Abstract: Background and purpose: The treatment planning of conventional intensity-modulated radiotherapy (IMRT) is a time-consuming optimization process. The aim of this work was to study the feasibility of quick auto-planning (QAP) technique using a new beam morphing method to improve the treatment planning efficiency (planning time <10 s) in rectal cancer radiotherapy. Methods: The QAP method was used to analyze the boundary deformation from the planning target volume (PTV) of source plan (template plan) to the PTV of the treatment plan to be designed for a new patient (target plan) in order to derive the source-totarget shape mapping function. The derived shape mapping function was used to morph the source beam aperture to the target beam aperture over all segments in each beam direction. Secondly, the beam weights of the target plan were re-calculated. The approach was tested on nine rectal cancer patients (one patient with template plan, eight target patients for QAP and manual planning) for proof-of-principle purpose. The planning time and the plan quality (dose endpoints of PTV and organs at risk) of the QAP plans were evaluated. Results: The QAP planning time of eight patients was less than 6 s each. For the template plan, QAP plans (x±s) and manual plans (x±s), the PTV D_mean was 51.70, (52.90±0.40) and (52.00±0.30) Gy. The conformity index (CI) was 1.12, 1.13±0.05 and 1.05±0.06. The bladder D_mean was 35.00, (35.80±5.00) and (36.70±5.10) Gy. The left femoral head D_mean was 27.40, (29.40±3.70) and (24.70±2.70) Gy. The right femoral head D_mean was 29.20, (28.40±2.20) and (24.20±2.80) Gy. Conclusion: The QAP with a new developed beam morphing method can be applied to the IMRT planning for rectal cancer radiotherapy. The plan quality by QAP is close to the plan quality of the template plan and manual plans. Compared with the conventional optimization method, the planning efficiency of the QAP is significantly improved, and its feasibility is verified.

Key words: Intensity-modulated radiotherapy, Quick auto-planning, Beam morphing method, Rectal cancer, Radiotherapy