Jerry Battista

X‐ray sources of medical linear accelerators: Focal and extra‐focal radiation

A computerized tomography (CT) reconstruction technique has been used to make quantitative measurements of the size and shape of the focal spot in medical linear accelerators. Using this technique, we have measured the focal spots in a total of nine accelerators, including (i) two Varian Clinac 2100cs, (ii) two Atomic Energy of Canada Ltd. (AECL) Therac-25s, (iii) two AECL Therac 6s, (iv) a Siemens KD-2, (v) a Varian Clinac 600c (4 MV), and (vi) an AECL Therac-20. Some of these focal spots were monitored for changes over a 2-yr period. It has been found that (i) the size and shape of the source spot varies greatly between accelerators of different design ranging from 0.5 to 3.4 mm in full width at half maximum (FWHM); and (ii) for accelerators of the same design, the focal spots are very similar. In addition to the measurements of the focal spot, a new technique for measuring the magnitude and distribution of extra-focal radiation originating from the linear accelerator head (flattening filter, primary collimator) has also been developed. The extra-focal radiation produced by a Varian Clinac 2100c accelerator was measured using this technique and it was found that the extra-focal radiation accounts for as much as 8% of the total photon fluence reaching the isocenter. The majority (75%) of this extra-focal radiation originates from within a circle 6 cm in diameter at the target plane. The source MTFs for each of the measured focal spots have been calculated in order to assess their influence on the spatial resolution of verification images. The limiting spatial resolution (i.e., 10% modulation) for all the source MTFs is 1.8 mm-1 or greater when used for transmission radiography at a magnification of 1.2. The extra-focal radiation, which produces a low-frequency drop in the source MTFs of up to 8%, changes with field size. As a result, the source MTFs of linear accelerators depend not only on the design of individual accelerators and image magnification, but also on the field size used when forming an image.

Tracking the dose distribution in radiation therapy by accounting for variable anatomy

The goal of this research is to calculate the daily and cumulative dose distribution received by the radiotherapy patient while accounting for variable anatomy, by tracking the dose distribution delivered to tissue elements (voxels) that move within the patient. Non-linear image registration techniques (i.e., thin-plate splines) are used along with a conventional treatment planning system to combine the dose distributions computed for each 3D computed tomography (CT) study taken during treatment. For a clinical prostate case, we demonstrate that there are significant localized dose differences due to systematic voxel motion in a single fraction as well as in 15 cumulative fractions. The largest positive dose differences in rectum, bladder and seminal vesicles were 29%, 2% and 24%, respectively, after the first fraction of radiation treatment compared to the planned dose. After 15 cumulative fractions, the largest positive dose differences in rectum, bladder and seminal vesicles were 23%, 32% and 18%, respectively, compared to the planned dose. A sensitivity analysis of control point placement is also presented. This method provides an important understanding of actual delivered doses and has the potential to provide quantitative information to use as a guide for adaptive radiation treatments.

Daily monitoring and correction of radiation field placement using a video-based portal imaging system: A pilot study

We have developed a video-based portal imaging system for radiotherapy localization. The system can acquire high quality portal images automatically using short (1-3 monitor unit) irradiations and immediately display the images. The major advantage of the imaging system is that it can be used routinely to check and correct patient positioning before much of the daily irradiation has been delivered. The portal imaging system has been used in a pilot study to monitor five patients during each of their daily treatments. The study has shown that: (i) image quality is sufficiently high to detect discrepancies in field placement from that prescribed on the simulator film; (ii) discrepancies in field placement occur frequently; and, (iii) routine correction of patient and block positioning can reduce the size of these discrepancies. This is the first time that field placement in radiation therapy has been checked and corrected routinely, before the treatment irradiation. However, limitations in the size of the field of view and in the methods of extracting and presenting the geometric information to the users limits the clinical utility of the imaging system. Solutions to these limitations are currently under development.

X-ray scatter in megavoltage transmission radiography: physical characteristics and influence on image quality.

The physical characteristics of x rays scattered by the patient and reaching the imaging detector, as well as their effect on verification (portal) image quality, were investigated for megavoltage (0.1-20 MeV) x-ray beams. Monte Carlo calculations and experimental measurements were used to characterize how the scatter and primary fluences at the detector plane were influenced by scattering geometry and the energy spectrum of the incident beam. The calculated scatter fluences were differentiated according to photon energy and scattering process. Scatter fractions were measured on a medical linear accelerator (Clinac 2100c, 6 MV) for a typical imaging geometry using an ionization chamber and a silicon diode. After correction for the energy dependence of the chamber and diode, the scatter fractions generated by the Monte Carlo simulations were found to be in excellent agreement with the measured results. In order to estimate the effect of scatter on image quality, the scatter and primary signals (i.e., energy deposited) produced in five different types of portal imaging detectors (lead plate/film, storage phosphor alone, lead plate/storage phosphor, compton recoil-electron detector, and a copper plate/Gd2O2S phosphor) were calculated. The results show that, for a specified geometry, the scatter fraction can vary by an order of magnitude, depending on the sensitivity of the imaging detector to low-energy (< 1 MeV) scattered radiation. For a common portal imaging detector (copper plate/Gd2O2S phosphor), the scattered radiation (i) reduced contrast by much as 50% for a fixed display-contrast system, and (ii) decreased the differential-signal-to-noise ratio (DSNR) by 10%-20% for a quantum-noise-limited portal imaging system. For currently available TV-camera-based portal imaging systems, which have variable display contrast, the reduction in DSNR depends on the light collection efficiency and the noise characteristics of the TV camera. Overall, these results show that scattered radiation can reduce contrast significantly in portal films while deteriorating image quality only moderately in on-line systems.

VARIABILITY OF TARGET VOLUME DELINEATION IN CERVICAL ESOPHAGEAL CANCER

PURPOSE Three-dimensional (3D) conformal radiation therapy (CRT) assumes and requires the precise delineation of the target volume. To assess the consistency of target volume delineation by radiation oncologists, who treat esophageal cancers, we have performed a transCanada survey. MATERIALS AND METHODS One of three case presentations, including CT scan images, of different stages of cervical esophageal cancer was randomly chosen and sent by mail. Respondents were asked to fill in questionnaires regarding treatment techniques and to outline boost target volumes for the primary tumor on CT scans, using ICRU-50 definitions. RESULTS Of 58 radiation oncologists who agreed to participate, 48 (83%) responded. The external beam techniques used were mostly anterior-posterior fields, followed by a multifield boost technique. Brachytherapy was employed by 21% of the oncologists, and concurrent chemotherapy by 88%. For a given case, and the three volumes defined by ICRU-50 (i.e., gross tumor volume [GTV], clinical target volume [CTV], and planning target volume [PTV]) we determined: 1. The total length in the cranio-caudal dimension; 2. the mean diameter in the transverse slice that was located in a CT slice that was common to all participants; 3. the total volume for each ICRU volume; and 4. the (5, 95) percentiles for each parameter. The PTV showed a mean length of 14.4 (9.6, 18.0) cm for Case A, 9.4 (5.0, 15.0) cm for Case B, 11.8 (6.0, 16.0) cm for Case C, a mean diameter of 6.4 (5.0, 9.4) cm for Case A, 4.4 (0.0, 7.3) cm for Case B, 5.2 (3.9, 7.3) cm for Case C, and a mean volume of 320 (167, 840) cm3 for Case A and 176 (60, 362) cm3 for Case C. The results indicate variability factors (95 percentile divided by 5 percentile values) in target diameters of 1.5 to 2.6, and in target lengths of 1.9 to 5.0. CONCLUSION There was a substantial inconsistency in defining the planning target volume, both transversely and longitudinally, among radiation oncologists. The potential benefits of 3D treatment planning with high-precision dose delivery could be offset by this inconsistency in target-volume delineation by radiation oncologists. This may be particularly important for multicenter clinical trials, for which quality assurance of this step will be essential to the interpretation of results.

Limitations of a convolution method for modeling geometric uncertainties in radiation therapy. I. The effect of shift invariance

Convolution methods have been used to model the effect of geometric uncertainties on dose delivery in radiation therapy. Convolution assumes shift invariance of the dose distribution. Internal inhomogeneities and surface curvature lead to violations of this assumption. The magnitude of the error resulting from violation of shift invariance is not well documented. This issue is addressed by comparing dose distributions calculated using the Convolution method with dose distributions obtained by Direct Simulation. A comparison of conventional Static dose distributions was also made with Direct Simulation. This analysis was performed for phantom geometries and several clinical tumor sites. A modification to the Convolution method to correct for some of the inherent errors is proposed and tested using example phantoms and patients. We refer to this modified method as the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over different beam arrangements in the various phantom examples) was 21% with the Static dose calculation, 9% with Convolution, and reduced to 5% with the Corrected Convolution. The average maximum dose error in the calculated volume (averaged over four clinical examples) was 9% for the Static method, 13% for Convolution, and 3% for Corrected Convolution. While Convolution can provide a superior estimate of the dose delivered when geometric uncertainties are present, the violation of shift invariance can result in substantial errors near the surface of the patient. The proposed Corrected Convolution modification reduces errors near the surface to 3% or less.

Three-Dimensional Dose Verification for Intensity-Modulated Radiation Therapy in the Radiological Physics Centre Head-and-Neck Phantom Using Optical Computed Tomography Scans of Ferrous Xylenol–Orange Gel Dosimeters

PURPOSE To extend the Radiological Physics Centre (RPC) intensity-modulated radiation therapy dose verification protocol to three dimensions using optical computed tomography (CT) scans of ferrous xylenol-orange (FX) gels. METHODS AND MATERIALS The dosimetry insert in the RPC head-and-neck phantom was replaced with an FX cylindrical gel dosimeter. Two gels were calibrated, independently irradiated with 6-MV X-ray beams and scanned using laser and cone-beam (Vista) optical CT, respectively. For matching dose slices, measured dose distributions were compared with Pinnacle3 computed distributions. RESULTS Within high-dose regions and low gradients, doses measured using laser CT were 2% to 3% less than the computed dose, whereas with cone-beam CT they were 4% to 5% less. Inside the central 90% of the gel cylinder diameter, the fraction of voxels satisfying the two-dimensional gamma analysis (5% dose difference, 3-mm distance to agreement) with laser-beam- and cone-beam-measured dose distributions were 98.4% and 99.0%, respectively. A three-dimensional gamma analysis with cone-beam data revealed that 96.7% of voxels within the central 90% gel volume satisfied the above criteria. Within the axial and sagittal planes through the primary planning target volume (PTV), computed and measured doses using GAFChromicEBT film (RPC measured) and cone-beam scanned FX gel generally agreed. At equivalent points in the planning target volumes, computed, thermoluminescent dosimeter (RPC-measured), and gel point doses agreed to within 5.1% in absolute dose. CONCLUSIONS Laser and cone-beam CT yield comparable dose distributions in high-dose regions. The RPC head phantom and optical CT-scanned FX gels can be used for accurate intensity-modulated radiation therapy dose verification in three dimensions.

Limitations of a convolution method for modeling geometric uncertainties in radiation therapy. II. The effect of a finite number of fractions

Convolution methods can be used to model the effect of geometric uncertainties on the planned dose distribution in radiation therapy. This requires several assumptions, including that the patient is treated with an infinite number of fractions, each delivering an infinitesimally small dose. The error resulting from this assumption has not been thoroughly quantified. This is investigated by comparing dose distributions calculated using the Convolution method with the result of Stochastic simulations of the treatment. Additionally, the dose calculated using the conventional Static method, a Corrected Convolution method, and a Direct Simulation are compared to the Stochastic result. This analysis is performed for single beam, parallel opposed pair, and four-field box techniques in a cubic water phantom. Treatment plans for a simple and a complex idealized anatomy were similarly analyzed. The average maximum error using the Static method for a 30 fraction simulation for the three techniques in phantoms was 23%, 11% for Convolution, 10% for Corrected Convolution, and 10% for Direct Simulation. In the two anatomical examples, the mean error in tumor control probability for Static and Convolution methods was 7% and 2%, respectively, of the result with no uncertainty, and 35% and 9%, respectively, for normal tissue complication probabilities. Convolution provides superior estimates of the delivered dose when compared to the Static method. In the range of fractions used clinically, considerable dosimetric variations will exist solely because of the random nature of the geometric uncertainties. However, the effect of finite fractionation appears to have a greater impact on the dose distribution than plan evaluation parameters.

Dose calculations using convolution and superposition principles: the orientation of dose spread kernels in divergent x-ray beams.

The convolution/superposition method of dose calculation has the potential to become the preferred technique for radiotherapy treatment planning. When this approach is used for therapeutic x-ray beams, the dose spread kernels are usually aligned parallel to the central axis of the incident beam. While this reduces the computational burden, it is more rigorous to tilt the kernel axis to align it with the diverging beam rays that define the incident direction of primary photons. We have assessed the validity of the parallel kernel approximation by computing dose distributions using parallel and tilted kernels for monoenergetic photons of 2, 6, and 10 MeV; source-to-surface distances (SSDs) of 50, 80, and 100 cm; and for field sizes of 5 x 5, 15 x 15, and 30 x 30 cm2. Over most of the irradiated volume, the parallel kernel approximation yields results that differ from tilted kernel calculations by 3% or less for SSDs greater than 80 cm. Under extreme conditions of a short SSD, a large field size and high incident photon energy, the parallel kernel approximation results in discrepancies that may be clinically unacceptable. For 10-MeV photons, we have observed that the parallel kernel approximation can overestimate the dose by up to 4.4% of the maximum on the central axis for a field size of 30 x 30 cm2 applied with a SSD of 50 cm. Very localized dose underestimations of up to 27% of the maximum dose occurred in the penumbral region of a 30 x 30-cm2 field of 10-MeV photons applied with a SSD of 50 cm.

Ytterbium‐169: Calculated physical properties of a new radiation source for brachytherapy

Seeds containing radioactive Ytterbium-169 (169Yb) have recently been manufactured for possible application to brachytherapy. Ytterbium-169 emits photons with an average energy of 93 keV (excluding energies less than 10 keV), and decays with a half-life of 32 days. Analytic and Monte Carlo computations have been used to predict physical quantities useful in treatment planning and radiation protection. Analytic calculations based on the primary photon spectrum of 169Yb (excluding energies less than 10 keV) yield an air-kerma rate constant of 0.0427 cGy cm2 h-1 MBq-1, and an exposure rate constant of 1.80 R cm2 mCi-1 h-1 for this radionuclide. Calculated fmed factors are 0.922 cGy/R for soft tissue and 2.12 cGy/R for bone. The first half-value layer in lead is 0.2 mm; the first tenth-value layer is 1.6 mm. Using Monte Carlo simulations, the relative dose distributions around 169Yb seeds (Amersham, prototypes 4 and 5) are provided, and are then compared with those around an 125I seed (3M model 6702). The 169Yb seeds produce more isotropic dose distributions, and for permanent implants, can deliver it at a greater initial dose rate. A value of 1.19 cm-2 was also calculated for the specific dose constant D0, a value which is applicable to both seed types. Radiation protection is not as easily achieved for permanent implants with 169Yb because of the higher energy emissions (vs 125I). However, for temporary implants, Ytterbium-169 may prove to be a useful substitute for 192Ir or 137Cs because of its relatively lower energy emissions. It is concluded that 169Yb merits further investigation, including dosimetry, radiobiological, and clinical studies.