Radiobiologically based analysis of the influence of breathing motion and DIR method on lung treatment plans

Abstract

Objective

The purposes of this work are to investigate the influence of breathing motion on the quality of delivered radiation therapy treatment for lung cancer patients, to study the effect of tumor volume movement and anatomy change (due to the breathing motion) on the dose distribution through a radiobiological analysis, and finally, to examine the influence of the deformable image registration (DIR) method used on the 4D dose accumulation.

Methods

The work is based on the 4D CT image sets and treatment plans of ten lung cancer patients. For each patient, the actual delivered dose was approximated through the development of the “4D” treatment plan, which was created by applying the plan of the average intensity projection (AIP) CT on the ten respiratory phases of the 4D CT dataset. Additionally, the “4D optimal” plan was created, which was based on the optimized plans of each of the 4D CT phases using the same beam setup and objectives as for the 4D AIP plan. Equal weights were used for the different phases, and for both the 4D and 4D optimal cases, the individual phase plans were exported from the treatment planning system (TPS) to the Velocity AI where the dose distributions were summed using DIR together with the extended deformable multipass (EXDMP) and deformable multipass (DMP) methods. A radiobiological analysis was performed based on the radiobiological parameters of the involved organs at risk (OARs) and planning target volume (PTV).

Results

Using the complication-free tumor control probability (P ₊) index, non-negligible differences in P ₊ were observed between the 4D and 4D optimal dose distributions as well as between the dose distributions that were generated with different DIR methods. The optimal P ₊ values ranged from 9.4 to 98.8 % for the 4D case and from 17.1 to 99.1 % for the 4D optimal case. The clinical P ₊ values ranged from 4.3 to 81.4 % for the 4D case using the EXDMP method and from 4.3 to 81.5 % for the DMP method. These differences emphasize the significance of using the proper DIR algorithm and its influence on the final dose distribution.

Conclusion

The radiobiological and dosimetric analyses showed the importance of taking into account the breathing motion during treatment planning. That was especially evident for smaller tumors and tumors with large motion extent. Also, the importance of choosing the proper DIR method was found to be great since it may lead to large differences in the accumulated 4D dose distribution.

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