Session Item

In our department, H&N patients are mainly treated with tomotherapy. Patients have up to 33 treatment sessions to complete for a total dose of 69.96 Gy. For each session, an image merge between the initial planning CT and the MVCT is performed. RTTs are responsible for ensuring reproducibility throughout treatment. It is only for the first session that the radiation oncologist validates the fusion. During the treatment process, visible differences in the image fusion occur. The doctor is called when the RTTs see a major difference and assess the need to replan.

Retrospective analysis of the time between the treatment start date and the CT replanning date, as well the analysis of the weight change. Reproduction of the dose distribution on the MVCT of the day before the decision to replan (on Raystation), and analysis of the differences observed in the dose distribution, concerning the coverage of PTVs, the dose on OARs and hot/cold possible areas. New H&N cases that started in a period corresponding to 4 months will be analyzed.

Of 44 H&N patients, 12 (27%) underwent replanning. The average weight loss is 2.8 kg. The average number of days elapsed between the treatment start date and the replanning CT date is 21 days. On the dose distribution evaluating plans applied on the MVCTs, we observe considerable hot regions at the PTVs level and therefore also at the skin level with an average relative max isodose of 111% and absolute max doses up to 78.91 Gy. Generally, the dose on OARs remains under the dose constraints. On the brachial plexus, we see an increase that exceeds the dose constraints limits, with D1 max at 70.55 Gy in one case. Note that these values ​​correspond to a potentially extreme situation, only if we did not do the replanning. We also see a difference in the volume of the external contour of the patient, especially in the neck region, with a visible decrease in the external contour on the MVCT compared to the initial planning CT.

This analysis shows the importance of controlling the weight for H&N patients but also of being vigilant to visible differences on image fusion process. RTTs should be aware of the potential dosimetric implications demonstrated in this analysis. The difference observed between the volume of the external contour on the MVCT and the planning CT, we will call it "intersection volume", could give us more information for a future analysis, in particular, from what difference of the "intersection volume” we would have significant differences (PTV with hot areas, doses on OAR excessive compared to the initial distribution) and create an indicator to help in the decision to replan or not the treatment for H&N patients.