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SU‐FF‐T‐77: Accuracy of Gated IMRT Delivery On the Varian Linac Using the Real‐Time Position Management System
Author(s) -
Hsu A,
Thorndyke B,
Pawlicki T,
Xing L
Publication year - 2006
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.2241003
Subject(s) - imaging phantom , linear particle accelerator , nuclear medicine , amplitude , physics , position (finance) , gating , dosimetry , biomedical engineering , medicine , beam (structure) , optics , finance , economics , physiology
Purpose: To investigate the accuracy of gated IMRT delivery on a Varian linear accelerator equipped with the Realtime Position Management (RPM) camera and software. Method and Materials: A non‐uniform dose distribution within a solid water phantom was contoured and planned with IMRT. A sinusoidally oscillating platform simulated superioinferior respiratory motion, and a reflecting block was placed on the surface of the platform to provide a “respiratory” signal to the RPM camera. First, the phantom was stationary while the platform served only to provide the respiratory signal. The respiratory period was 5 sec, and the treatment was delivered in phase‐gated intervals of 6%, 10%, 25% and 50%. Second, the phantom was placed on the platform, with motion amplitude of 6 cm. Here, dose was delivered to the phantom during a small amplitude‐defined interval at end‐expiration, with periods 1.7 sec, 5.3 sec and 12.6 sec. Dose distributions were captured on film. Results: Dose profiles generally showed variation between configurations less than 2% the maximal dose, with shorter‐interval delivery providing slightly less dose than longer‐interval delivery. The only notable difference occurred for the phantom moving with respiratory period of 1.7 sec, where dose fluctuations of nearly 6% occurred at regions of high dose gradient in the direction of motion. It should be noted that the gating interval spanned 15% the respiratory cycle, implying the beam was delivered in only 1.7 × 0.15 = 0.25 sec intervals. Conclusion: Gated IMRT delivery provided dose distributions equivalent to ungated delivery to within clinically acceptable limits. This result held for significant motion amplitude, under a wide range of respiration frequencies and gating intervals. While discrepancies up to 6% arose at high gradient borders for configurations of extremely rapid motion and short beam‐on time, these parameters are very unlikely to be seen in any clinical situation.