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Illustration of Electron Beam Loss of Field Edge Fidelity (Beam Shape) and Uniformity with Air Gap


D Lightfoot

D Lightfoot*, Grand View Hospital, Sellersville, PA

SU-E-T-219 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:
Clinical use of electron beams frequently involves elaborate field shapes and/or matching of edges with adjacent poton fields. Treatment on accasion requires an extended Source Skin Distance (SSD). The optical projection of the field shape on the patient at the extended SSD does not depict the impact of the increased distance on shape and uniformity. The purpose of this presentation is to illustrate some examples of the impact.

Methods:
Shape and dosimetric quantities measured on films exposed cross-beam at 1 cm depth in plastic water at standard and extended SSD are compared with optical projections of shape and TPS calculations of dosimetric quantities.

Results:
Small field sizes of low energy electron beams have a small Effective SSD or f-air and show a greater impact of extended distance on field shape and uniformity. For example, an f-air value of 0.9 indicates that a net 10 percent of the beam directed towards the measurement point at the standard SSD is scattered away at the extended SSD. The scattering causes an outward bowing of straight edges, a rounding of sharp corners, loss of shape detail and a loss of uniformity of dose.

Conclusion:
When substantial departure from Inverse Square Law (ISL) is evident in extended SSD Electron Beam calculations it is advisable to evaluate the altered shape and uniformity so that clinical relevance can be judged. Substantial departure from ISL may mean more than a 5 or 10 percent departure from ISL or an air gap of more than 5 or 6 cm. Facilities ought to establish acceptable f-air and air gap limits or action levels. Careful attention to minimizing air gaps as much as possible minimizes discrepancy between optically projected and actual shape and uniformity.

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