Monte Carlo simulation of respiratory motion induced penumbral broadening in dose distribution using PENELOPE

Background: Image guided respiratory gating for lung and liver cancers was commissioned at William Buckland Radiotherapy Centre. Treatments are delivered on a Novalis Classic accelerator with Brainlab ExacTrac^TM robotic couch capable of tracking implanted fiducial markers. Objective: Commissioning was performed to understand the characteristics of gated beams. The effect of the dosedistribution penumbral broadening caused by respiration was analysed as function of the motion amplitude in free-breathing. The effect of gating was evaluated as function of the duty-cycle. Method: The PENELOPE Monte Carlo (MC) code validated against Gafchromic film was used. The film was placed on the MotionSimXY/4D^TM moving bed and was irradiated with a 6MV 5x5 cm2 field size. The device was programmed with sinusoidal patterns to reproduce breathing movements in the superior-inferior direction with amplitudes ranging from 3 to 45 mm. MC calculations were performed with similar set-up and breathing motion was simulated by combining discrete movements according to their weights. The broadening width was estimated in the 90-10% and 80-20% half-width-nominalmaximum penumbral regions with free-breathing and gating with 33% and 50% duty-cycles. Results: In the non-gated case, in the 90-10% region films and MC are in agreement for all motions within MC statistical uncertainties. In the 80-20% region MC results are in agreement up to 18 mm motion amplitude. Similar results can be found for the gated case both for 33% and 50% windows, and absolute differences between the two levels are within 1 mm. Conclusions: Monte Carlo models were validated with film measurements and were used to study treatment delivered in free-breathing or with gated beams. The results showed that for patients breathing regularly and SI motion amplitudes up to 20 mm it may be worth using a wide duty-cycle of 50% in order to improve efficiency of treatments and preserve dosimetric characteristics of the beam.