The clinical application of dynamic shielding and imaging in moving table total body irradiation.

The moving table technique for total body irradiation (MTT TBI) has some advantages in regard to dose homogeneity, patient positioning and comfort. However, divergence of the radiation field coupled with patient motion necessitates corresponding motion of the shielding blocks and verification film so that penumbra is minimized. MTT TBI system is presented, together with dose calculations, incorporating moving trays for shields and film to ensure dose delivery with minimal penumbra of the blocked field.

Phantom assessment of lung dose from proton arc therapy.

PURPOSE: To compare resultant lung dose from proton arc therapy of the chest wall to that from electron arc therapy. METHODS AND MATERIALS: A 200 MeV proton beam from the Indiana University Cyclotron was range shifted and modulated to provide a spread out Bragg peak extending from the surface to a depth of 4 cm in water. The chest wall of an Alderson Rando phantom was irradiated by this beam, collimated to a 20 x 4 cm field size, while it rotated on a platform at approximately 1 rpm. For comparison, electron arc therapy of the Rando phantom chest wall was similarly performed with 12 MeV electrons and the resultant lung dose measured in each case. RESULTS: Dose-volume histograms for the Rando phantom left lung indicate a reduced volume of irradiated lung for protons at all dose levels and an integral lung dose that is half that for electron arc therapy in the case studied. In addition, a more uniform dose coverage of the target volume was achieved with the proton therapy. CONCLUSION: This study demonstrates a potential role for proton arc therapy as an alternative to electron arc therapy when lung dose must be minimized.

Matching photon and electron fields in the treatment of head and neck tumors.

Radiotherapy of head and neck tumors frequently involves joining photon and electron fields. In such situations narrow penumbras combined with relatively small positioning errors can lead to significant "hot" and "cold" spots in the vicinity of the join-up. The objective of this work was to devise penumbra spreading techniques which lead to a relatively uniform dose distribution in the join-up region of these fields and which reduce the effect of positioning errors on dose uniformity. A stepped edge attenuator was used to obtain a wider penumbra for the 4-MV x-ray beam and a Lucite scatterer was used for the 10-MeV electron beam. The resulting composite beam profiles from these "modified" abutting photon and electron fields are provided and the effects of positioning errors on dose uniformity across the junction are illustrated. These profiles are compared with those resulting from "unmodified" adjacent electron and photon beams.

The calculation of transmission through a photon beam attenuator using sector integration.

A calculational scheme is presented for the prediction of the transmitted fraction (TR) through an attenuator of known material and physical dimensions, at any point in a photon beam, for a beam of any shape or size. The method considers the total TR to be composed of scatter and primary components and computes the scatter component by sector integration. The input for the calculations consists of measured narrow- and broad-beam transmitted fractions through lead in air for various circular fields, thicknesses of the attenuator, and angles from the central axis, in a geometry approximating typical treatment conditions. The method has been tested for the case of a uniform half slab and a 45 degree wedge in a 4-MV photon beam. It was found that the use of TR values obtained by the above method reduced the maximum absorbed dose computation error from 8% computed with a commercially available algorithm to 3%, in a typical treatment setup. This method is generally applicable to any shaped attenuator such as a wedge or compensator covering whole or part of a radiation field.