This study explored using a novel diffuse correlation spectroscopy (DCS) flow-oximeter to noninvasively monitor blood flow and oxygenation changes in head and neck tumors during radiation delivery. A fiber-optic probe connected to the DCS flow-oximeter was placed on the surface of the radiologically/clinically involved cervical lymph node. The DCS flow-oximeter in the treatment room was remotely operated by a computer in the control room. From the early measurements, abnormal signals were observed when the optical device was placed in close proximity to the radiation beams. Through phantom tests, the artifacts were shown to be caused by scattered x rays and consequentially avoided by moving the optical device away from the x-ray beams. Eleven patients with head and neck tumors were continually measured once a week over a treatment period of seven weeks, although there were some missing data due to the patient related events. Large inter-patient variations in tumor hemodynamic responses were observed during radiation delivery. A significant increase in tumor blood flow was observed at the first week of treatment, which may be a physiologic response to hypoxia created by radiation oxygen consumption. Only small and insignificant changes were found in tumor blood oxygenation, suggesting that oxygen utilizations in tumors during the short period of fractional radiation deliveries were either minimal or balanced by other effects such as blood flow regulation. Further investigations in a large patient population are needed to correlate the individual hemodynamic responses with the clinical outcomes for determining the prognostic value of optical measurements.
Grid radiation therapy with megavoltage x-ray beam has been proven to be an effective technique for management of large, bulky malignant tumors. The clinical advantage of GRID therapy, combined with conventional radiation therapy, has been demonstrated using a prototype GRID block [Mohiuddin, Curtis, Grizos, and Komarnicky, Cancer 66, 114-118 (1990)]. Recently, a new GRID block design with improved dosimetric properties has become commercially available from Radiation Product Design, Inc. (Albertive, MN). This GRID collimator consists of an array of focused apertures in a cerrobend block arranged in a hexagonal pattern having a circular cross-section with a diameter and center-to-center spacing of 14.3 and 21.1 mm, respectively, in the plane of isocenter. In this project, dosimetric characteristics of the newly redesigned GRID block have been investigated for a Varian 21EX linear accelerator (Varian Associates, Palo Alto, CA). These determinations were performed using radiographic films, thermoluminescent dosimeters in Solid Water phantom materials, and an ionization chamber in water. The output factor, percentage depth dose, beam profiles, and isodose distributions of the GRID radiation as a function of field size and beam energy have been measured using both 6 and 18 MV x-ray beams. In addition, the therapeutic advantage obtained from this treatment modality with the new GRID block design for a high, single fraction of dose has been calculated using the linear quadratic model with alpha/beta ratios for typical tumor and normal cells. These biological characteristics of the new GRID block design will also be presented.
The nucleus '~Sm has been studied with the (n, n'y) reaction and lifetimes of many states have been extracted from the observed Doppler shifts of the deexciting y rays. A large number of fast E1 transitions have been observed and have led to the identification of possible members of the octupole-octupole and the quadrupole-octupole multiplets.Substantial fragmentation of the twophonon octupole strength is indicated.
The intermediate dose spill for a stereotactic radiosurgery (SRS) plan can be quantified with the metric R50%, defined as the 50% isodose cloud volume (VIDC50%) divided by the volume of the planning target volume (PTV). By coupling sound physical principles with the basic definition of R50%, we derive an analytical expression for R50% for a spherical PTV. Our analytical expression depends on three quantities: the surface area of PTV (SAPTV), the volume of PTV (VPTV), and the distance of dose drop‐off to 50% (Δr). The value of ∆r was obtained from a simple set of cranial phantom plan calculations. We generate values from our analytical expression for R50% (R50%Analytic) and compare the values to clinical R50% values (R50%Clinical) extracted from a previously published SRS data set that spans the VPTV range from 0.15 to 50.1 cm3. R50%Analytic is smaller than R50%Clinical in all cases by an average of 15% ± 7%, and the general trend of R50%Clinical vs VPTV is reflected in the same trend of R50%Analytic. This comparison suggests that R50%Analytic could represent a theoretical lower limit for the clinical SRS data; further investigation is required to confirm this. R50%Analytic could provide useful guidance for what might be achievable in SRS planning.
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