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Patients undergoing radiotherapy are positioned to restrict motion to ensure treatment reproducibility and accuracy. Internal anatomy, including the uterus, moves independent of bony surrogates and surrounding soft tissue. Currently, a large target volume is irradiated allowing for intra-fraction uterine motion; however, this may not provide adequate PTV coverage. A retrospective feasibility study was planned to explore uterine movement in patients with full and empty bladders, in preparation for adaptive radiotherapy for cervical cancer.       

A retrospective feasibility study included a descriptor of uterine position (Fig.1), geometric analysis of uterine motion with a full bladder (n=6 patients) versus an empty bladder (n=6 patients), followed by a comparison of dose constraints respectively (whether they were met or not) . Geometric measurements were taken as per Fig.2; measurements of uterine movement in superior-inferior (SI) and anterior-posterior (AP) direction. Systematic and random errors were analysed with respect to the average geometric displacement with standard deviation per patient and per population between the planning CT scan and CBCT scan.

The modal uterine position was anteverted for full and empty bladder protocols.  The systematic and random errors of uterine motion for full and empty bladder can be observed in Tab.1.A vector systematic error was more notable in the empty bladder group, similarly the random error was more notable in the full bladder group. Dose constraints were met in both groups however PTV margins (covering uterine fundus movement) were insufficient in 15 to17% of CBCTs.  

 Table 1. Systematic & random errors of uterine motion

Our findings were consistent with the available evidence regarding the normal uterine position. However, the systemic displacement of the uterus in (SI) and (AP) directions were higher than the reported normal. Further work improving bladder and bowel preparation will be crucial in the short term to ensure PTV coverage and avoidance of healthy tissue. In the medium term, an adaptive PoD protocol should be introduced to ensure a more individual tailored plan to cover PTV and avoid healthy tissue. In the longer term, a plan to prescribe PoD or standard radiotherapy to those with mobile or fixed uteri, respectively, would ensure patient tailored treatment and efficiency.