Biomedical subjects

Thomas LoSasso

Publications and source records attributed to Thomas LoSasso.

4 recordsLinked to original sources

PubMed2006

Pencil beam approach for correcting the energy dependence artifact in film dosimetry for IMRT verification.

The higher sensitivity to low-energy scattered photons of radiographic film compared to water can lead to significant dosimetric error when the beam quality varies significantly within a field. Correcting for this artifact will provide greater accuracy for intensity modulated radiation therapy (IMRT) verification dosimetry. A procedure is developed for correction of the film energy-dependent response by creating a pencil beam kernel within our treatment planning system to model the film response specifically. Film kernels are obtained from EGSnrc Monte Carlo simulations of the dose distribution from a 1 mm diameter narrow beam in a model of the film placed at six depths from 1.5 to 40 cm in polystyrene and solid water phantoms. Kernels for different area phantoms (50 x 50 cm2 and 25 x 25 cm2 polystyrene and 30 x 30 cm2 solid water) are produced. The Monte Carlo calculated kernel is experimentally verified with film, ion chamber and thermoluminescent dosimetry (TLD) measurements in polystyrene irradiated by a narrow beam. The kernel is then used in convolution calculations to, predict the film response in open and IMRT fields. A 6 MV photon beam and Kodak XV2 film in a polystyrene phantom are selected to test the method as they are often used in practice and can result in large energy-dependent artifacts. The difference in dose distributions calculated with the film kernel and the water kernel is subtracted from film measurements to obtain a practically film artifact free IMRT dose distribution for the Kodak XV2 film. For the points with dose exceeding 5 cGy (11% of the peak dose) in a large modulated field and a film measurement inside a large polystyrene phantom at depth of 10 cm, the correction reduces the fraction of pixels for which the film dose deviates from dose to water by more than 5% of the mean film dose from 44% to 6%.

Algorithms↗

PubMed2005

Influence of phantom material and phantom size on radiographic film response in therapy photon beams.

The energy dependence of radiographic film can introduce dosimetric errors when evaluating photon beams. The variation of the film response, which is attributed to the changing photon spectrum with depth and field size, has been the subject of numerous publications in recent years. However, these data show large unexplained differences in the magnitude of this variation among independent studies. To try to resolve this inconsistency, this study assesses the dependence of radiographic film response on phantom material and phantom size using film measurements and Monte Carlo calculations. The relative dose measured with film exposed to 6 MV x rays in various phantoms (polystyrene, acrylic, Solid Water, and water; the lateral phantom dimensions vary from 25 to 50 cm square; backscatter thickness varies from 10 to 30 cm) is compared with ion chamber measurements in water. Ranges of field size (5 x 5 to 40 x 40 cm2) and depth (dmax to 20 cm) are studied. For similar phantom and beam configurations, Monte Carlo techniques generate photon fluence spectra from which the relative film response is known from an earlier study. Results from film response measurements agree with those derived from Monte Carlo calculations within 3%. For small fields (< or = 10 x 10 cm2) and shallow depths (< or = 10 cm) the film response variation is small, less than 4%, for all phantoms. However, for larger field sizes and depths, the phantom material and phantom size have a greater influence on the magnitude of the film response. The variation of film response, over the ranges of field sizes and depths studied, is 50% in polystyrene compared with 30% in water. Film responses in Solid Water and water phantoms are similar; acrylic is between water and polystyrene. In polystyrene the variation of film response for a 50 cm square phantom is nearly twice that observed in a 25 cm square phantom. This study shows that differences in the configuration of the phantoms used for film dosimetry can explain much of the inconsistency for film response reported in the literature.

Dose-Response Relationship, Radiation↗

PubMed2004

Predicting energy response of radiographic film in a 6 MV x-ray beam using Monte Carlo calculated fluence spectra and absorbed dose.

The advantage of radiographic film is that it allows two-dimensional, high-resolution dose measurement. While there is concern over its photon energy dependence, these problems are considered acceptable within small fields, where the scatter component is small. The application of film dosimetry to intensity modulated radiotherapy (IMRT) raises additional concern since the primary fluence may vary significantly within the field. The varying primary fluence in combination with a large scatter fraction, present for large fields and large depths, causes the spectrum at various points within the IMRT field to differ from the spectrum in the uniform fields typically used for calibrating the film. As a result, significant artifacts are introduced in the measured dose distribution. The purpose of this work is to quantify and develop a method to correct for these artifacts. Two approaches based on Monte Carlo (MC) simulations are examined. In the first method, the film artifact, as quantified by film and ion chamber output measurements in uniform square fields, is derived from the MC calculated ratio of absorbed doses to film and to water. In the second method, the measured film artifact is correlated with MC calculated photon spectra, revealing a strong correlation between the measured artifact and the "scatter"-to-"primary" ratio, defined by the ratio of the number of photons below to the number of photons above 0.1 MeV, independent of field size and depth. These methods are evaluated in high- and low-dose regions of a large intensity-modulated field created with a central block. The spectral approach is also tested with a clinical IMRT field. The absorbed dose method accurately corrects the measured film dose in the open part of the field and in points under the block and outside the field. The dose error is reduced from as much as 16% of the open field dose to less than 1%, as verified with an ion chamber. The spectral method accurately corrects the measured film dose in the open region of the centrally blocked field, but does not fully correct for the film artifact for points under the block and outside the field, where the spectrum is substantially different. Applied to the clinical field, the corrected film measurement shows good agreement with data obtained with a two-dimensional diode array.

Dose-Response Relationship, Radiation↗

PubMedDate not supplied

Intensity-modulated radiotherapy.

Intensity-modulated radiotherapy represents a recent advancement in conformal radiotherapy. It employs specialized computer-driven technology to generate dose distributions that conform to tumor targets with extremely high precision. Treatment planning is based on inverse planning algorithms and iterative computer-driven optimization to generate treatment fields with varying intensities across the beam section. Combinations of intensity-modulated fields produce custom-tailored conformal dose distributions around the tumor, with steep dose gradients at the transition to adjacent normal tissues. Thus far, data have demonstrated improved precision of tumor targeting in carcinomas of the prostate, head and neck, thyroid, breast, and lung, as well as in gynecologic, brain, and paraspinal tumors and soft tissue sarcomas. In prostate cancer, intensity-modulated radiotherapy has resulted in reduced rectal toxicity and has permitted tumor dose escalation to previously unattainable levels. This experience indicates that intensity-modulated radiotherapy represents a significant advancement in the ability to deliver the high radiation doses that appear to be required to improve the local cure of several types of tumors. The integration of new methods of biologically based imaging into treatment planning is being explored to identify tumor foci with phenotypic expressions of radiation resistance, which would likely require high-dose treatments. Intensity-modulated radiotherapy provides an approach for differential dose painting to selectively increase the dose to specific tumor-bearing regions. The implementation of biologic evaluation of tumor sensitivity, in addition to methods that improve target delineation and dose delivery, represents a new dimension in intensity-modulated radiotherapy research.

Abdominal Neoplasms↗