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Results for “Photon radiotherapy”

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At least 19 recordsLinked to original sources
PubMed2026

Robust optimisation for photon radiotherapy: A scoping review of models, paradigms, and reporting.

BACKGROUND AND PURPOSE: Robust optimisation offers an alternative to conventional margin-based photon radiotherapy planning by explicitly modelling uncertainty, but practice is variable and not standardised. MATERIALS AND METHODS: A scoping review was conducted to map robust optimisation for photon external beam radiotherapy. Electronic searches of Scopus, PubMed and Google Scholar (2000-2025, English language) identified planning studies that incorporated modelled uncertainties into the optimisation process and reported at least one robustness-related outcome. Data were charted on clinical context, uncertainty models, optimisation paradigms, robustness metrics and evidence for clinical implementation. RESULTS: Seventy-one studies were included. Most investigated prostate, breast or lung cancer and used intensity-modulated radiotherapy or volumetric-modulated arc therapy in commercial or research treatment planning systems. Scenario-based worst-case (minimax) optimisation was the dominant paradigm in clinically oriented work, while chance-constrained, conditional value at-risk, distributionally robust and adaptive formulations were confined to small methodological series. Uncertainty modelling focused mainly on rigid set-up error; fewer studies incorporated respiratory motion, inter-fraction anatomical change, dose-calculation uncertainty or biological variation. Robustness was evaluated with diverse scenario-based dose-volume metrics, probabilistic coverage measures, composite robustness indices and, less often, biological endpoints. Direct clinical implementation reports were scarce. CONCLUSION: Robust photon planning is technically feasible and generally maintains or improves target coverage and organ sparing compared with margin-based planning. However, heterogeneity in uncertainty models, optimisation configuration and robustness reporting limits comparison and synthesis. Pragmatic minimum standards are proposed to support future consensus and wider clinical adoption.

Humans↗
PubMedDate not supplied

Computer generated scatter dose distributions for 6-MV radiotherapy photon beams.

The time required for accurate computer dose distributions is of major concern in radiation therapy. Isodose distributions were obtained by calculating the primary and scatter radiation reaching a point. A description of the primary dose distribution, especially at the edge of the beam, requires the calculation of the dose on a finely spaced grid. However, the scatter dose distributions can be described on a much coarser grid. The primary and scatter dose distribution calculations were separated into two computer subroutines which permitted choosing the appropriate grid spacing for each dose component. A more rational partition of the computing time for the primary and scatter dose was obtained. In addition, the separation of the calculation of the scatter dose from the total dose allows one to examine the calculated scatter dose distribution as the radial and angular increments used in the Clarkson-Cunningham summation are varied. Accurate scatter dose distributions were generated with a tenfold time reduction by optimization of grid spacing and radial and angular increments.

Computers↗
PubMedDate not supplied

Measurement of photon dose fraction in a neutron radiotherapy beam.

Photon dose fractions (PDFs) have been measured in and around a neutron radiotherapy beam with a tissue-equivalent proportional counter (TEPC) and with paired ion chambers. The PDFs were found to increase linearly with increasing field size and width depth in phantom. PDFs were shown to decrease with decreasing phantom size and to be larger in the shielded region of the phantom than in the direct beam. Uncertainties in the PDF values were estimated to be 10%-15% for the TEPC measurements but about 50% for the measurement made with ion chambers.

Elementary Particles↗
PubMed1976

Bone scanning with 99m-Tc-phosphates: a comparison and problems in the detection of tumor metastasis.

A comparative study on 99m-Tc-phosphate compounds (TcPP) in detecting tumor metastasis to bone and problems accompanying it are reported. TcPP revealed metastatic foci which are unrecognized by conventional bone survey. To recognize these foci, exclusion of following problems is necessary: Accumulation at front of neck, asymmetrical image of joint, increased bone density of the aged, Tc-photon absorption and radiotherapy effect. The mechanism of TcPP accumulation is discussed.

Absorption↗
PubMedDate not supplied

Neutron spectral measurements in an intense photon field associated with a high-energy x-ray radiotherapy machine.

High-energy x-ray radiotherapy machines in the supermegavoltage region generate complex neutron energy spectra which make an exact evaluation of neutron shielding difficult. Fast neutrons resulting from photonuclear reactions in the x-ray target and collimators undergo successive collisions in the surrounding materials and are moderated by varying amounts. In order to examine the neutron radiation exposures quantitatively, the neutron energy spectra have been measured inside and outside the treatment room of a Sagittaire medical linear accelerator (25-MV x rays) located at Yale-New Haven Hospital. The measurements were made using a Bonner spectrometer consisting of 2-, 3-, 5-, 8-, 10- and 12-in.-diameter polyethylene spheres with 6Li and 7Li thermoluminescent dosimeter (TLD) chips at the centers, in addition to bare and cadmium-covered chips. The individual TLD chips were calibrated for neutron and photon response. The spectrometer was calibrated using a known PuBe spectrum Spectrometer measurements were made at Yale Electron Accelerator Laboratory and results compared with a neutron time-of-flight spectrometer and an activation technique. The agreement between the results from these independent methods is found to be good, except for the measurements in the direct photon beam. Quality factors have been inferred for the neutron fields inside and outside the treatment room. Values of the inferred quality factors fall primarily between 4 and 8, depending on location.

Neutrons↗
PubMed1977

Implications of computed tomography for inhomogeneity corrections in photon beam dose calculations.

Patient inhomogeneity information is investigated for use in radiotherapy planning. Absorbed doses measured in a phantom are compared to doses calculated for photon beams using various treatment planning inhomogeneity correction methods. Delineation of inhomogeneities with a spatial resolution of 5 mm and with an electron density accuracy of 2% in usually sufficient to allow doses to be calculated with a mean accuracy of better than 2% for 60Co and 3% for 25-MV x rays if the authors' Equivalent Tissue-Air Ratio Method is used.

Cobalt Radioisotopes↗
PubMed1978

Water/air mass stopping power ratios for megavoltage photon and electron beams.

Water/air mass stopping power ratios have been calculated for 5, 10, 20 and 30 MeV electron beams and various photon beams from 60Co to 31 MV betatron. The stopping power ratios have been evaluated by applying a modified version of the Spencer-Attix theory to depth-dependent electron flux spectra computed by the Monte Carlo method. Results are presented for three values of the cavity size parameter delta, 0.001, 0.01 and 0.1 MeV, and also for a Bragg-Gray cavity. For the electron beams, the results are compared to Berger's comprehensive computations. In the case of the photon beams, such a rigorous evaluation of the stopping power ratio has not been carried out previously. It is shown that the currently used approximate Bragg-Gray ratios are as much as 2% too low for high energy photon beams, and that there is a difference of about 1% between values for betatron (thin target) and linac (thick target) beams of the same maximum photon energy.

Air↗
PubMedDate not supplied

Empirical equation for tissue-maximum ratios/scatter-maximum ratios for indirectly ionizing radiotherapy beams.

A six-parameter equation has been fitted to the tissue-maximum ratio/scatter maximum ration (TMR/SMR) tables for 60Co, 4-, 6-, and 10-MV photon beams as well as the 35 MeV d leads to Be neutron beam. The empirical equation has been shown to fit the 60Co tissue-air ratio/scatter-air ratio (TAR/SAR) data equally well. The TMR/SMR model was developed from a three-parameter equation designed to eliminate the graphical extrapolation of zero-field size TARs.

Cobalt Radioisotopes↗
PubMedDate not supplied

Fast and thermal neutron profiles for a 25-MV x-ray beam.

High-energy x-ray radiotherapy machines generate neutrons by photonuclear reactions in the target and the treatment head and expose the patient to a neutron flux. In order to evaluate the neutron exposure quantitatively, fast and thermal neutron profiles for 25-MV x-ray beams of the Sagittaire accelerator have been measured. An activation technique, using the reactions 31P(n, gamma)32P (thermal neutrons) and 31P(n, p)31Si (fast neutrons, E greater than 0.7 MeV), has been developed to measure fast- and thermal-neutron fluxes in an intense high-energy photon flux. The sensitivity of this activation detector to high-energy photons, which has plagued many previous neutron measurements, was carefully measured and found to be less than 4%. Neutron fluxes for various photon field sizes ranging from 5 X 5 cm to 30 X 30 cm have been measured. The fast-neutron profiles were observed to have rounded edges and the thermal fluxes were found to be relatively uniform. In the central part of the x-ray beam, the ratio of neutron dose equivalent to photon absorbed dose was found to be between 0.2% and 0.5%. Outside of the photon field, the ratio of neutron dose equivalent to the central-axis photon absorbed dose was 0.12%.

Fast Neutrons↗
PubMed1979

Scattered radiation from beam modifiers used with megavoltage therapy units.

The magnitude and distribution of scattered radiation produced by scatterers inserted into megavoltage therapy beams, including the beams from an 8MV medical linear accelerator and a 60Co teletherapy unit, were investigated. The intensity distribution of scatter depends on the distance from the scatterer to the measurement plane (retraction distance) and also, to a lesser extent, on the atomic number of the scattering material. The effective energy of the scattered radiation was determined by depth dose measurements in tissue equivalent material using thermoluminescent dosimeters and was found to increase with photon beam energy.

Cobalt Radioisotopes↗
PubMed1977

[Consideration of inhomogeneities in irradiation planning. 2. Influence of inhomogeneities on the shape of the depth dose curve in water with 42 MeV X-rays (author's transl)].

With 42 MeV high-energy radiation, inhomogeneities such as bones, fat, plaster or palacos have no particular influence upon the depth dose curve in water because of the relatively insignificant differences of their atomic numbers, and there is no need to consider them in calculation of the focal dose. Metallic inhomogeneities, however, their effective atomic numbers being relatively high, show a different behaviour in 42 MeV X-ray bremsstrahlung: Just behing the metallic inhomogeneity an increase of the dose as compared to water is to be seen (e.g. directly behind a metallic plate, 4.5 mm thick, the augmentation of the dose amounts to 17%). This difference between 42 MeV X-ray bremsstrahlung and 60Co gamma rays (see part I of the present study) in the shape of the depth dose curve following permeation of metal by the radiations is due to enhanced pair formation within the high-energy radiation of 42 MeV photons.

Adipose Tissue↗
PubMed1976

Fast neutron beam radiotherapy of glioblastoma multiforme.

Twenty-one patients with glioblastoma multiforme were treated with fast neutron beam irradiation of the whole brain. Therapy was well tolerated up to calculated doses of 1.850 radn+y in 12-18 increments over 6 weeks. The survival rate 6 month after initiation of treatment was 62%, not significantly different from conventional photon therapy; average posttreatment survival appears to be shortened compared to photon therapy. No improvement or prolonged maintenance of existing neurologic function was observed. Autopsy findings in seven patients showed replacement of tumor by coagulative necrosis persisting at least 16 months posttreatment, paucity of tumor cells with infrequent mitosis, and suppression of macrophage response. These findings differ from those in conventionally irradiated patients. No treatment-related changes were documented by conventional gross and histologic studies of the irradiated brains distant from the tumors. Thus the deaths of patients in this study appear to be related to unexplained causes other than progressive growth of tumor.

Brain Neoplasms↗
PubMed1979

Bremsstrahlung dose to patients in rotational electron therapy.

Dose and integral dose from bremsstrahlung in a 10-MeV electron beam were measured for irradiation of large areas with 120 degrees-arc rotational fields. The maximum bremsstrahlung dose ranged from 2% to 7% of the maximum electron dose for the different beam arrangements, while the integral dose showed the same range of variation. The concomitant bremsstrahlung beam should be collimated by the x-ray photon collimators and the use of narrow field rotations avoided.

Electromagnetic Phenomena↗