Small field segments surrounded by large areas only shielded by a multileaf collimator: comparison of experiments and dose calculation.

PURPOSE: Complex radiotherapy fields delivered using a tertiary multileaf collimator (MLC) often feature small open segments surrounded by large areas of the beam only shielded by the MLC. The aim of this study was to test the ability of two modern dose calculation algorithms to accurately calculate the dose in these fields which would be common, for example, in volumetric modulated arc treatment (VMAT) and study the impact of variations in dosimetric leaf gap (DLG), focal spot size, and MLC transmission in the beam models. METHODS: Nine test fields with small fields (0.6-3 cm side length) surrounded by large MLC shielded areas (secondary collimator 12 × 12 cm(2)) were created using a 6 MV beam from a Varian Clinac iX linear accelerator with 120 leaf MLC. Measurements of output factors and profiles were performed using a diamond detector (PTW) and compared to two dose calculations algorithms anisotropic analytical algorithm [(AAA) and Acuros XB] implemented on a commercial radiotherapy treatment planning system (Varian Eclipse 10). RESULTS: Both calculation algorithms predicted output factors within 1% for field sizes larger than 1 × 1 cm(2). For smaller fields AAA tended to underestimate the dose. Profiles were predicted well for all fields except for problems of Acuros XB to model the secondary penumbra between MLC shielded fields and the secondary collimator. A focal spot size of 1 mm or less, DLG 1.4 mm and MLC transmission of 1.4% provided a generally good model for our experimental setup. CONCLUSIONS: AAA and Acuros XB were found to predict the dose under small MLC defined field segments well. While DLG and focal spot affect mostly the penumbra, the choice of correct MLC transmission will be essential to model treatments such as VMAT accurately.

Electron scattering filter design for a single field rotational total skin irradiation.

The aim of radiotherapy treatment of cutaneous T-cell lymphoma is to irradiate the skin with an appropriately homogeneous dose distribution up to a few millimetres in depth. This can be achieved by applying one of the total skin electron irradiation techniques. An aluminium/polystyrene foam electron scattering filter was designed so that the incident beam is broadened and degraded sufficiently to achieve a mean dose uniformity in a rectangular field of 180 cm height and 40 cm width. This paper reports on the development and construction of the electron scattering filter for use with a Varian 2100C accelerator, without MLCs, with a dose uniformity, over a useful field dimension of 180 cm height and 40 cm width, of +/- 7% about the mean, and an x-ray contamination of less than 2.4% beyond a depth of 3 cm.

Levels of leakage radiation from electron collimators of a linear accelerator.

The leakage radiation from electron applicators used with our linear accelerator has been measured. For the applicators 6 X 6 to 25 X 25 cm size, the leakage was measured in the plane of the patient and on the sides of the applicators with the available electron energies of 6, 9, 12, 15 and 18 MeV. The levels were significant. The highest leakage on the side was for the combination of 6 X 6-cm applicator and 9-MeV electrons (32%) and in the plane of the patient for 25 X 25-cm applicator with 18 MeV (10%) relative to the peak dose. Adding lead 1-2 mm, at appropriate locations inside the applicators has reduced the leakages to acceptable levels without affecting the beam parameters.

Implementation of enhanced dynamic wedge using Theraplan.

The article discusses how to use the standard utility program on Theraplan to create physical wedges which are equivalent to each of the enhanced dynamic wedges. The user must first create segmented treatment tables for each wedge, using a spreadsheet, and then convert these tables to equivalent thicknesses of an arbitrary wedge material. These thicknesses are then supplied to Theraplan. The paper discusses the agreement achieved between Theraplan and measured data, and the quality assurance procedures which should be adopted.

Comparative evaluation of Wellhöfer ion chamber array and Scanditronix diode array for dynamic wedge dosimetry.

Normalised profiles have been measured using the Scanditronix diode array and the Wellhöfer ion chamber array for the Varian dynamic wedge. Agreement was of the order of 0.1% of central axis peak dose for an open beam at depth, 0.3% for a dynamic wedge field at depth, and up to 0.6% at the peak depth. The use of the arrays for data acquisition is discussed, including user interface limitations. Data reproducibility is determined to be of the order of 0.1% for both systems. The issue of beam hardening within dynamic wedges is discussed and resolved in terms of the dose-gradient effect. A method for interpolation between dynamic wedge profiles using open beam data is presented that allows construction of isodoses to an estimated accuracy of 0.7%. Finally a benchmark for comparison of different measuring systems based on quality assurance requirements for the enhanced dynamic wedge is suggested.

Simple methods to reduce patient dose in a Varian cone beam CT system for delivery verification in pelvic radiotherapy.

Cone-beam computed tomography (CBCT) is a three-dimensional imaging modality that has recently become available on linear accelerators for radiotherapy patient position verification. It was the aim of the present study to implement simple strategies for reduction of the dose delivered in a commercial CBCT system. The dose delivered in a CBCT procedure (Varian, half-fan acquisition, 650 projections, 125 kVp) was assessed using a cylindrical Perspex phantom (diameter, 32 cm) with a calibrated Farmer type ionisation chamber. A copper filter (thickness, 0.15 mm) was introduced increasing the half value layer of the beam from 5.5 mm Al to 8 mm Al. Image quality and noise were assessed using an image quality phantom (CatPhan) while the exposure settings per projection were varied from 25 ms/80 mA to 2 ms/2 mA per projection. Using the copper filter reduced the dose to the phantom from approximately 45 mGy to 30 mGy at standard settings (centre/periphery weighting 1/3 to 2/3). Multiple CBCT images were acquired for six patients with pelvic malignancies to compare CBCTs with and without a copper filter. Although the reconstructed image is somewhat noisier with the filter, it features similar contrast in the centre of the patient and was often preferred by the radiation oncologist because of greater image uniformity. The X-ray shutters were adjusted to the minimum size required to obtain the desired image volume for a given patient diameter. The simple methods described here reduce the effective dose to patients undergoing daily CBCT and are easy to implement, and initial evidence suggests that they do not affect the ability to identify soft tissue for the purpose of treatment verification.

Phantom measurements and computed estimates of breast dose with radiotherapy for Hodgkin's lymphoma: dose reduction with the use of the involved field.

The risk of breast cancer following radiotherapy for Hodgkin's lymphoma appears to be dose related. In this study we compared breast dose in an anthropomorphic phantom for conventional 'mantle'; upper mediastinal/bilateral neck (minimantle) and unilateral neck fields, and evaluated the accuracy of computer planned dose estimates for out-of-field doses. For each field, computer-planned breast dose (CPD) estimates were compared with thermoluminescence dosimetry measurements in five locations within 'breast tissue'. CPD were also compared with ion chamber measurements in a slab phantom. Measured dose and CPD were within 20% of each other up to approximately 10 cm from the field edge. Beyond 10 cm, the CPD underestimated dose by a factor of 2 or more. The mini-mantle reduced the breast dose by a factor of approximately 10 compared with the mantle treatment. Treating the neck field lowered the breast dose by a further 50% or more. Modern involved-field radiotherapy for lymphoma substantially reduces breast dose compared with mantle fields. Computer dosimetery underestimated dose at larger distances from the field. This needs to be considered if computer dosimetery is used to estimate breast dose and, by extrapolation, breast cancer risk.