Purpose: Organ motion may have an impact on the radiation dose administered in
radiotherapy. Motion can occur during treatment (intrafractional motion) or between fractions
of treatment (interfractional motion). A clinically relevant dosimetric impact on OAR would mean that additional measures such as 4D-CT planning or a PRV are needed. In this study, the
dosimetric impact of intra- and interfractional motion is studied for TBI delivered with VMAT is studied.
Method: For the intrafractional part, motion magnitude was determined with 4D-CT data, and this magnitude was used to expand and reduce the volumes of the OAR in 27 VMAT-TBI plans, dose was analyzed in these volumes. For the interfractional part, the CBCT images taken right before treatment are registered with the planning CT. The planning CT is then deformed to match the anatomy of the CBCT for each fraction. The dose is recalculated on this deformed CT and the mean dose in OAR volumes is evaluated for each fraction.
Results: For the intrafractional part, on average, an expansion with a typical respiratory motion magnitude results in a 0.3 Gy higher mean dose in the kidneys. This expansion results in a 0.6 Gy higher mean dose in the left lung and a 0.8 Gy higher dose in the right lung. For the interfractional part, the mean dose difference between the fraction with the lowest mean dose and the one with the highest mean dose was 0.04 Gy on average.
Conclusion: After discussing these dose differences with a radiooncologist, it was concluded that these differences were not clinically relevant. Therefore, 4D-CT planning and a PRV are
not necessary.