ESTRO 2021 Abstract Book

S60

ESTRO 2021

OC-0086 PTV margins for intrafraction motion in MRI-guided online adaptive radiotherapy in rectal cancer C. Kensen 1 , T. Janssen 1 , A. Betgen 1 , L. Wierseman 1 , F. Peters 1 , P. Remeijer 1 , C. Marijnen 1 , U. Van der Heide 1 1 Netherlands Cancer Institute, Radiotherapy, Amsterdam, The Netherlands Purpose or Objective In MRI-guided radiotherapy (MRgRT) with online adaptive replanning on the daily anatomy, the primary remaining uncertainties are intrafraction motion and delineation uncertainty. As adaptive replanning is time consuming and non-periodic motion can occur during this period, a second plan adaptation prior to starting the irradiation, could improve accuracy and limit the necessary PTV margins. The aim of this work was first to determine the PTV margins required for intrafraction motion for MRgRT of rectal cancer and second to determine an action level for 2 nd adaptation to reduce the required PTV margin by 50%. Materials and Methods Sixteen patients with rectal cancer received short course radiotherapy (n=9) or long course (chemo)radiotherapy (n=7) on a 1.5T MR-linac. During each fraction T2-weighted images were acquired for adaptation (MRI adapt ), for verification prior to starting the irradiation (MRI ver ) and after irradiation (MRI post ). On the images of 5 treatment fractions per patient, the mesorectum CTV was delineated. The CTV on MRI adapt was expanded to PTV adapt . All margins were anistropic with the anterior expansion a factor 1.6 times all other expansion directions. Per patient we determined the required size of PTV adapt such that for all fractions 95% of CTV on MRI post was covered. We focused on intrafraction motion, disregarding contouring and other uncertainties. The required population margin was determined by taking the 90 th percentile of these values. To study the effect of a 2 nd plan adaptation after MRI ver , we determined the margin required between MRI adapt and MRI ver following the same method. We studied the tradeoff between a possible margin reduction and the number of fractions where a 2 nd adaptation would be needed by varying the action level on when to replan (Figure 1). We then determined the number of 2 nd adaptations required to reduce PTV margins by 50%.

Results Median time between MRI adapt

and MRI ver

was 12 minutes (range, 6-44) and 12 minutes (range, 8-18) between

MRI ver . PTV margin for online adaptation without 2 nd replanning were 6.4 mm in the anterior direction and 4.0 mm in all other directions. The effect of performing 2 nd adaptations is shown in Figure 2. In 32% of the cases there was motion between MRI adapt and MRI ver . Correcting all these cases (action level = 0), would lead to a PTV margin of 2.4 mm/1.5 mm. For a 50% reduction of the PTV margin to 3.2 mm/2 mm, a 2 nd adaptation would have been necessary in 17% of the fractions where the motion exceeded 1.2 mm. and MRI post.

Conclusion We demonstrated the potential benefit of intrafraction motion monitoring and 2 nd replanning, for PTV margins in online adaptive MRgRT in rectal cancer. At an action level of 1.3 mm (2 mm in the anterior direction), 2 nd adaptations are needed in 17% of the fractions. This could result in a 50% margin reduction. For clinical practice other uncertainties should be incorporated in the margin. OC-0087 Benchmarking daily plan adaptation on the Unity MR-Linac T. Jagt 1 , T. Janssen 1 , A. Betgen 1 , L. Wiersema 1 , R. Verhage 1 , S. Garritsen 1 , T. Vijlbrief-Bosman 1 , P. de Ruiter 1 , F. Peters 1 , P. Remeijer 1 , C. Marijnen 1 , J. Sonke 1 1 Netherlands Cancer Institute, Radiation Oncology, Amsterdam, The Netherlands