Program Information
Dose Perturbation Caused by Sun Nuclear QED Diode When Used for Very Small Electron Fields
S Klash1*, J Steinman2 , T Stanley3 , (1) Radphys Oncology Services, LLC, Knoxville, TN, (2) Radphys Oncology Services, LLC, Knoxville, TN, (3) CCS Oncology Center, Kenmore, NY
Presentations
SU-E-P-34 (Sunday, July 12, 2015) 3:00 PM - 6:00 PM Room: Exhibit Hall
Purpose:Diodes are utilized by radiotherapy departments to help verify that treatment fields are being delivered correctly to the patient. Some treatment fields utilize electron beams along with a cerrobend cutout to shape the beam to the area to be treated. Cerrobend cutouts can sometimes be very small < 2x2-cm2. Some published work has addressed diode perturbation for cutout sizes down to 1.5-cm, this work addresses the diode perturbation of the Sun Nuclear QEDTM diode for cutouts as small as 0.5-cm in diameter.
Methods:Measurements were taken with an A16 Exradin micro-chamber in Solid Water to 100-cm SSD. Dmax was determined for each cutout using various amounts of Solid Water in 1-2 mm increments to account for the dmax shifting in small fields. The diode was placed on top of the solid water to 100-cm SSD in the center of the cutout. Measurements were taken with no diode for comparison. The cutouts ranged in diameter from 0.5-cm to 5.0-cm and included the open 6x6 insert. Measurements were made for energies 6, 9, 12, 15, & 18 MeV.
Results:For 6 MeV, the percent dose reduction from the diode in the cutout field compared to the field without the diode ranged from 35% to 25% as a function of cutout size. For higher energies, this percentage decreased and generally was 25% to 15%. It was observed that dmax shifts significantly upstream for very small cutouts (<2-cm diameter) to less than 1 cm for all energies.
Conclusion:The presence of diodes in small electron fields is enough to cause significant dose perturbation to the target volume. It is recommended that diodes for very small electron fields be used sparingly or possibly with a dose correction per treatment fraction(s), if the total projected delivered dose is going to be significantly different from that prescribed by the physician.
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