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

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.

[1]  J. Unkelbach,et al.  Inclusion of organ movements in IMRT treatment planning via inverse planning based on probability distributions. , 2004, Physics in medicine and biology.

[2]  S Webb,et al.  The effect on IMRT conformality of elastic tissue movement and a practical suggestion for movement compensation via the modified dynamic multileaf collimator (dMLC) technique , 2005, Physics in medicine and biology.

[3]  Steve B. Jiang,et al.  Towards fluoroscopic respiratory gating for lung tumours without radiopaque markers , 2005, Physics in medicine and biology.

[4]  Ajl Harrison,et al.  Technical overview of geometric uncertainties in radiotherapy , 2003 .

[5]  J. Wong,et al.  The use of active breathing control (ABC) to reduce margin for breathing motion. , 1999, International journal of radiation oncology, biology, physics.

[6]  P J Keall,et al.  The application of the sinusoidal model to lung cancer patient respiratory motion. , 2005, Medical physics.

[7]  Nzhde Agazaryan,et al.  An evaluation of gating window size, delivery method, and composite field dosimetry of respiratory-gated IMRT. , 2002, Medical physics.

[8]  M. V. van Herk,et al.  Precise and real-time measurement of 3D tumor motion in lung due to breathing and heartbeat, measured during radiotherapy. , 2002, International journal of radiation oncology, biology, physics.

[9]  Sartaj Sahni,et al.  Leaf sequencing algorithms for segmented multileaf collimation. , 2003, Physics in medicine and biology.

[10]  Warren D D'Souza,et al.  A stochastic convolution/superposition method with isocenter sampling to evaluate intrafraction motion effects in IMRT. , 2005, Medical physics.

[11]  Steve B. Jiang,et al.  An experimental investigation on intra-fractional organ motion effects in lung IMRT treatments. , 2003, Physics in medicine and biology.

[12]  M. Butson,et al.  Post-irradiation colouration of Gafchromic EBT radiochromic film , 2005, Physics in medicine and biology.

[13]  E. Larsen,et al.  A method for incorporating organ motion due to breathing into 3D dose calculations. , 1999, Medical physics.

[14]  J. R. Turner,et al.  High-resolution dosimetry using radiochromic film and a document scanner. , 1996, Physics in medicine and biology.

[15]  S. Webb,et al.  Segmentation of IMRT plans for radical lung radiotherapy delivery with the step-and-shoot technique. , 2004, Medical physics.

[16]  Gikas S. Mageras,et al.  Interfractional anatomic variation in patients treated with respiration‐gated radiotherapy , 2005, Journal of applied clinical medical physics.

[17]  GafChromic RTQA film for routine quality assurance of high-energy photon beams. , 2006, Physics in medicine and biology.

[18]  Lech Papiez,et al.  DMLC leaf-pair optimal control for mobile, deforming target. , 2005, Medical physics.

[19]  J Debus,et al.  Influence of intra-fractional breathing movement in step-and-shoot IMRT. , 2004, Physics in medicine and biology.

[20]  Uwe Oelfke,et al.  Compensation for respiratory motion by gated radiotherapy: an experimental study , 2005, Physics in medicine and biology.

[21]  R. Mohan,et al.  Motion adaptive x-ray therapy: a feasibility study , 2001, Physics in medicine and biology.

[22]  J. Adler,et al.  Robotic Motion Compensation for Respiratory Movement during Radiosurgery , 2000, Computer aided surgery : official journal of the International Society for Computer Aided Surgery.

[23]  H. Shirato,et al.  Four-dimensional treatment planning and fluoroscopic real-time tumor tracking radiotherapy for moving tumor. , 2000, International journal of radiation oncology, biology, physics.