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W Kwa

Publications and source records attributed to W Kwa.

5 recordsLinked to original sources
PubMed1997

Clinical use of asymmetric collimators.

PURPOSE: To illustrate some of the radiation treatment techniques with asymmetric collimators in one field dimension. METHODS AND MATERIALS: Treatment planning for various sites is done with an in-house developed treatment planning system. Dose distributions in the central plane are illustrated. RESULTS: The use of asymmetric collimation, in addition to being a replacement for cerrobend and lead blocks, can facilitate treatment setup with boost fields and with half-beam asymmetric fields as in matching two adjacent fields, in avoiding nearby critical organ or tissue, and in tangential breast treatment. The use of asymmetric collimators would alter the dose distribution across the radiation field and should be accounted for during treatment planning. In conjunction with arc rotation or multiple asymmetric fields, two-dimensional conformal radiotherapy is possible. CONCLUSION: The full potential of asymmetric collimation requires the use of a proper treatment planning algorithm. Some of the treatment techniques with asymmetric collimation in one field dimension are shown here.

Breast Neoplasms↗
PubMed1994

Dosimetry for asymmetric x-ray fields.

Conventional linear accelerators have four field-defining jaws or collimators. Usually, one set of the two opposing jaws moves concurrently to define the field width and the other set defines the field length. The resultant square or rectangular field will have the field centerline coincide with the collimator axis. However, some modern linacs have independent collimators or jaws that can be set asymmetrically. In this case, one of the two opposing jaws can be closed down independently of the other one to define an asymmetric field of smaller dimension. The field center now does not coincide with the collimator axis. Asymmetric collimators have found many clinical applications, but have complicated the dosimetry for physicists. Data acquisition and treatment planning implementations are tedious and complicated. An algorithm has been developed to correct for the reduced dose in the smaller asymmetric field. The approach used is similar in principle to the Day's equivalent field calculation. The difference in dose between an asymmetric and a symmetric radiation field is accounted for by a correction factor that is a function of the asymmetric and symmetric field sizes, off axis distance, and depth of measurement. The correction method presented here applies only to the closing down of one independent jaw. Beam profiles for asymmetric fields are measured for both the 6 and 10 MV photon beams.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1984

Electron beam dosimetry study of the digits in total body surface irradiation.

Total body surface electron beam therapy has been used for treatment of patients with mycosis fungoides. Using a 3 MeV electron beam, four fields and a moving couch technique, it was noted that patients developed marked erythema of the hands with bullous reaction along the lateral aspects of their fingers. Cylindrical phantoms were devised to study thermoluminescent dosimetry of the digits in electron beam therapy. At the lateral surface, dosage as high as 154% of the nominal dose was noted. However, dosage at 3 mm from the periphery was more homogeneous. To exploit the more even dosage at depth, the dosimetry study was repeated with a layer of 2 mm Uvex covering the phantom digits. The variation in surface dose was much reduced without jeopardizing the dose at depth. Uvex hand moulds of 2 mm thickness were constructed for patients on total body surface electron treatment with similar reduction of the much enhanced dose at the lateral borders of the digits.

Dose-Response Relationship, Radiation↗
PubMedDate not supplied

Comparison of ferrous sulfate (Fricke) and ionization dosimetry for high-energy photon and electron beams.

Dose measurements using Fricke and ionization methods were compared for 60Co gamma rays, 4-25-MV photons, and 10-25-MeV electrons. Fricke derived doses based on a constant yield (epsilon mG) were in good agreement with ionization derived doses based on the American Association of Physicists in Medicine Task Group 21 protocol and the National Research Council of Canada (NRC) ND calibration or the NRC proposed Nx. These measurements also confirmed the validity of the double-voltage technique in the collection efficiency correction, even for swept electron beams. Assuming the correctness of the ionization derived doses, the radiation yield appeared to be 1% higher and to increase with photon energy when irradiation vessels were made of Pyrex but not with polystyrene cells. These glass wall effects could be due to the scattering perturbation of electrons between inhomogeneous materials and, in particular for photon beams, due to the mismatch in mass energy absorption ratios and mass collision stopping power ratios between the Fricke dosimeter and the wall materials.

Cobalt Radioisotopes↗