Improved field edge definition in electron arc therapy with dynamic collimation techniques

The diffuse shape of the electron beam used for arc therapy requires collimation on the patient's surface to sharply define treatment field edges. The electron beam arcs 10-15 degrees past field edges defined by a custom fitted and manufactured cast which acts as a tertiary collimator. This allows the entire beam profile to be integrated at the field edge. The tertiary collimator is heavy and bulky, requiring two therapists to lift and position the cast. An alternative technique for field edge definition would require the electron arc collimators to dynamically close to zero, while maintaining the projection of the leading edge of the field coincident with the geometric edge of the treatment field. This would allow integration of the entire electron arc profile and maintain a sharply defined treatment edge at the medial and lateral margins of the arc. The present customized cast could be replaced by generic lead strips at only the superior and inferior treatment field borders. This study investigates the dosimetry of dynamically collimated electron arc treatment volumes at field margins and its potential for eliminating the need for tertiary collimation at the arc field margins.

Electron arc isodose distributions were calculated using a pencil beam algorithm for treatment volumes defined by tertiary collimation at the surface of a cylindrical phantom and compared to distributions generated by simulating dynamic collimation to define the same field edges. Phantom measurements were performed using film densitometry to verify computer predictions.

Penumbra width is one measure of the sharpness of dose fall off at a treatment field edge. We define it as the distance between the 90% and 20% isodose lines at the field edge measured orthogonal to the incident electron beam. Calculations and phantom film densitometry measurements were performed for electron energies from 6-20 MeV. Dynamic and tertiary collimation both reduce penumbra width by approximately 50% compared to no collimation. There is a small advantage in minimizing penumbra width at low electron energy with tertiary collimation. This shifts to a small advantage with dynamic collimation at high electron energy.

Dynamic collimation produces a field edge isodose distribution equivalent to tertiary collimation for clinical purposes. These results suggest that tertiary collimation at medial and lateral electron are treatment field margins can be eliminated with dynamic collimation. This should result in greater clinical acceptance of breast electron arc therapy as the capacity for dynamic collimation is added to the next generation of linear accelerators.