SU‐E‐T‐555: Small Field Dosimetry of the Flattening Filter Free (FFF) X‐Ray Photons
Purpose: The dosimetry of small field is critical in high‐resolution photonradiation therapy, such as the highly conformal stereotactic radiosurgery(SRS) and stereotactic body radiotherapy(SBRT). The flattening filter free (FFF) linac as an emerging technique can offer highly efficient photon use and enhanced dose rate, which benefits SRS and SBRT. This study investigates the small field photondosimetry of the FFF X‐rays using diode detectors and compares results to the conventional flattened X‐ rays. Methods: A Varian's TrueBeam (TB) linac with 6X‐FFF and 6X‐flat X‐rays and a Novalis TX (NTX) linac with 6X‐flat X‐rays have been fully commissioned. Output factors (6X‐flat and 6X‐FFF of TB and 6 X‐flat NTX) were measured by a diode detector (Edge detector, Sun Nuclear) and solid water phantom with different field sizes (FS) created in Eclipse Treatment Planning System. Different FS were formed by MLC patterns rather than jaws (FS=5, 10, 20, 30, 40, 60, 100mm by MLC and the corresponding jaw size=7, 12, 22, 32, 42, 60, 100mm). A regular SSD=100cm and extended SSD=210cm with dmax=1.5cm buildup were setup for the small field dosimetry measurement and all these relative dosimetry was normalized to FS=10cm.Results: Good repeatability of the measurement was demonstrated. The regular and extended SSD results were comparable, which showed the OF of 6X‐flat was smaller than the OF of 6X‐FFFfor FS=1–10cm, likely due to the absence of flattening filter. Moreover, the OF of TB's 6X‐flat was generally lower than the OF of NTX's 6X‐flat. One possible cause is that leaf thickness of NTX is thinner than TB. Conclusions: The 6X‐FFF photons have higher output than the 6X‐ flat in the TrueBeam linac, maybe due to the removal of flattening filter. The TB's 6X‐flat photons have lower output than NTX's 6X‐flat X‐ray photons probably because NTX has thinner leaves.