SU‐E‐T‐781: Reduction of Thread Effect for Superficial Tomotherapy Treatments

Purpose: Helical tomotherapy can be useful in the treatment of superficial treatment volumes. As a consequence of the helical delivery, a thread pattern is evident in measured dose distributions. This goal of this work is to reduce this effect.Methods: Radiation treatment plans were created utilizing pitches of 0.287, 0.360, 0.430 and 0.500 with a 5 cm field width with the helical tomotherapy treatment planning system (TPS). The plans were delivered to a cylindrical solid water phantom with 1.0 cm bolus. EDR2 film was placed between the solid water and the bolus in the target volume region. Back‐to‐back delivery of procedures with different pitch to constitute a single treatment fraction was considered. Results: The measured percent difference of minima and maxima of the thread pattern was determined to be 5%, 12%, 10%, and 16% for pitch values of 0.287, 0.360, 0.430 and 0.500, respectively. The calculated thread magnitude was within 2% of the measured. Combining procedures with pitches of 0.287 & 0.360 and 0.287 & 0.430 resulted in interference patterns in the threading. For the pitch combination of 0.287 & 0.430, reduced variation (∼3%) was largely observed; however, limited regions of greater magnitude minima and maxima (± 5% of the mean dose) were also observed. An attempt was made to modify the target volume in order to create two plans with equal pitch values that combine to form a destructive interference pattern and, hence, a uniform dose. However, this was not readily achievable with the clinical version of the TPS Conclusions: The thread pattern can be mitigated with the use of a combination of delivery procedures with differing pitch, specifically 0.287 and 0.430. The TPS was able to predict the magnitude of the thread effect within 2% of the measured for the superficial target for all cases considered.