A Monte Carlo evaluation of beam characteristics for total body irradiation at extended treatment distances
The aim is to study beam characteristics at large distances when focusing on the electron component. In particular, to investigate the utility of spoilers with various thicknesses as an electron source, as well as the effect of different spoiler‐to‐surface distances (STSD) on the beam characteristics and, consequently, on the dose in the superficial region. A MC model of a 15 MV Varian accelerator, validated earlier by experimental data at isocenter and extended distances used in large‐field total body irradiation, is applied to evaluate beam characteristics at distances larger than 400 cm. Calculations are carried out using BEAMnrc/DOSXYZnrc code packages and phase space data are analyzed by the beam data processor BEAMdp. The electron component of the beam is analyzed at isocenter and extended distances, with and without spoilers as beam modifiers, assuming vacuum or air surrounding the accelerator head. Spoiler thickness of 1.6 cm is found to be optimal compared to thicknesses of 0.8 cm and 2.4 cm. The STSD variations should be taken into account when treating patients, in particular when the treatment protocols are based on a fixed distance to the patient central sagittal plane, and also, in order to maintain high dose in the superficial region. PACS numbers: 87.55.D‐, 87.55.de, 87.55.K‐
Monte Carlo Dosimetry of Organ Doses from a Sweeping-Beam Total Body Irradiation Technique: Feasibility and First Results
Total body irradiation (TBI) is a radiation treatment often purposed to suppress the immune system prior to a bone marrow transplant. Several toxicities can arise in TBI, and high-quality dose volume data for organs at risk are required if one is considering any change from a well-established technique. We present a novel Monte Carlo (MC) dosimetry technique to acquire this data based on our current TBI technique that accounts for a sweeping-beam Cobalt-60 delivery, a stationary flattening filter, patient-specific lung compensators, and two patient treatment positions. For each patient, a virtual MC phantom is created including the planning CT image in each treatment position (supine and prone). The results from dose simulations on each phantom were summed together geometrically with a deformable registration tool. Dose volume statistics for lungs, liver, thyroid, and kidneys are obtained. The preliminary results of a retrospective study using this technique on patients who have received TBI at our clinic indicate that, for a total body prescription dose of 12 Gy ± 10%, the mean body dose ranged from 11.19 to 12.15 Gy with smaller patients receiving lower mean body doses than larger patients. The mean dose delivered to the thyroid was the highest of the contoured organs receiving up to 12.84 Gy, and the lung doses were the most heterogeneous, with standard deviations up to 0.73 Gy in individual patients. This high-quality dose data shows promise for use in both routine quality assurance of our current technique, and to provide baseline data for development of a new technique. The technique could also be adapted to TBI techniques at other clinics that include compensators, flattening filters, moving beams, and/or multiple treatment positions.
Chemical, biological, and immunological characterization of mono- and diiodothyrotropin releasing factor.
Abstract Iodination of thyrotropin releasing factor (TRF) with chloramine-T and NaI at pH 7.5 produces both mono- and diiodo-TRF as well as unreacted TRF and I − . The four compounds can be separated by ion-exchange chromatography on SP-Sephadex C-25, using a stepwise gradient of 0.01 m MH 4 OAc at pH 3.5, 5.2 and 7.2. The monoiodo-TRF was characterized by nuclear magnetic resonance, mass spectra, and elemental analysis as [Im-5-iodo-His 2 ]-TRF while the diiodo-TRF was similarly characterized as [Im-2,5-diiodo-His 2 ]-TRF. Both iodination products showed no thyrotropin releasing activity at doses up to 1 μg in the in vivo mouse bioassay. The specific binding of mono- and diiodo-TRF was measured against three different TRF antisera. In the absence of TRF, antisera A and C bound more diiodo-TRF than monoiodo-TRF while antiserum B seemed to bind both tracers equally. In all cases steeper curves were produced when diiodo-TRF was displaced by synthetic TRF.