Accounting for tumour rotation in real time

Tumours are not static inside the human body but are moving continuously. Prostate tumours were observed to translate (move from side-to-side) up to 25 mm and lung tumours to rotate (tilt) by up to 45 degrees. These motions cause the radiation beam to misalign with the tumour, thus reducing the likelihood of curing the cancer and increasing toxicity to normal tissue. However, none of the 44,000 cancer patients receiving radiotherapy in Australia each year have had tumour motion accounted for during treatment, as the technology is not yet clinically available.

The rationale of this study is to adapt the radiation beam to the moving tumour (red area) by dynamically moving the MLC leaves (yellow blocks) in real time, so that the irradiated healthy tissue (blue area) surrounding the tumour can be greatly reduced by the proposed strategy (right) relative to the current clinical practice (left).

Tumour rotation

We will develop a real-time six degree of freedom (6 DoF; 3 DoF of translation and 3 DoF of rotation) tumour motion management system. The goals of this project are to:

  • measure the tumour 6 DoF motion in real-time from kilovoltage (kV) images,
  • continuously track and adjust the radiation beam to target the tumour as it moves, and
  • determine the clinical benefit of real-time management of 6 DoF tumour motion

Method

The figure below illustrates the 6 DoF tumour motion management strategy. As the linac gantry rotates around the patient during treatment (1), the kV imager acquires 2D projections of the prostate (2). The 6 DoF motion measurements were determined from the individual 3D marker positions, after using methods to reject spurious and smooth noisy data, using an iterative closest point algorithm (3), and (4) tumour motion is adapt with MLC tracking.

Tumour rotation

The 6 DoF Kilovoltage Intrafraction Monitoring (KIM) software has been used in a world-first clinical trial of 6 DoF tumor motion monitoring using KIM method at the Royal North Shore Hospital.

Tumour rotation

Results

Continuous kilovoltage (kV) projections of tumors with gold fiducial markers were acquired during radiotherapy for 267 fractions from 10 prostate cancer patients and immediately prior or after radiotherapy for 50 fractions from 3 lung cancer patients.

There were large variations in the magnitude of the tumour rotation among different fractions and patients. Various rotational patterns were observed. The average prostate rotation angles around the LR, SI and AP axes were 1.0±5.0, 0.6±3.3 and 0.3±2.0 degrees respectively. For 35% of the time, the prostate rotated more than 5° about the LR axis indicating the need for intrafractional adaptation during radiation delivery.

For lung patients, the average LR, SI and AP rotation angles were 0.8±4.2, -0.8±4.5 and 1.7±3.1 degrees respectively. For about 30% of the time, the lung tumours rotated more than 5° around the SI axis. Respiration induced rotation was detected in two of the three lung patients.

Tumour rotation
Tumour rotation

Figures below show kV images of a rotated prostate (up) and lung (down). Compared to the initial position (left) the prostate has rotated -15.2°, about the LR axis and the lung tumour has rotated -12.3° around SI axis. Blue triangles represent the tumour initial position. Yellow arrows and dashed triangles denote the rotation direction and the rotated tumour positions.

Tumour rotation
Tumour rotation

Tumour 6 DoF motion trajectory data for download

Lung tumour 6 DoF intrafractional motion trajectory
Prostate 6 DoF intrafractional motion trajectory

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Disucssion and conclusion

This project is a world-first study addressing both rotation and translation during radiotherapy. 95% of the cancer radiotherapy systems purchased today incorporate kV imagers, therefore the proposed system can be readily translated into the practice of clinical radiotherapy as it can be performed with no additional requirements in terms of clinical hardware or workflow, therefore maximising the benefit of the resources currently available for cancer control.

We are looking for outstanding students major in physics, engineering and IT to join the world-leading tumour motion adaptation team. If you are interested in doing an honour, master or PhD in any of the broad topics of this project, please contact [http://sydney.edu.au/medicine/people/academics/profiles/chen-yu.huang.php||Dr Chen-Yu Huang]].