Dosimetric characterization of a new surface-conforming electron MLC prototype

Abstract

The purpose is to reduce normal tissue radiation toxicity for electron therapy through the creation of a surface-conforming electron multileaf collimator (SCEM). The SCEM combines the benefits of skin collimation, electron conformal radiotherapy, and modulated electron radiotherapy. An early concept for the SCEM was constructed. It consists of leaves that protrude towards the patient, allowing the leaves to conform closely to irregular patient surfaces. The leaves are made of acrylic to decrease bremsstrahlung, thereby decreasing the out-of-field dose. Water tank scans were performed with the SCEM in place for various field sizes for all available electron energies (6, 9, 12, and 15 MeV) with a 0.5 cm air gap to the water surface at 100 cm source-to-surface distance (SSD). These measurements were compared with Cerrobend cutouts with the field size-matched at 100 and 110 cm SSD. Output factor measurements were taken in solid water for each energy at d_max for both the cerrobend cutouts and SCEM at 100 cm SSD. Percent depth dose (PDD) curves for the SCEM shifted shallower for all energies and field sizes. The SCEM also produced a higher surface dose relative to Cerrobend cutouts, with the maximum being a 9.8% increase for the 3 cm × 9 cm field at 9 MeV. When compared to the Cerrobend cutouts at 110 cm SSD, the SCEM showed a significant decrease in the penumbra, particularly for lower energies (i.e., 6 and 9 MeV). The SCEM also showed reduced out-of-field dose and lower bremsstrahlung production than the Cerrobend cutouts. The SCEM provides significant improvement in the penumbra and out-of-field dose by allowing collimation close to the skin surface compared to Cerrobend cutouts. However, the added scatter from the SCEM increases shallow PDD values. Future work will focus on reducing this scatter while maintaining the penumbra and out-of-field benefits the SCEM has over conventional collimation.

Keywords: electron therapy; multi-leaf collimator; skin collimation.

FIGURE 1

Comparison of MLC positioning and leaf motion relative to the patient surface for stationary leaves (a), leaves moving tangentially (b), and leaves moving normally and tangentially (c). MLC, multileaf collimator.

FIGURE 2

Design of the SCEM for a single leaf. SCEM, surface‐conforming electron multileaf collimator.

FIGURE 3

A beam's eye view (a) and sideview (b) of the SCEM in a bank with the jaws. The width of the bank of leaves is fixed a 9 cm in the in‐plane direction and the leaves can open to a maximum of 20 cm in the cross‐plane direction.

FIGURE 4

The SCEM attached to the linac with one solid water “jaw” removed.

FIGURE 5

PDD curves for the SCEM (dashed) and Cerrobend cutouts (solid). The y‐axis is the central‐axis percent depth dose relative to the maximum and the x‐axis is the central axis depth in millimeters. PDD, percent depth dose.

FIGURE 6

A comparison of the isodose lines for the cerrobend cutout at 100 cm SSD, cerrobend cutout at 110 cm SSD, and the SCEM for the 3 cm × 9 cm field size for each energy. The isodose lines shown from lowest to highest are 10%, 30%, 50%, 80%, and 90%. SCEM, surface‐conforming electron multileaf collimator; SSD, source‐to‐surface distance.

FIGURE 7

A comparison of the isodose lines for the cerrobend cutout at 100 cm SSD, cerrobend cutout at 110 cm SSD, and the SCEM for the 5 cm × 9 cm field size for each energy. The isodose lines shown from lowest to highest are 10%, 30%, 50%, 80%, and 90%. SCEM, surface‐conforming electron multileaf collimator; SSD, source‐to‐surface distance.

FIGURE 8

A comparison of the isodose lines for the cerrobend cutout at 100 cm SSD, cerrobend cutout at 110 cm SSD, and the SCEM for the 10 cm × 9 cm field size for each energy. The isodose lines shown from lowest to highest are 10%, 30%, 50%, 80%, and 90%. SCEM, surface‐conforming electron multileaf collimator; SSD, source‐to‐surface distance.

FIGURE 9

Output factors for each energy for the SCEM (dashed) and Cerrobend cutouts (solid), normalized to the 10 cm × 9 cm field size. The x‐axis represents the cross‐plane dimension (i.e., the variable dimension) of the SCEM.

FIGURE 10

MiniPhantom output factors for each energy for the SCEM (dashed) and Cerrobend cutouts (solid), normalized to the 10 cm × 9 cm field size. The x‐axis represents the cross‐plane dimension (i.e., the variable dimension) of the SCEM.

FIGURE 11

Cross‐plane profiles for the SCEM for the 10 cm × 9 cm field size at 1 mm depth (near‐surface) and d_max for each energy.