Conclusion: The choice of model contributes to SC risk fluctuating in favour of either IMPT or VMAT. Large variations were seen across rCTs, indicating that day-to-day variations in anatomy lead to fluctuations in SC risk estimates that are at least of the same magnitude as the inter-patient variations. Organ motion effects should therefore also be accounted for in SC risk estimates. Electronic Poster: Physics track: Treatment plan optimisation: algorithms EP-1625 A dosimetric analysis of

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... utomated knowledge-based VMAT planning for rectal cancer patients Purpose or Objective: To compare the dosimetric features of the semi-automated knowledge-based vs. conventional experience-based VMAT planning for pre-operative rectal cancer patients treated with simultaneous-integratedboosting (SIB) radiotherapy.