Abstract
Abstract Aim: The dosimetric and clinical advantages offered by implementation of pencil beam scanning (PBS) proton therapy for moving thoracic tumours is hindered by interplay effect. The purpose of this study is to evaluate the impact of large proton beam spot size along with adaptive aperture (AA) and various motion mitigation techniques on the interplay effect for a range of motion amplitudes in a three-dimensional (3D) respiratory motion phantom. Materials and Methods: Point doses using ionisation chamber (IC) and planner dose distributions with radiochromic film were compared against the corresponding treatment planning system (TPS) information. A 3D respiratory motion phantom was scanned either for static or 4D computed tomographic (CT) technique for 6-, 10- and 14-mm motion amplitudes in SI direction. For free breathing (FB) treatment, a tumour was contoured on maximum intensity projection scan and an average scan was used for treatment planning. Each FB treatment was delivered with one, three and five volumetric repaintings (VRs). Three phases (CT40–60%) were extracted from the 4D-CT scans of each motion amplitude for the respiratory-gated treatment and were used for the treatment planning and delivery. All treatment plans were made using AA and robustly optimised with 5-mm set-up and 3·5% density uncertainty. A total of 26 treatment plans were delivered to IC and film using static, dynamic and respiratory-gated treatments combinations. A percent dose difference between IC and TPS for the point dose and gamma indices for film–TPS planner dose comparison was used. Results: The dose profile of film and TPS for the static phantom matched well, and percent dose difference between IC and TPS was 0·4%. The percent dose difference for all the gated treatments were below 3·0% except 14-mm motion amplitude-gated treatment. The gamma passing rate was more than 95% for film–TPS comparison for all gated treatment for the investigated gamma acceptance criteria. For FB treatments, the percent dose difference for 6-, 10- and 14-mm motion amplitude was 1·4%, −2·7% and −4·1%, respectively. As the number of VR increased, the percent difference between measured and calculated values decreased. The gamma passing rate met the required tolerance for different acceptance criteria except for the 14-mm motion amplitude FB treatment. Conclusion: The PBS technique for the FB thoracic treatments up to 10-mm motion amplitude can be implemented with an acceptable accuracy using large proton beam spot size, AA and robust optimisation. The impact of the interplay effect can be reduced with VR and respiratory-gated treatment and extend the treatable tumour motion amplitude.
Highlights
Proton therapy with pencil beam scanning (PBS) technique has been more susceptible to the interplay effect[3] which refers to deviation of delivered dose distributions from the planned distributions due to combined effects of inter-field tumour motion and spot scanning
We quantitatively evaluated the interplay effect using 3D motion phantom for Mevion’s S250i system for the range of tumour motion for free breathing (FB) and motion mitigation techniques such as volumetric repaintings (VRs) and respiratory-gated treatments
An interplay effect depends on a number of factors, including beam spot size, spot spacing, tumour motion amplitude, size of the tumour, among others
Summary
Proton therapy has demonstrated dosimetric advantages over photon therapy for the treatment of thoracic tumours.[1,2] Proton therapy with pencil beam scanning (PBS) technique has been more susceptible to the interplay effect[3] which refers to deviation of delivered dose distributions from the planned distributions due to combined effects of inter-field tumour motion and spot scanning. Treatment plans made using the PSPT technique were relatively robust against the interplay effect which lacks intensity modulation and the ability to adjust modulation width for various thicknesses of tumour resulting in additional dose to nearby critical structures.[4] Motion mitigation techniques reduce the impact of the interplay effect for PBS, lowering the critical organ doses and aid in therapeutic dose escalation for tumours while maintaining critical organ doses.[5]
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
