat panel portal imager for exit-beam dosimetry

Amorphous-silicon flat panel detectors are currently used to acquire digital portal images with excellent image quality for patient alignment before external beam radiation therapy. As a first step towards interpreting portal images acquired during treatment in terms of the actual dose delivered to the patient, a calibration method is developed to convert flat panel portal images to the equivalent water dose deposited in the detector plane and at a depth of 1.5 cm. The method is based on empirical convolution models of dose deposition in the flat panel detector and in water. A series of calibration experiments comparing the response of the flat panel imager and ion chamber measurements of dose in water determines the model parameters. Kernels derived from field size measurements account for the differences in the production and detection of scattered radiation in the two systems. The dissimilar response as a function of beam energy spectrum is characterized from measurements performed at various off-axis positions and for increasing attenuator thickness in the beam. The flat panel pixel inhomogeneity is corrected by comparing a large open field image with profiles measured in water. To verify the accuracy of the calibration method, calibrated flat panel profiles were compared with measured dose profiles for fields delivered through solid water slabs, a solid water phantom containing an air cavity, and an anthropomorphic head phantom. Open rectangular fields of various sizes and locations as well as a multileaf collimator-shaped field were delivered. For all but the smallest field centered about the central axis, the calibrated flat panel profiles matched the measured dose profiles with little or no systematic deviation and approximately 3% (two standard deviations) accuracy for the in-field region. The calibrated flat panel profiles for fields located off the central axis showed a small -1.7% systematic deviation from the measured profiles for the in-field region. Out of the field, the differences between the calibrated flat panel and measured profiles continued to be small, approximately 0%-2% of the mean in-field dose. Further refinement of the calibration model should increase the accuracy of the procedure. This calibration method for flat panel portal imagers may be used as part of a validation scheme to verify the dose delivered to the patient during treatment.

7-Tesla Susceptibility-Weighted Imaging to Assess the Effects of Radiotherapy on Normal-Appearing Brain in Patients With Glioma

PURPOSE To evaluate the intermediate- and long-term imaging manifestations of radiotherapy on normal-appearing brain tissue in patients with treated gliomas using 7T susceptibility-weighted imaging (SWI). METHODS AND MATERIALS SWI was performed on 25 patients with stable gliomas on a 7 Tesla magnet. Microbleeds were identified as discrete foci of susceptibility that did not correspond to vessels. The number of microbleeds was counted within and outside of the T2-hyperintense lesion. For 3 patients, radiation dosimetry maps were reconstructed and fused with the 7T SWI data. RESULTS Multiple foci of susceptibility consistent with microhemorrhages were observed in patients 2 years after chemoradiation. These lesions were not present in patients who were not irradiated. The prevalence of microhemorrhages increased with the time since completion of radiotherapy, and these lesions often extended outside the boundaries of the initial high-dose volume and into the contralateral hemisphere. CONCLUSIONS High-field SWI has potential for visualizing the appearance of microbleeds associated with long-term effects of radiotherapy on brain tissue. The ability to visualize these lesions in normal-appearing brain tissue may be important in further understanding the utility of this treatment in patients with longer survival.

Peripheral dose in ocular treatments with CyberKnife and Gamma Knife radiosurgery compared to proton radiotherapy

Peripheral radiation can have deleterious effects on normal tissues throughout the body, including secondary cancer induction and cataractogenesis. The aim of this study is to evaluate the peripheral dose received by various regions of the body after ocular treatment delivered with the Model C Gamma Knife, proton radiotherapy with a dedicated ocular beam employing no passive-scattering system, or a CyberKnife unit before and after supplemental shielding was introduced. TLDs were used for stray gamma and x-ray dosimetry, whereas CR-39 dosimeters were used to measure neutron contamination in the proton experiments. Doses to the contralateral eye, neck, thorax and abdomen were measured on our anthropomorphic phantom for a 56 Gy treatment to a 588 mm(3) posterior ocular lesion. Gamma Knife (without collimator blocking) delivered the highest dose in the contralateral eye, with 402-2380 mSv, as compared with 118-234 mSv for CyberKnife pre-shielding, 46-255 mSv for CyberKnife post-shielding and 9-12 mSv for proton radiotherapy. Gamma Knife and post-shielding CyberKnife delivered comparable doses proximal to the treatment site, with 190 versus 196 mSv at the thyroid, whereas protons doses at these locations were less than 10 mSv. Gamma Knife doses decreased dramatically with distance from the treatment site, delivering only 13 mSv at the lower pelvis, comparable to the proton result of 4 to 7 mSv in this region. In contrast, CyberKnife delivered between 117 and 132 mSv to the lower pelvis. In conclusion, for ocular melanoma treatments, a proton beam employing no double scattering system delivers the lowest peripheral doses proximally to the contralateral eye and thyroid when compared to radiosurgery with the Model C Gamma Knife or CyberKnife. At distal locations in the pelvis, peripheral doses delivered with proton and Gamma Knife are of an order of magnitude smaller than those delivered with CyberKnife.

Peripheral doses in CyberKnife radiosurgery

The purpose of this work is to measure the dose outside the treatment field for conformal CyberKnife treatments, to compare the results to those obtained for similar treatments delivered with gamma knife or intensity-modulated radiation therapy (IMRT), and to investigate the sources of peripheral dose in CyberKnife radiosurgery. CyberKnife treatment plans were developed for two hypothetical lesions in an anthropomorphic phantom, one in the thorax and another in the brain, and measurements were made with LiF thermoluminescent dosimeters (TLD-100 capsules) placed within the phantom at various depths and distances from the irradiated volume. For the brain lesion, gamma knife and 6-MV IMRT treatment plans were also developed, and peripheral doses were measured at the same locations as for the CyberKnife plan. The relative contribution to the CyberKnife peripheral dose from inferior- or superior-oblique beams entering or exiting through the body, internally scattered radiation, and leakage radiation was assessed through additional experiments using the single-isocenter option of the CyberKnife treatment-planning program with different size collimators. CyberKnife peripheral doses (in cGy) ranged from 0.16 to 0.041% (+/- 0.003%) of the delivered number of monitor units (MU) at distances between 18 and 71 cm from the field edge. These values are two to five times larger than those measured for the comparable gamma knife brain treatment, and up to a factor of four times larger those measured in the IMRT experiment. Our results indicate that the CyberKnife peripheral dose is due largely to leakage radiation, however at distances less than 40 cm from the field edge, entrance, or exit dose from inferior- or superior-oblique beams can also contribute significantly. For distances larger than 40 cm from the field edge, the CyberKnife peripheral dose is directly related to the number of MU delivered, since leakage radiation is the dominant component.

Communication and sampling rate limitations in IMRT delivery with a dynamic multileaf collimator system

The delivery of an intensity modulated radiation field with a dynamic multileaf collimator (MLC) requires precise correlation between MLC positions and cumulative monitor units (MUs). The purpose of this study is to investigate the precision of this correlation as a function of delivered MUs and dose rate. A semi-Gaussian shaped intensity profile and a simple geometric intensity pattern consisting of four square segments were designed to deliver a total of 1, 4, 16, 64, and 100 MUs at three different dose rates of 100, 400, and 600 MU/min. The semi-Gaussian intensity pattern was delivered using both sliding window and step and shoot techniques. The dose profiles of this intensity pattern were measured with films. The four square intensity pattern was delivered using step and shoot and conventional delivery techniques for comparison. Because of geometrical symmetry, the dose to each segment in this intensity pattern is expected to be the same when the same MU is assigned to each segment. An ionization chamber was used to measure the dose in the center of each of the four square segments. For the semi-Gaussian shaped profile, significant artifacts were observed when the profile was delivered with small MUs and/or at a high dose rate. For the four square intensity pattern, the dose measured in each segment presented a large variation when delivered with small MUs and a high dose rate. The variation increases as the MU/segment decreases and as the dose rate increases. These MU and dose rate dependencies were not observed when the intensity pattern was delivered using a conventional delivery technique. The observed distortion of the semi-Gaussian profile and dose variations among the segments of the four square intensity pattern are explained by considering the sampling rate and the communication time lag between the control systems. Finally, clinical significance is discussed.

Potential value of MR spectroscopic imaging for the radiosurgical management of patients with recurrent high-grade gliomas

Previous studies have shown that metabolic information provided by 3D Magnetic Resonance Spectroscopy Imaging (MRSI) could affect the definition of target volumes for radiation treatments (RT). This study aimed to (i) investigate the effect of incorporating spectroscopic volumes as determined by MRSI on target volume definition, patient selection eligibility, and dose prescription for stereotactic radiosurgery treatment planning; (ii) correlate the spatial extent of pre-SRS spectroscopic abnormality and treatment volumes with areas of focal recurrence as defined by changes in contrast enhancement; and (iii) examine the metabolic changes following SRS to assess treatment response. Twenty-six patients treated with Gamma Knife radiosurgery for recurrent glioblastoma multiforme (GBM) were retrospectively evaluated. All patients underwent both MRI and MRSI studies prior to SRS. Follow-up MRI exams were available for all 26 patients, with MRI/MRSI available in only 15/26 patients. We observed that the initial CNI 2 contours extended beyond the pre-SRS CE in 25/26 patients ranging in volume from 0.8 cc to 18.8 cc (median 5.6 cc). The inclusion of the volume of CNI 2 extending beyond the CE would have increased the SRS target volume by 5–165% (median 23.4%). This would have necessitated decreasing the SRS prescription dose in 19/26 patients to avoid increased toxicity; the resultant treatment volume would have exceeded 20cc in five patients, thus possibly excluding those from RS treatment per our institutional practice. MRSI follow-up studies showed a decrease in Choline, stable Creatine, and increased NAA indicative of response to SRS in the majority of patients. When combined with patient survival data, metabolic information obtained during follow-up MRSI studies seemed to indicate the potential to help to distinguish necrosis from new/recurrent tumor; however, this should be further verified by biopsy studies.

Peripheral dose measurement for CyberKnife radiosurgery with upgraded linac shielding

The authors investigated the peripheral dose reduction for CyberKnife radiosurgery treatments after the installation of a linac shielding upgrade. As in a previous investigation, the authors considered two treatment plans, one for a hypothetical target in the brain and another for a target in the thorax, delivered to an anthropomorphic phantom. The results of the prior investigation showed that the CyberKnife delivered significantly higher peripheral doses than comparable model C Gamma Knife or IMRT treatments. Current measurements, after the linac shielding upgrade, demonstrate that the additional shielding decreased the peripheral dose, expressed as a percentage of the delivered monitor units (MU), by a maximum of 59%. The dose reduction was greatest for cranial-caudal distances from the field edge less than 30 cm, and at these distances, the CyberKnife peripheral dose, expressed as a percentage of the delivered MU, is now comparable to that measured for the other treatment modalities in our previous investigation. For distances between 30 and 70 cm from the field edge, the additional shielding reduced the peripheral dose by between 20% and 55%. At these distances, the CyberKnife peripheral dose remains higher than doses measured in our previous study for the model C Gamma Knife and IMRT.