No abstract

The contribution of organ and tumour motion to the degradation of planned dose distributions during radiotherapy to the breathing lung has been experimentally investigated and quantified. An anthropomorphic, tissue-equivalent breathing phantom with deformable lungs has been built, in which the lung tumour can be driven in any arbitrary 3D trajectory. The trajectory is programmed into a motion controller connected to a high-precision moving platform that is connected to the tumour. The motion controller is connected to the accelerator's dose counter and the speed of motion is scaled to the dose rate. This ensures consistent delivery despite variation in either the dose rate or inter-segment timing. For this study, the phantom was made to breathe by a set of periodic equations representing respiratory motion by an asymmetric, trigonometric function. Several motion amplitudes were selected to be applied in the primary axis of motion. Five three-dimensional, geometrically conformal (3DCRT) fractions with different starting phases (spaced uniformly in the breathing cycle) were delivered to the phantom and compared to a delivery where the phantom was static at the end-expiration position. A set of intensity-modulated radiotherapy plans (IMRT) was subsequently delivered in the same manner. Bigger amplitudes of motion resulted in a higher degree of dose blurring. Severe underdosages were observed when deliberately selecting the PTV wrongly, their extent being correlated with the degree of margin error. IMRT motion-averaged dose distributions exhibited areas of high dose in the gross tumour volume (GTV) which were not present in the static irradiations, arising from booster segments that the optimizer was creating to achieve planning target volume (PTV) homogeneity during the inverse-planning process. 3DCRT, on the other hand, did not demonstrate such effects. It has been concluded that care should be taken to control the delivered fluence when delivering IMRT to the breathing lung, even when the PTV margin has been adequately chosen to include the extent of the breathing motion.

The benefits of using Synchrony Respiratory Tracking System (RTS) in conjunction with the CyberKnife robotic treatment device to treat a "breathing tumor" in an anthropomorphic, tissue-equivalent, thoracic phantom have been investigated. The following have been studied: (a) Synchrony's ability to allow the CyberKnife to deliver accurately a planned dose distribution to the free-breathing phantom and (b) the dosimetric implications when irregularities in the breathing cycle and phase differences between internal (tumor) and external (chest) motion exist in the course of one treatment fraction. The breathing phantom PULMONE (phantom used in lung motion experiments) has been used, which can imitate regular or irregular breathing patterns. The breathing traces from two patients with lung cancer have been selected as input. Both traces were irregular in amplitude, frequency, and base line. Patient B demonstrated a phase difference between internal and external motion, whereas patient A did not. The experiment was divided into three stages: In stage I-static, the treatment was delivered to the static phantom. In stage II-motion, the phantom was set to breathe, following the breathing trace of each of the two patients. Synchrony was switched off, so no motion compensation was made. In stage III-compensation, the phantom was set to breathe and Synchrony was switched on. A linear correspondence model was chosen to allow for phase differences between internal and external motion. Gafchromic EBT film was inserted in the phantom tumor to measure dose. To eradicate small errors in film alignment during readout, a gamma comparison with pass criteria of 3%/3 mm was selected. For a more quantitative approach, the percentage of pixels in each gamma map that exceeded the value of 1 (P1) was also used. For both breathing signals, the dose blurring caused by the respiratory motion of the tumor in stage II was degraded considerably compared with stage I (P1 = 15% for patient A and 8% for patient B). The motion compensation via the linear correspondence model was sufficient to provide a dose distribution that satisfied the set gamma criteria (P1=3% for patient A and 2% for patient B). Synchrony RTS has been found satisfactory in recovering the initial detail in dose distribution, for realistic breathing signals, even in the case where a phase delay between internal tumor motion and external chest displacement exists. For the signals applied here, a linear correspondence model provided an acceptable degree of motion compensation.