Intrafractional Tumour-Tracked Irradiation using a Linac-MR

• Author / Creator

  Yun, Jihyun

• Intrafractional tumour tracking is of considerable interest as a means to minimize the PTV in treating mobile tumours. By utilizing the intrafractional MR imaging feature of linac-MR, this thesis seeks to develop a direct, non-surrogate based intrafractional tumour tracking system, and physically demonstrate its feasibility by delivering highly conformal dose to a moving target undergoing simulated lung tumour motions. An autocontouring algorithm was developed to determine the shape and position of a lung tumour from each intrafractional MR image. Because our linac-MR systems are equipped with low field MRI (0.2/0.5 T), the algorithm was initially evaluated using a lung motion phantom simulating low field MR images by using a single 3 T scanner. Also, an initial in-vivo study was performed to verify the feasibility of lung tumour autocontouring using real patient data. Motion prediction software was developed to compensate for the tumour motions during system delay (time interval between detection of current tumour position and beam delivery) in MRI-based tracking. Prediction accuracy was evaluated using 1D superior–inferior lung tumour motions of 29 lung cancer patients for system delays of 120 – 520 ms. In our prototype linac-MR, MLC motors are operated in the close proximity of the MRI. Due to this, we investigated (1) appropriate RF shielding around the motors to mitigate the negative effects of RF motor noise in MR images, and (2) the effect of strong external magnetic field on the functionality of MLC motors. Intrafractional tumour-tracked irradiation to a moving target was physically demonstrated using the prototype linac-MR. Two different motion patterns (sine and modified cosine) were used to simulate lung tumour motions. Comparing the film measurement results from moving target irradiation with our tracking system to static target irradiation, 50 % beam width revealed minimal differences of < 0.5 mm, while the increase in 80 % - 20 % penumbra width was limited to 0.4 and 1.7 mm in the sine and modified cosine patterns, respectively. The performance of our tracking system shown in this research illustrates potential dosimetric advantages of intrafractional MR tumour tracking in treating mobile tumours as shown for the phantom study.

• Subjects / Keywords

  • Intrafractional
  • Linac-MR
  • Tumour
  • Tracking

• Graduation date

  Fall 2013

• Type of Item

  Thesis

• Degree

  Doctor of Philosophy

• DOI

  https://doi.org/10.7939/R3CZ32D3Q

• License

  This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.