James C.H. Chu

Three-dimensional photon dosimetry: a comparison of treatment of the intact breast in the supine and prone position.

PURPOSE To compare the adequacy of target coverage, dose homogeneity, and volume of normal tissue irradiated in treatment of the intact breast in the supine and prone position. METHODS AND MATERIALS Fifteen patients with early breast cancer who presented for treatment to the intact breast after excisional biopsy were studied. A specially designed device was used for the prone setup to displace the contralateral breast away from the tangential field borders. Treatment planning computed tomography was performed for each patient in both the supine and prone positions. Dosimetric data were obtained in both positions and isodose distributions were calculated for each patient in both positions. RESULTS The volume of breast receiving greater than 5% of the prescribed dose was significantly less in the prone position. Medial wedges were either not used or their angles were reduced for all patients in the prone position compared with the supine position. The average volume of lung receiving >10 Gy and >20 Gy was significantly less in the prone positions. The volume of heart irradiated at critical dose levels did not vary consistently in the prone and supine positions. The integral dose delivered to the contralateral breast was not significantly different. CONCLUSION Treatment of the intact breast in the prone position may result in improved dose homogeneity within the target volume as well as sparing of normal lung compared with treatment in the conventional supine position.

CT and PET lung image registration and fusion in radiotherapy treatment planning using the chamfer-matching method

PURPOSE We present a validation study of CT and PET lung image registration and fusion based on the chamfer-matching method. METHODS AND MATERIALS The contours of the lung surfaces from CT and PET transmission images were automatically segmented by the thresholding technique. The chamfer-matching technique was then used to register the extracted lung surfaces. Arithmetic means of distance between the two data sets of the pleural surfaces were used as the cost function. Matching was then achieved by iteratively minimizing the cost function through three-dimensional (3D) translation and rotation with an optimization method. RESULTS Both anatomic thoracic phantom images and clinical patient images were used to evaluate the performance of our registration system. Quantitative analysis from five patients indicates that the registration error in translation was 2-3 mm in the transverse plane, 3-4 mm in the longitudinal direction, and about 1.5 degree in rotation. Typical computing time for chamfer matching is about 1 min. The total time required to register a set of CT and PET lung images, including contour extraction, was generally less than 30 min. CONCLUSION We have implemented and validated the chamfer-matching method for CT and PET lung image registration and fusion. Our preliminary results show that the chamfer-matching method for CT and PET images in the lung area is feasible. The described registration system has been used to facilitate target definition and treatment planning in radiotherapy.

Dose to contralateral breast: A comparison of four primary breast irradiation techniques

PURPOSE Contralateral breast dose from primary breast irradiation has been implicated in the risk of second breast malignancies. It has been previously shown that the use of half-beam blocking can increase the opposite breast dose by a factor of five. This study evaluates four different breast treatment techniques to compare the radiation dose to the contralateral breast. METHODS AND MATERIALS Dose measurements were made using thermoluminescent dosimeters (TLD) capsules, which were placed in the Rando phantom in the following locations in the contralateral breast: seven along the central axis plane, on at 5 cm superior to, and one 5 cm inferior to the central axis plane. One TLD capsule was placed in the midcenter of the treated breast. The following radiation techniques were used: (a) half-beam with a custom block (HB+CB), (b) half-beam using asymmetric collimator jaw (HB/AJ), (c) half-beam using asymmetric collimator jaw with custom block (H/AJ+CB), and (d) isocentric technique with nondivergent posterior borders [Joint Center for Radiation Therapy (JCRT) techique]. For each technique, isodose distributions for the Rando phantom were optimized using (a) 15 degree medial and lateral compensating wedges, and (b) a single 30 degree lateral compensating wedge. The phantom was treated with 6 MV photons. Each technique was repeated six times, and the TLD readings were averaged. RESULTS The custom cerrobend half-beam block technique gives the highest contralateral breast dose, regardless of wedge. The remaining techniques give results in a similar range, with the asymmetric jaw with no medial wedge technique giving the lowest total dose (p = not significant). The use of a medial wedge increases the opposite breast dose for all techniques. The asymmetric half-beam technique gives significantly less dose than the cerrobend half-beam technique, due to decreased transmission through the asymmetric collimators. The asymmetric jaw vs JCRT technique results in similar contralateral breast dose. CONCLUSIONS As expected, dose to the contralateral breast increases with the use of a medial wedge. Cerrobend half-beam blocking gives the highest opposite breast dose. The lowest contralateral breast dose is with the asymmetric jaw with no medial wedge and no block. The asymmetric jaw technique with block yields equivalent contralateral breast doses to the JCRT technique.

Applications of simulator computed tomography number for photon dose calculations during radiotherapy treatment planning

PURPOSE To study the variation of computed tomography (CT) number from a simulator-based scanner and the effect of this variation on photon-dose calculations. METHOD AND MATERIALS CT images of a cylindrical phantom with multiple inserts were obtained using a commercially-available simulator-CT (Ximatron: Varian, Palo Alto, CA). The linear correlation coefficient and Chi-square methods were used to determine the X-ray effective energy in a phantom. CT numbers in Hounsfield units (HU) were measured as a function of phantom size, orientation, field of view (FOV), distance from the center, and time for various inserts. The change of dose calculations due to the CT number variations was then determined using the equivalent path-length (EPL) and collapsed cone convolution methods. RESULTS AND DISCUSSION A significant beam-hardening effect was observed for the simulator-CT. Consequently, the CT number from the sim-CT was more sensitive to the size of the phantom than those from a conventional CT. The sim-CT number is not sensitive to the locations within the phantom and is stable over a 6-week period. It is important to use the proper FOV for sim-CT studies; scanning a small polystyrene phantom using a large FOV may result in an increase of l20 HU in CT number at the center of the field. However, the dose-calculation variations, due to the CT number uncertainty, do not exceed 2-3% for 6-18 MV photon beams. CONCLUSION The simulator CT images were acquired with patients in the treatment position, and these CT numbers are useful for CT-based dose calculations.

Cosmetic Outcome and Incidence of Infection with the MammoSite Breast Brachytherapy Applicator

Abstract:  We present our results regarding the cosmetic outcome achieved and the rate of infection using the MammoSite breast brachytherapy applicator to treat patients with partial breast irradiation. In addition, factors associated with cosmetic outcome and infection are analyzed. The study population consisted of 30 patients with early stage breast cancer treated using the MammoSite device from October 28, 2002, to February 13, 2004. Cosmetic outcome was analyzed for its association with the following parameters: volume of the balloon, balloon‐to‐skin distance, maximal skin point dose per fraction, V100 (percent of volume that received 100% of the prescription dose), V150 (percent of volume that received 150% of the prescription dose), and V200 (percent of volume that received 200% of the prescription dose). The occurrence of infection at the time of treatment and during follow‐up was also recorded. At a median follow‐up of 13 months (range 1–16 months), 53.3% of the patients (16/30) were reported to have an excellent cosmetic outcome and 40.0% (12/30) had a good cosmetic outcome. Excellent cosmetic outcome was associated with a greater mean balloon‐to‐skin distance compared to those who achieved a good cosmetic outcome (1.5 cm versus 1.2 cm) (p = 0.164). The mean V100, V150, and V200 of those in the excellent cosmetic outcome group were 92.1%, 34.5%, and 7.6% versus 93.0%, 34.7%, and 7.6% in the good cosmetic outcome group (p = 0.642, 0.926, and 0.853), The mean balloon volumes were 47.7 cm3 and 56.9 cm3, respectively (p = 0.063) in the excellent and good outcome groups. The mean maximal skin doses per fraction in the excellent and good outcome groups were 354.8 cGy and 422.3 cGy (p = 0.286), respectively. Infection occurred in 13.3% of the patients (4/30). An excellent or good cosmetic outcome was achieved in 93.3% of patients and infection occurred in 13.3% of patients treated with the MammoSite breast brachytherapy applicator. Excellent cosmetic outcome was associated with a greater balloon‐to‐skin distance, lower maximal skin dose per fraction, and smaller mean balloon volume; however, the results did not reach statistical significance. 

Dosimetric characteristics of the Leipzig surface applicators used in the high dose rate brachy radiotherapy

The nucletron Leipzig applicator is designed for (HDR) 192Ir brachy radiotherapy of surface lesions. The dosimetric characteristics of this applicator were investigated using simulation method based on Monte Carlo N-particle (MCNP) code and phantom measurements. The simulation method was validated by comparing calculated dose rate distributions of nucletron microSelectron HDR 192Ir source against published data. Radiochromic films and metal-oxide-semiconductor field-effect transistor (MOSFET) detectors were used for phantom measurements. The double exposure technique, correcting the nonuniform film sensitivity, was applied in the film dosimetry. The linear fit of multiple readings with different irradiation times performed for each MOSFET detector measurement was used to obtain the dose rate of each measurement and to correct the source transit-time error. The film and MOSFET measurements have uncertainties of 3%-7% and 3%-5%, respectively. The dose rate distributions of the Leipzig applicator with 30 mm opening calculated by the validated MC method were verified by measurements of film and MOSFET detectors. Calculated two-dimensional planar dose rate distributions show similar patterns as the film measurement. MC calculated dose rate at a reference point defined at depth 5 mm on the applicators central axis is 7% lower than the film and 3% higher than the MOSFET measurements. The dose rate of a Leipzig applicator with 30 mm opening at reference point is 0.241+/-3% cGy h(-1) U(-1). The MC calculated depth dose rates and profiles were tabulated for clinic use.

Dose perturbation induced by radiographic contrast inside brachytherapy balloon applicators

Phantom measurements and Monte Carlo calculations have been performed for the purpose of characterizing the dose perturbation caused by radiographic contrast inside the MammoSite breast brachytherapy applicator. Specifically, the dose perturbation is quantified as a heterogeneity correction factor (HCF) for various balloon radii and contrast concentration levels. The dose perturbation is larger for larger balloon radii and higher contrast concentrations. Based on a validated Monte Carlo simulation, the calculated HCF values are 0.99 for a 2 cm radius balloon and 0.98 for a 3 cm radius balloon at 6% contrast concentration levels, and 0.89 and 0.87 for 2 and 3 cm radius balloons, respectively, at 100% contrast concentrations. For a typical implanted balloon radius of 2.4 cm, the HCF values decrease from 0.99 at 6% contrast concentration to 0.90 at 100% contrast concentration. For balloons implanted in patients at our institution, the mean HCF is 0.99, corresponding to a dose reduction of approximately 1%. The contrast effect results in a systematic reduction in the delivered dose, therefore the minimal amount of radiographic contrast necessary should be used.

Evaluation and characterization of parallel plate microchamber’s functionalities in small beam dosimetry

A parallel plate microchamber (PPMC) has been designed to specifically address the problems of small beam dosimetry. The chambers extremely small volume and tissue equivalency theoretically make it possible for the chamber to perform an ideal measurement for small field dosimetry. Results show the PPMC to be a simple and reproducible detector for the measurements of total scattering factors, percentage depth doses, and off-axis ratios. Even with its unique geometry, the PPMC requires a correction factor when measuring total scatter factors of fields smaller than 2.5 cm in diameter. Results obtained with the PPMC for fields greater than 2.5 cm diameter closely match those of alternative measurement modalities. The exceptionally small volume of the chamber increases the effect of radiation-induced cable currents. With careful experimental technique, this problem can be resolved. Monte Carlo simulations of a Sun Nuclear QED low build-up diode were done to show that no correction factor is needed for the diode in measuring total scatter factors of small fields. However, the scattering factors measured with the PPMC should be corrected for cone fields smaller than 2.5 cm in diameter. With the correction factor, the scattering factor obtained with the PPMC matches that with the QED diode within 0.7%. The percent depth dose data taken with the PPMC for a 40 x 40 cm2 field closely matches that taken with the PTW chamber with the largest deviation being approximately 1.2% at a depth of 30 cm. For a measurement of the off-axis ratio with stereotactic cones of diameter 1.25 and 4.0 cm, the data obtained with the PPMC have a good agreement (less than 0.5% difference) with the film measurement.

Dose enhancement by a thin foil of high-Z material: A Monte Carlo study

The purpose of this work is to study the dose enhancement by a thin foil (thickness of 0.2-4 mm) of high-Z material in a water phantom, irradiated by high-energy photon beams. EGS4 Monte Carlo technique was used. Perturbations on the beam spectra due to the presence of the foils, and dose enhancement dependence of photon-beam quality, beam incident angle, atomic number (Z), the thickness and size of the foil, and the depth of the foil situated in the phantom were studied. Analysis of photon and secondary-electron spectra indicates that the dose enhancement near an inhomogeneity interface is primarily due to secondary electrons. A calculation for 1-mm-thick planar lead foil in a water phantom shows that the dose enhancements at 0.25, 1, 2 and 3 mm away from the foil in the backward region were 58%, 37%, 24% and 17%, respectively, for a 15 MV beam. Calculations for a variety of planar foils and photon beams show that dose enhancement: (a) increases with Z; (b) decreases with decreasing foil thickness when the foils are thinner than a certain value (1 mm for lead foil for 15 MV); (c) decreases with decreasing incident photon-beam energies; (d) changes slightly for beam incident angles less than 45 degrees and more prominently for larger angles; (e) increases with size of foil; and (f) is almost independent of the depth at which the foil is situated when the foil is placed beyond the range of secondary electrons. The dose enhancement calculation is also performed for a cylindrically shaped lead foil irradiated by a four-field-box. The dose enhancement of 34%/13% was obtained at 0.25/2 mm away from the cylindrical outer interface for a 15 MV four-field-box.

Dynamic wedge versus physical wedge: A Monte Carlo study

The purpose of this study is to analyze the characteristics of dynamic wedges (DW) and to compare DW to physical wedges (PW) in terms of their differences in affecting beam spectra, energy fluence, angular distribution, contaminated electrons, and dose distributions. The EGS4/BEAMMonte Carlo codes were used to simulate the exact geometry of a 6 MV beam and to calculate 3-D dose distributions in phantom. The DW was simulated in accordance with the segmented treatment tables (STT). The percentage depth dose curves and beam profiles for PW, DW, and open fields were measured and used to verify the Monte Carlo simulations. The Monte Carlo results were found to agree within 2% with the measurements performed using film and ionizing chambers in a water phantom. The present EGS4 calculation reveals that the effects of a DW on beam spectral and angular distributions, as well as electron contamination, are much less significant than those for a PW. For the 6 MV photon beam, a 45° PW can result in a 30% increase in mean photon energy due to the effect of beam hardening. It can also introduce a 5% dose reduction in the build-up region due to the reduction of contaminated electrons by the PW. Neither this mean-energy increase nor such dose reduction is found for a DW. Compared to a DW, a PW alters the photon-beam spectrum significantly. The dosimetric differences between a DW and a PW are significant and clearly affect the clinical use of these beams. The data presented may be useful for DW commissioning.