ESTRO 2021 Abstract Book

S1272

ESTRO 2021

Conclusion This study demonstrates that during radiation there is gross changes of volumes in locally advanced head and neck cancers and thus adaptive radiation therapy plays a pivotal role in locally advanced head and neck cancer. PO-1548 Liver deformability as residual error in liver stereotactic radiotherapy F. Patani 1 , C. Di Carlo 1 , M. Valenti 2 , L. Caravatta 3 , S. Costantini 1 , F. Fenu 1 , V. Panni 1 , D. Genovesi 3 , G. Mantello 1 1 Azienda Ospedaliero Universitaria Ospedali Riuniti, Radiotherapy, Ancona, Italy; 2 Azienda Ospedaliero Universitaria Ospedali Riuniti, Medical Physics, Ancona, Italy; 3 Ospedale Clinicizzato "SS. Annunziata", Radiation Oncology, Chieti, Italy Purpose or Objective Breath control methods and image-guided radiotherapy (IGRT) are recommended techniques to online correct liver stereotactic radiotherapy (SRT) uncertainties. Liver deformability is a liver feature mainly influenced by neighboring OARs filling and position and it represents an important issue in the setting of liver SRT, since it is not online correctable by IGRT. Aim of this study was to assess and offline quantify liver deformability as residual error after online IGRT match in patients treated with liver SRT in our Department. Materials and Methods Twenty-six CBCTs/4DCBCTs from 6 patients with unresectable metastatic liver cancer were analyzed. For all the sample, CT simulation was performed with free breathing and short 4DCT (patients 1, 2, 3) or short 4DCT and breath hold CT (patients 4, 5, 6), both with Real-time Position Management (RPM) system. Liver silhouette was offline delineated on each daily CBCT and compared to liver contour on planning CT. On the basis of liver/liver match between CT planning and each daily CBCT, liver center of mass shift on 3 axes was evaluated (Figure 1). Dice Similarity Coefficient (DSC), as deformability index, was estimated. Both were used to calculate residual error and internal margin to compensate it.

Results Breath hold technique was tolerated and reproduced. Deformability was evaluated for all the sample: maximum mean center of mass shift was 0.78 cm (X) with a DSC of 0.85. In patients 1 and 3 liver deformability resulted with mean maximum values of -0.15 cm (X) and -0.33 cm (Y) and DSC of 0.89 and 0.84, respectively. Considering breath hold patients, patient 4 showed a maximum mean value of 1.07 cm on Z axis with a DSC of 0.82, which indicated a high liver deformability though breath hold technique application; patient 5 presented a maximum median value of 0.53 cm (Z axis). Patients 2, 4, 5 and 6, with optimal breath control system, and their mean standard deviations of DSC values were examined; standard deviation of these values was calculated with the result of at least 2 mm (Table 1). It was considered as a preliminary result of inter-