Potential toxicity benefit and inter-fraction robustness of proton arc therapy compared to IMPT and VMAT for nasopharyngeal cancer patients


Radiotherapy (RT) in nasopharyngeal cancer (NPC) patients presents challenges due to proximity of many anatomical structures to the target volume. Furthermore, inter-fractional changes must be considered to assure target coverage. Proton arc therapy (PAT) potentially reduces healthy tissue dose compared to IMPT and VMAT. The impact of PAT on dose to organs-at-risk (OARs), predicted acute- and late radiation toxicities and robust target coverage to inter-fraction changes in NPC patients were investigated.
Approach:
Robustly optimized PAT plans were compared to clinical VMAT and IMPT plans for 10 NPC patients treated with 70.00 Gy to the primary target (CTV 7000) and 54.25 Gy to the prophylactic lymph nodal area (CTV 5425). Integral body dose and mean and max in 0.03cc dose (Dmean and max D0.03cc) in OARs were compared. Normal tissue complication probability (NTCP) values for 22 acute and late radiation-induced toxicities were evaluated. A PAT "base approach" and nine PAT planning approaches to improve PAT inter-fraction robust target coverage were investigated. Target coverage was evaluated on in total 54 weekly repeated CT images (rCTs).
Main Results:
PAT integral dose reduced by on average 55% and 15% compared with clinical VMAT and IMPT, respectively. Compared to IMPT, average Dmean and max D0.03cc were significantly reduced in all evaluated neurological structures. Compared to IMPT, in the PAT plans Dmean was reduced most in the arytenoids, PCM medius and brainstem by on average 8.0 Gy, 6.4 Gy and 6.1 Gy, respectively and all evaluated NTCP's for both acute and late timepoint were significantly reduced. Compared with IMPT, PAT base approach target coverage on rCTs was worse. Approaches to improve PAT inter-fraction target coverage were successful, while maintaining the reduction in NTCP compared to IMPT.
Significance:
Compared to IMPT and VMAT, PAT reduces healthy tissue dose and subsequent estimated toxicity risks in NPC patients. PAT planning approaches to improve inter-fraction robustness were employed successfully, while NTCP benefits of PAT were maintained.&#xD.