Influence of Residual Motion Within the Gating Window on Dose Distribution in Respiratory-gated Proton Therapy

<h3>Background/Aim:</h3> Respiratory-gated proton therapy can mitigate interplay effects between the proton beam and target motion. However, the residual motion of the target within the gating window (GW) may still cause clinically relevant dosimetric variations. This study aimed to evaluate target and normal-tissue doses associated with residual motion within the GW. <h3>Patients and Methods:</h3> Eight patients with non-small cell lung cancer were analyzed. For each GW setting, the internal clinical target volume (iCTV) was defined as the union of clinical target volumes (CTVs) within the GW. Maximum residual motion (MRM) was defined as the maximum CTV motion within the GW relative to the 50% phase. Treatment plans were generated for single-field uniform dose (SFUD) and intensity-modulated proton therapy (IMPT). Dynamic 4D dose (D4DD) was calculated, and differences in dose metrics between the treatment plan and D4DD (ΔDM) were evaluated for CTV D_95%, D_98%, and D_2%, and for lungs V_20GyE and average dose (AD). <h3>Results:</h3> Increasing MRM was associated with decreased CTV D_95% and D_98% and increased CTV D_2% for both SFUD and IMPT. When MRM was ≤3 mm, CTV ΔDM remained within ±3%; however, some metrics exceeded 5% when MRM was &gt;5 mm. Lungs ΔAD showed a slight increasing trend with MRM but remained within 0.25%. Lungs ΔV_20GyE showed no clear correlation with MRM and varied from −5.7% to 7.1%; however, the values remained within ±3% when MRM was &lt;3 mm. <h3>Conclusion:</h3> When MRM was &lt;3 mm, ΔDM for both the target and lungs were within ±3%. Therefore, selecting a GW that limits MRM to &lt;3 mm may represent an appropriate criterion for both SFUD and IMPT.