Fifty-sixth annual meeting of the American association of physicists in medicine
SU-D-18A-07: Towards 6-Degree-Of-Freedom Real-Time Motion Management in Cancer Radiotherapy
Lung tumor motion has been identified as a major issue that deteriorates treatment efficacy for radiotherapy, especially for SBRT. As tighter PTV margins are applied due to translational compensation, tumor rotation will become the dominant factor limiting tumor targeting accuracy. This is the world-first study quantifies lung tumor rotation by utilizing kV images with fiducial markers and a step towards 6-degree-of-freedom real-time cancer radiotherapy.
Three or four gold coils were implanted as tumor surrogates in 3 lung cancer patients. 50 fractions of 8- minute, 10 Hz 4D CBCT projections were acquired for the patients immediately prior or after radiotherapy. The fiducial marker positions are segmented, reconstructed and used to determine tumour rotation by the iterative closest point algorithm. Different data acceptance and filtering methods were applied to accept data or smooth the marker trajectory.
The average rotation angles around the left/ right (LR), superior/inferior (SI), anterior/posterior (AP) rotations were found to be 0.8±4.2, -0.8±4.5 and 1.7±3.1 degrees respectively. For 28% of the treatment time, the lung tumors rotated more than 5° around the SI axis. Respiration-induced rotational motion was detected in 2 of the 3 lung patients. This can be explained by the patient developed atelectasis during the treatment period. Interestingly, no heart beating component of rotation was observed in the power spectrum. Different rotational types were observed within the patient cohort with large variations in the magnitude of the rotation between patients.
For the first time, continuous tumor rotation has been measured for lung patients with gold fiducial markers. Tumors were found to undergo rotations of more than 5° for almost a third of the total treatment time. The study also demonstrated the feasibility of using continuously kV images for real-time lung tumour motion adaptive radiotherapy which can potentially reduce treatment margins and side effects.
The authors acknowledge the financial support of an NHMRC Australia Fellowship.