Dosimetry characteristics of GAFCHROMIC EBT film responding to therapeutic electron beams.

For the last 50 years, high-energy electron beams have been used mainly for treatment of superficial targets. Accordingly, two-dimensional (2D) measurements are required to determine the margin between the surface and protected normal tissues in high-dose-gradient regions. As recommended by the AAPM, 2D electron beam dosimetry should be conducted primarily with films because of their high spatial resolution and because of the fact that they produce permanent records. In this work, the dosimetric characteristics of a newly developed radiochromic film, GAFCHROMIC EBT film were used to study treatment parameters for therapeutic electron beams. The dose-response curve was found to be weakly dependent on the electron beam energy (within +/-4%). The effect of fractionation, as well as electron beam dose rate, had small influence on the calibration curve of EBT films (+/-2.5% and 2%, respectively). For the investigated range of depth, dose-response curves are nearly independent of the calibration depth. As for the cone size dependence, we found that it is accurate to calibrate GAFCHROMIC EBT films using a 10 x 10 cm(2) cone and apply this calibration to other cone sizes.     

High-resolution measurements of small field beams using polymer gels.

Small field sizes are increasingly becoming important in radiotherapy particularly since the introduction of intensity-modulated radiation therapy (IMRT) techniques. It is normally a challenging task to reliably measure the delivered dose and to determine its distribution in a medium for such small fields using conventional-type dosimeters such as gas ionisation chambers. Recently, attempts have been made to use films, but they are not tissue equivalent, they measure the dose only in two dimensions and they are not as responsive to radiations. In the present work, polyacrylamide gel (PAG) dosimeters are employed to measure the dose and its distribution in three dimensions for very small field sizes, such as those typically used in stereotactic radiosurgery. Field sizes of 6 x 6 and 18 x 18 mm in width are investigated. The results show an agreement with radiochromic film and ionisation diode measurements, with some variation in measured doses near the edge of the field, where the gel data decreases more rapidly than the other methods.     

Practical considerations for electron beam small field size dosimetry.

Special care of superficial lesions surrounding critical structures, such as an eye, may require tight margins. When this is the case, small megavoltage electron treatment fields and nonstandard treatment distances become necessary. When the field size is found to be less than the practical range of the electron beam, dosimetric measurements should be performed. This research includes data proving that very small electron fields can be employed for treatment with appropriate beam flatness and penumbra. This is accomplished by first coming down the incident beam to a small field size, then secondly by adding a single lead sheet to the patient's skin surface. The aperture of the sheet is required to be greater than 2 x 2 cm2 in size, and must be cut properly to adequately confine the treatment area.     

Small electron beams in radiation therapy.

The use of lead cutouts to produce small beams in electron therapy results in a reduced dose to the patient. The authors investigated this effect for beams with energies less than 8 MeV and diameters of 3 cm or less. Dose measurements were obtained using film and an ionization chamber. Based on these values, corrections were established to account for dose reduction due to cutout spaces and air spaces between the end of the electron cone and the skin surface. Data were also obtained on the dose increase due to backscattering from internal eyeshields.     

Dosimetry of normally and obliquely incident cobalt-60 teletherapy beams

Standard isodose charts are generally given by the manufacturer for normal incidence of radiation beams of various qualities. These charts are used for routine treatment planning in a radiotherapy department. Due to the enormous variety of the cases encountered in the department, the physicist often runs short of appropriate charts required for specific treatment planning. Besides, the plan sometimes needs a beam directed at an angle to the patient's surface. As a result, isodose charts for individual treatment plans are desirable for precise delivery of the requisite dose to the tumour volume. In this paper a computer method is described for determining dose distributions for normally and obliquely incident 60Co beam. The experimental verification of the computed results using a 0.4 cm3 ionization chamber and a perspex water phantom assembly is also presented. The results are found to be in good agreement.     

Dosimetry of 70 MeV proton beams from the NIRS cyclotron

Dosimetry of 70 MeV proton beams for radiotherapy was carried out using various ionization chambers. The beam irradiation conditions, dose estimation with an ionization chamber, measurement of dose distributions, and calculation of isodose curves for the proton beams will be discussed.     

Dosimetry and treatment planning of complex electron arc therapy

Electron are therapy has been primarily utilized for treatment of the post-mastectomy chest wall. Its clinical usefulness and benefits have been proven. Casting procedures and blocking techniques have been discussed. We have successfully applied electron are therapy to the treatment of the nasal cavity and associated nodal regions. When compared to the chest wall, the smaller radii and pronounced irregularity of the surface anatomy of the facial region presents multiple new dosimetric and treatment planning challenges. We will discuss these challenges, along with those encountered in the casting procedures and reproduction of patient characteristics required for the execution of this treatment. Many aspects of dosimetry were utilized in unique ways to produce the treatment plans, cast and bolus needed. Due to the insertion of bolus into the nostrils, a unique hollow bite block was constructed to allow patient respiration during treatment. Film dosimetry was used to verify computer predictions and to compare this electron arc treatment technique with alternative fixed electron beam techniques. The potential benefits and difficulties of this technique will be discussed.     

Medulloblastoma therapy using electron beams

The use of megavoltage photons for the spinal component of cerebrospinal axis irradiation is responsible for most of the long-term sequelae in children with medulloblastomas who undergo this procedure. The technique and advantages of using electrons for this component of the procedure are described.     

Ionization chamber dosimetry for photon and electron beams. Theoretical considerations.

New Clambda-values (Cair,lambda) are proposed which should be applied for ionization chambers with an inner wall of air-equivalent material, air eq. plastic or graphite. The new values are up to approximately 3 per cent lower, and apply for a chamber with an inner wall lining of water-equivalent material.er cent higher than those published in the ICRU Report No. 14 and here named Cwater,lambda as the inner wall is considered water-equivalent. Also two sets of CE-values are proposed, namely Cair,E which is given in ICRU No. 21 and Cwater,E which is approximately 3 p