H (Parenica) Paschal*, C Kabat, T Martin, S Stathakis, M Bonnen, N Papanikolaou, N Kirby, UT Health San Antonio, San Antonio, TX

TU-I345-IePD-F7-1 (Tuesday, 7/12/2022) 3:45 PM - 4:15 PM [Eastern Time (GMT-4)]

Exhibit Hall | Forum 7

Purpose: We aim to reduce normal tissue radiation toxicity for anatomical sites accessible to electron therapy through the creation of a surface-conforming electron MLC (SCEM). The SCEM should combine the benefits of skin collimation, electron conformal radiotherapy, and modulated electron radiotherapy.

Methods: An early concept for the SCEM was constructed. It consists of leaves that protrude towards the patient and move tangentially and normally to the patient surface, 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 a 10x10 cm field using a PTW 60012 diode for all available electron energies on an Elekta Versa HD linac (6, 9, 12, and 15 MeV) with a 0.5 cm airgap to the water surface (100 cm SSD). These measurements were compared to a 10x10 cm Cerrobend cutout with the field size matched at 110 cm SSD (a clinically relevant treatment SSD). All field sizes matched at the surface and were within 1 mm.

Results: The SCEM provided superior dose fall-off for shallow and deep depths for all energies. For 6 MeV at dmax, there were 59% and 92% decreases in penumbra and out-of-field dose (3 cm from field edge), respectively. For 15 MeV, this effect was less pronounced, though there was still significant improvement with the SCEM (26% and 39% decrease in penumbra and out-of-field dose, respectively). The SCEM produced higher surface doses. This effect was more pronounced with increasing energy.

Conclusion: The SCEM provides significant improvement in penumbra and out-of-field dose by allowing collimation close to the skin-surface. It negates the need for patient-specific cutouts, saving time and eliminating exposure to Cerrobend. Additional field sizes and SSDs will be measured to fully characterize the SCEM.

Keywords

Electron Therapy, MLC, Beam Shaping

Taxonomy

TH- External Beam- Electrons: Development (new technology and techniques)

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