Quantifying the effect of respiratory motion on lung tumour dosimetry with the aid of a breathing phantom with deforming lungs

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.

Converging Stereotactic Radiotherapy Using Kilovoltage X-Rays: Experimental Irradiation of Normal Rabbit Lung and Dose–Volume Analysis With Monte Carlo Simulation

“…This analysis confirmed the phenomenon of ''dose blurring,'' which appeared as a stretched isodose distribution in the cranio-caudal direction and as a global reduction of absorbed dose in the PTV. These observations correspond with the results of other experimental studies showing dose blurring, underdosages, and penumbral spreading of the dose distribution because of respiratory movement of the target (20,21). In our experiments, the observed CT and histopathologic findings might have been blurred because of these presumed effects.…”

Performance of ultrasound based measurement of 3D displacement using a curvilinear probe for organ motion tracking

“…The probe was clamped to a mechanical articulated arm, which was mounted on a three-axis of motion translational stage (Nioutsikou et al 2006). The position of the transducer was controlled by three stepper motors driven by commands from a PC.…”

Towards accurate dose accumulation for Step-&-Shoot IMRT: Impact of weighting schemes and temporal image resolution on the estimation of dosimetric motion effects

“…[3][4][5][6][7][8]) or by means of planning studies making use of 4D (=3D+t) dose calculation concepts (cf. [9][10][11][12] as examples).…”