ESTRO 2022 - Abstract Book

S66

Abstract book

ESTRO 2022

Figure 1 - Parameters extracted from DVHs for NTCP model evaluation.

Results Median follow-up was 36 months (range 9-98 months). Grade G1 TLN was reported in 26 patients (24%), while G2 in 14 cases (13%). No TLN > G2 was recorded. 95% of patients had TLN within 50 months from the end of treatment. Among the dosimetric parameters analyzed, D 0.5cc , D 2cc and D Max better correlate with the risk of TLN. The corresponding NTCP curves are reported in Figure 2. For the G2 TLN models based on D 0.5cc , D 2cc and D Max , R 2 was 0.88, 0.95 and 0.92 whereas AUC was 0.80, 0.82 and 0.79, respectively. Moreover, when considering the TD reproducing experimental G2 TLN occurrence, the D 2cc -based TLN model reached the best predictive performance (accuracy = 0.71, sensitivity = 0.82, F-score = 0.33). Tolerance dose at 2 cm 3 of TL for 5% and 20% probability of developing G2 TLN in 5 years were 62.9 Gy (RBE) and 72 Gy (RBE), respectively, confirming the validity of the dose constraint used for plan optimization.

Figure 2 - Figure 1 - D2cc, D0.5cc, DMax NTCP models from univariate analysis. Shading limits the region within the 95% confidence interval. Solid line-open circles (yellow) TLN G > 0; dashed line-filled circles (red) TLN ≥ G2.

Conclusion Patterns of TLN occurrence in patients with skull-base chordoma and chondrosarcoma underwent PT were comparable with literature data. High doses to very small TL volumes were the major predictors of TLN. This analysis showed that the TL dose constraint, currently in use in clinical practice, correctly predicted TLN G2 occurrence.

OC-0091 Brainstem toxicity after proton or photon therapy in children with localized intracranial ependymoma

C. Dalmasso 1 , C. Alapetite 2 , S. Bolle 3 , F. Tensaouti 4 , A. Lusque 5 , J. Desrousseaux 4 , L. Claude 6 , J. Doyen 7 , S. Supiot 8 , V. Bernier-Chastagner 9 , P. Leblond 10 , A. Ducassou 4 , P. Péran 11 , A. Sévely 12 , M. Roques 12 , A. Laprie 4 1 Institut Universitaire du Cancer Toulouse, Radiation therapy, Toulouse, France; 2 Institut Curie, Radiation Therapy, Paris, France; 3 Gustave Roussy, Radiation Therapy, Villejuif, France; 4 Institut Universitaire du Cancer Toulouse, Radiation Therapy, Toulouse, France; 5 Institut Universitaire du Cancer Toulouse, Biostatistic, Toulouse, France; 6 Centre Léon Bérard, Radiation Therapy, Lyon, France; 7 Centre Antoine Lacassagne, Radiation Therapy, Nice, France; 8 Institut de Cancérologie de l'Ouest, Radiation Therapy, Nantes, France; 9 Institut de Cancérologie de Lorraine, Radiation Therapy, Nancy, France; 10 Centre Léon Bérard, Paediatry, Lyon, France; 11 INSERM, Neuroscience, Toulouse, France; 12 Centre Hospitalier Universitaire de Toulouse, Radiology, Toulouse, France Purpose or Objective Ependymomas are the 3rd cause of children’s intracranial tumours and achieve a 5-years local control rate of 50% with a standard post-operative dose of 59.4 Gy. Proton beam therapy (PBT) is the best alternative technic so far to decrease the dose to organs at risk. Recently, retrospective studies reported higher rates of radionecrosis and imaging modifications of the brainstem after PBT than photon radiation therapy (XRT). This led several centers to decrease the prescribed dose to 54 Gy in PBT for posterior fossa ependymomas under 3 years old. The benefit-risk’s balance of the dose reduction is unclear especially since the real incidence of clinically relevant brainstem’s toxicity appears low. This prompted us to compare the incidence of clinical and imaging brainstem toxicity between XRT and PBT and to correlate the MRI changes to the dosimetric data in the retrospective PEPPI French cohort.

Materials and Methods