S. Kumar

SU‐E‐T‐522: Evaluation of EDW and Sliding Window IMRT in the Presence of Organ Motion with Gating

Purpose: To investigate the interplay effect of intrafractional motion when using Enhanced Dynamic Wedge (EDW) and Intensity Modulated Radiotherapy(IMRT) with gated delivery. Methods: EDW of angles 30, 45 & 60 (field size 10×10sq.cm) and sliding window IMRT bench‐mark fluences were delivered using 6MV beams from a Varian CL2100CD with millennium 120 MLC at 100, 400 and 600MU/min dose‐rates for static and gated operation using RPM. The IMRT MatriXX system and 0.6cc ionization chamber were placed at 5 cm depth on the Quasar motion platform to measure 2D fluence and point doses. The motion platform was programmed for breathing frequencies 0.125, 0.17 and 0.25Hz with the jaw/MLC movement in the same direction. During the gated mode, duty cycles were 25%, 50%, 75% and 100% (un‐gated). The 2D fluence comparison and variation in point dose of gated operation with static measurement for various breathing frequencies, duty cycles and dose rates was performed and fluence comparisons analyzed using gamma criteria of 3%/3mm. Results: Fluence and point‐dose deviations of EDW and IMRT increased with increasing duty cycle and breathing frequency. The gamma evaluation of the 25% duty cycle was found to be closest to the static measurements and the point dose deviation was <2%, while for larger duty cycles, the deviations were upto 15% which increased with increasing EDW angles, although with no apparent relationship with the dose rate. At lower dose rates, more number of pixels passed the gamma evaluation criteria. The deviations for 100% duty cycle with breathing frequency of 0.25Hz and dose rate of 100MU/min were −5.7%, −8.9% and −15.8% for 30, 45 and 60 EDW angles respectively. Conclusions: The interplay effect demonstrates considerable error in the delivery of dynamic intensity modulated treatments, which is reduced by appropriate gating. Acknowledgement: Department of Science & Technology ‐ Grant No: IR/SO/LS 02/2003, Government of India

SU‐GG‐T‐161: Effect of Leaf Motion Calculator Parameters in Head and Neck IMRT

Purpose: To investigate the dosimetric effect of the Leaf Motion Calculator (LMC) parameters using sliding window technique in head and neck IMRT.Materials and Methods: Two head and neck IMRT patients (T1/T2,N0) were selected for this study. The actual fluence of the IMRT plans were calculated by varying the LMC parameters which includes, dose rates from 100 to 600 MU/min, MLC limiting leaf speed (0.5 to 3.0 cm/sec) and minimum dynamic leaf gap (0.05 to 0.2cm) from the same optimal fluence. The doses to OAR, total monitor unit (MU), low and high dose volumes were analyzed. Results: The doses to OAR, low and high dose volume (10%, 50% & 95% of isodose) increases with increase in dose rate and decreases with increase in limiting leaf speed. The total MU also increases with increase in dose rate and decreases with increase in leaf speed. When the dose rate increases from 300 MU/min to 600MU/min, the total MU significantly increase by 12% at a limiting leaf speed of 2.5cm/sec. There was no significant difference in doses to OAR, total MU, low and high dose volumes for minimum dynamic leaf gap between 0.05 to 0.10 cm. Conclusion: OAR sparing is observed with lower dose rate and higher leaf speed. There is no significant difference in PTV coverage and high dose volumes with dose rate as well as limited leaf speed. However, in the low dose volumes significant difference was observed for leaf speed less than 1.5cm/sec.