AbstractID: 9072 Title: Dosimetry of Narrow (5 mm-20 mm diameter) Photon Radiation
Fields Using Micro Ionization Chambers and Silicon Diodes
In recent years several cylindrical Micro ionization chambers with collecting volumes ranging from 0.007 cm3 to 0.015 cm3 and
silicon diodes with effective measuring points of 0.5 mm and 2.5 mm have been introduced. These radiation detectors with high
spatial resolutions are suitable for measurements of narrow radiation beams (5-20 mm) usually encountered in stereotactic
radiosurgery beams and in IMRT subfields. We have used these detectors to characterize circular radiation fields with diameters of 5
mm, 12.5, and 20 mm and compare the results. Tissue maximum ratio (TMR), beam off-axis ratios, and output factors were measured
with a 6 MV photon beam from a Varian 2100 CD linear accelerator. The TMR measurements and output factors were performed with
plastic water and the beam off-axis ratios were measured in water. The detectors used are as follows: The A16 EXRADIN slimline
microchamber thimble ionization chamber with collection volume of 0.007 cm3, the PTW PinPoint Ionization chamber with collecting
volume of 0.015 cm3, and two SCANDITRONIX Silicon diode detectors with effective measuring diameters of 0.5 mm and 2.5 mm.
The TMR measurements for the 5 mm beam show agreements within 3%. For the larger fields, the agreement is within 2%. The offaxis ratio measurements for 5 mm field show variations not exceeding 1 mm above 20% relative dose level. For larger fields the
maximum is within 0.5 mm. For very small fields (” PP WKH KLJKHU UHVROXWLRQ GHWHFWRUV DUH SUHIHUDEOH LH $ DQG PP
diode).