Enhanced dynamic wedge factors at off-axis points in asymmetric fields.

Several recent reports have described methods for calculating enhanced dynamic wedge factors (EDWFs). Many of these reports use the monitor-unit (MU) fraction method to predict EDWFs as a function of field size. Although simple in approach, MU fraction methods do not produce accurate EDWFs in large or asymmetric fields. A recently described technique, based on the MU fraction method works well for large and asymmetric fields, but only when the calculation point is in the center of the field. Other existing methods based on beam-segment superposition do not have this limitation. These beam summation methods, however, are difficult to implement in routine clinical MU calculation schemes. In this paper, we present a simple calculation method that estimates EDWFs at off-axis calculation points in both symmetric and asymmetric fields. Our method, which also is based on the MU fraction method, similarly uses empirically determined field-size corrections but also applies wedged-field profiles to estimate EDWFs that are independent of calculation-point location and field symmetry. EDWF measurements for a variety of field sizes and calculation-point locations for both 6- and 18-MV x-ray beams were performed to validate our calculations and those of our ADAC Pinnacle3 Treatment Planning System. The disagreement between the calculated and measured EDWFs over the useful clinical range of field sizes and calculation-point locations was less than 2%. The worst disagreement was 3% and occurred at a point 8.5 cm from the center of an asymmetric 25 (wedged direction)x20 cm2 60 degrees-wedged field. Detailed comparisons of measurements with calculations and wedge factors obtained from the ADAC Pinnacle3 Treatment Planning System will be presented. In addition, the strengths and weaknesses of this calculation method will be discussed.

Fetal dose during radiotherapy: clinical implementation and review of the literature.

Two pregnant patients received radiation therapy, one for the treatment of mediastinal Hodgkin's lymphoma and the other for a head and neck squamous cell carcinoma. The fetuses were both protected by additional shielding which reduced the unshielded exposure of the first fetus by 20-40%, and that of the second by 20-60%. The first child received an estimated maximum dose of 42 cGy, the second a maximum dose of 9 cGy. Treatment details are reported and a review of the literature that addresses the possible irradiation-induced side effects at low doses is included.

Verification of the accuracy of 3D calculations of breast dose during tangential irradiation: measurements in a breast phantom.

This report specifically describes the use of a unique anthropomorphic breast phantom to validate the accuracy of three-dimensional dose calculations performed by a commercial treatment-planning system for intact-breast tangential irradiation. The accuracy of monitor-unit calculations has been corroborated using ionization chamber measurements made in this phantom. Measured doses have been compared to those calculated from a variety of treatment plans. The treatment plans utilized a 6-MV x-ray beam and incorporated a variety of field configurations and wedge combinations. Dose measurements at several clinically relevant points within the breast phantom have confirmed the accuracy of calculated doses generated from the variety of treatment plans. Overall agreement between measurements and calculations averaged 0.998+/-0.009. These results indicate that the dose per monitor-unit calculations performed by the treatment-planning system can be confidently utilized in the fulfillment of clinical dose prescriptions.

Clinical implementation of the AAPM Task Group 36 recommendations on fetal dose from radiotherapy with photon beams: a head and neck irradiation case report.

We present the results of our efforts in estimating and diminishing the fetal dose expected when a 29-year-old patient, 22 weeks pregnant, received external beam radiation therapy for a squamous cell carcinoma of the tongue. We explain our use of the information contained, and recommendations made, in the Report of the American Association of Physicists in Medicine Radiation Therapy Committee Task Group 36 [Med. Phys. 22, 63-82 (1995)]. We also explain our dose estimation, describe our validation measurements, and demonstrate the effectiveness of supplemental shielding. Consequently, this case report will serve as a guide to radiation oncologists and medical physicists who may encounter similar cases.

A quest for accuracy and reproducibility. Design of a simulation record.

This paper describes a simulation record that has been designed for use in most radiation therapy centers. The format is such that it can be used for all machine types and can be transferred easily between centers. The simulation record is separate from the treatment documentation record. It can be reused for several simulations with the addition of one or two inserts. All information needed to reproduce the treatment setup (i.e., positioning, immobilization technique, use of blocks, and photographs of the patient) is well documented. The record prompts the simulation technologist to record field and patient parameters, which are necessary for dose calculations along the central axis and at any other points of interest.

Asymmetric field calculations.

A simple method for performing manual dose calculations in asymmetric fields is proposed. The method is based on a generalized central-axis dose calculation equation for which open- and wedged-field off-axis provisions have been made. A calculation form designed to document the calculation and simplify the calculation process is presented. Lastly, the required off-axis dosimetry data obtained from a dual-energy accelerator are shown.

Measurement of yield and cross section for the 27Al (gamma,2p) 25Na reaction with a clinical linear accelerator.

A clinical electron linear accelerator was adapted for continuous energy selection and used to measure bremsstrahlung yield for the photonuclear reaction 27Al (gamma,2p) 25Na from 24.0 to 32.0-MeV in 0.25 MeV steps. A germanium detector was used to measure the number of photoproduct gamma-rays emitted. The yield values were analyzed for photonuclear cross section using PHONUC II, a program especially adapted for use with yield data from thick-target bremsstrahlung. The cross section shows distinct resonance peaks at 26.3 +/- 0.3, 27.3 +/- 0.4 and 30.3 +/- 0.5 MeV. Possible theoretical origins of these resonances and applications of the reaction in accelerator energy calibration are discussed.

Photonuclear activation ratios in fluorine compounds; an index of bremsstrahlung quality.

Samples of potassium hexafluorosilicate (K2SiF6) and polytetrafluoroethylene (C2F4)n were irradiated in the bremsstrahlung beam from a clinical linear electron accelerator at integral values of endpoint energy between 13 and 32 MeV. Sample positron activities produced by the reactions 39K(gamma, n) 38K, 19F(gamma, n) 18F, and 12C(gamma, n)11C, were determined by counting the gamma radiation. Saturation activities per target nucleus were calculated from these data. For the K2SiF6 samples, the photonuclear activation ratio (PAR) of the 7.6 min 39K saturation activity per nucleus divided by the 18F result increased by more than 20% per MeV between 15 and 21 MeV. For the (C2F4)n samples the ratio (PAR) of the 11C saturation activity per nucleus to that for 18F increased by more than 24% per MeV between 22 and 26 MeV. Quality changes caused by the lead flattening filter were easily detectable using the PAR technique. Because of the high sensitivity to bremsstrahlung spectrum changes these PAR values can serve as sensitive indices of quality for high-energy beams.