KEY POINTS
- Fifteen cervical cancer patients treated on Ethos with 50.4 Gy in 28 fractions were retrospectively replanned using five strategies: standard nine-field intensity-modulated radiotherapy and dual-arc volumetric modulated arc therapy with 2°, 3°, 4°, or 5° angular resolution. All plans used the same anatomy, objectives and automated optimization settings.
- Target coverage and conformity were similar across techniques. However, the homogeneity index was significantly better with intensity-modulated treatment (0.07±0.01 versus 0.09–0.10; p<0.001), and near-maximum target dose was lower than with most arc configurations.
- Several pelvic organ-at-risk metrics favored intensity-modulated treatment: rectal V40 was 49.85% versus 52.17–54.91%, bladder V40 44.60% versus 47.26–48.19%, and marrow V30 48.60% versus 51.77–52.46%.
- Bowel sparing showed a clear trade-off. Arc therapy reduced bowel V30 from 22.47% to approximately 20.1–20.5%, but intensity-modulated treatment reduced bowel D2cc from 53.24–53.66 Gy to 52.33 Gy.
- Automatic optimization was dramatically faster with intensity-modulated radiotherapy: 122.8 seconds versus 705.9–742.9 seconds for arc therapy. Arc delivery itself was much faster, however, at 158.1 versus 305.3 seconds.
- Once optimization, final calculation and delivery were combined, the estimated total workflow still favored intensity-modulated treatment: 472 seconds compared with 893–937 seconds for the four arc strategies. Thus, faster arc delivery did not compensate for substantially longer online plan generation.
- All strategies exceeded 98% gamma passing at 3%/3 mm. The comparison was retrospective and platform-specific, used planning rather than continuously changing intrafraction anatomy, and provides no evidence that the modest dosimetric differences translate into clinical toxicity differences.
CLINICAL TAKEAWAY
For Ethos cervical online adaptation, the fastest beam delivery is not necessarily the fastest adaptive workflow. On the tested software version, nine-field intensity-modulated radiotherapy offered a better overall balance of plan generation speed and dosimetry, although these findings should be re-evaluated as optimization software evolves.