Individualized lung shields constructed from lead shot embedded in plastic.

Abstract

Earle and Bagshaw (2) described a rapid optical method for preparation of complex field shapes for megavoltage therapy. In their technic, treatment simulation is carried out under conditions which are exactly similar to the therapeutic conditions. With optical methods, it is possible to produce a minified image of required size and shape, to guide construction of lung shields for the purpose of reducing dosage to these radiosensitive normal structures. At the same time prescribed dosage is delivered to areas of or presumed areas of tumor involvement. While it is a simple matter to prepare the complex field shape, the preparation of an individualized shield is a separate and more difficult problem. Highly desirable features include ease and rapidity of construction and also simplicity of modification, since modification is often necessary to obtain the final desired shape. We describe here a rapid, simply constructed lung shield which is readily modifiable and which fulfills the above requirements for a highly individualized lung shield. With minor change it can be used for any shielding problem of complex shape, involving external beam, megavoltage x-ray or γ-ray therapy. Moreover, it also permits one to take account of beam divergence and differences in anterior and posterior lung block shape. Finally, it is practical and economical. Materials And Methods Transparent cutouts of film outlines were prepared, using an optical method. The tracings were transferred to a 10 em thickness of polyethylene foam. This can then be cut with a band saw or, if beam divergence correction is necessary, by appropriately designed devices, using either a hot wire or angle blade. Once cut, the mold serves as a receptacle for pouring the lung shields. Lead, in the form of small shot, was used for the shielding material. Heated themoplastic compound- is initially and concurrently poured into the mold until the entire form is filled. Agitation aids the settling process. After setting, the form can be cut from the polyethylene, and after additional port films, minor alterations and trimming to the required shape can easily be accomplished with a band saw. The final, completed product is shown in Figure 1, along with a port radiograph of the mantle field. Results and Preliminary Evaluation A 10-cm-thick lung shield can be made easily for reversible anteroposterior and postero-anterior usage, or shields may be constructed for each port to be treated. Beam-divergence correction may also be easily taken into account, owing to the simplicity of construction of each shield. Such considerations are of greater importance for blocks placed close to the collimator, and of less importance for blocks placed close to the patient. A transmission curve for the lead-shot-in-plastic shield is shown in Figure 2, relative to that for lead metal (3). The resulting half-value layer was about 2.0 cm.