ESTRO 2020 Abstract Book

S1096 ESTRO 2020

physique et d'optique, Quebec, Canada ; 2 CHU de Québec, Université Laval- Département de radio- oncologie et Axe Oncologie du CRCHU de Québec, Quebec, Canada Purpose or Objective To access any potential catheter reconstruction limits of an afterloader (Flexitron - Elekta AB, Stockholm, Sweden) equipped with electromagnetic (EM) tracking capabilities. Material and Methods A Flexitron research unit was equipped with a special check-cable integrating an EM sensor (NDI Aurora V3), enable tracking and reconstruction capability. Catheter r econstructions were performed with two methods: continuous fixed-speed check cable retraction and stepping through each dwell position. A catheter was connected to the afterloader and moved perpendicular from its axis with increment of 1mm. The reconstruction were 23.8 cm long. A Meca500 robotic arm (Mecademic, Montreal) (Fig.1A) with 0.01mm accuracy was used to move one of the catheters between each reconstruction. Reconstruction using dwell-positions was performed with the tracking at 5 mm step. Catheters were reconstructed using different check cable speed (1.0, 2.5, 5.0, 10.0, 25.0, 50.0 cm/s). Reconstructions were obtained with a locally weighted scatterplot smoothing algorithm. To quantify the reconstruction accuracy, distances between two catheters were computed along the reconstruction track with a 10 mm step. Reconstruction in a phantom was used to access the feasibility of curved catheter reconstruction, parallel and perpendicular to the field generator. Finally, the check cable was also used to detect simulated error of 0.1 to 1 mm of the indexer length. Results Using a 50 cm/s check cable speed does not allowed accurate reconstruction (Fig. 1C). A 1 cm/s speed can clearly differentiate all reconstructions down to a 1 mm inter-catheter distance (Fig. 1B). A slower check cable speed further resulted in smaller standard deviation (Fig. 1E). Being in the optimum operating distance from the field generator resulted in a lower absolute mean deviation from expected value (0.2±0.1mm) than being closer to the edge of the electromagnetic sensitive detection volume (0.6±0.3mm). A speed of 5 cm/s provides a good compromise between reconstruction accuracy and speed (0.27 ± 0.08mm). Reconstruction of curved catheter with a check cable speed under 5 cm/s gives a 0.8mm± 0.3mm error. Comparing dwells position with a one standard deviation of 0.1mm, 96% of the 1.0mm shift were detected.

Conclusion The EM-equipped Flexitron afterloader is able tracked and reconstruct catheters with high accuracy. A speed under 5cm/s is recommended for catheter reconstruction. At that speed all catheter swaps down to 1 mm inter-catheter distance are detected. The check cable can also be used to detect shift errors. OC-1045 Dose-volume effect relationships for rectal morbidity after brachytherapy for pediatric cancers. E. Romano 1 , R. Simon 2 , V. Martin 1 , S. Bolle 1 , M. Andraud 1 , G. Boulle 1 , M. Kissel 1 , T. Kumar 1 , H. Martelli 3 , F. Guerin 3 , E. Deutsch 1 , C. Haie-Meder 1 , C. Chargari 1 1 Gustave Roussy Institute, Department of Radiation Oncology, Villejuif, France ; 2 Hospices Civils de Lyon, Public Health, Lyon, France ; 3 Kremlin Bicetre University Hospital, Department of Pediatric Surgery, Kremlin Bicêtre, France Purpose or Objective To establish dose-volume effect relationships predicting rectal morbidity in children with pelvic tumors treated with image-guided brachytherapy. Material and Methods Medical records of all consecutive children with pelvic tumors treated in our center and receiving three- dimensional image-based pulsed-dose-rate (PDR) brachytherapy with or without external beam radiation therapy (EBRT) between 2005 and 2018 were reviewed. Morbidity was scored according to the CTC-AE 5.0. We retrospectively examined the impact of D 0.5cm 3 , D 1cm 3 , D 2cm 3 (doses converted into equivalent doses per 2-Gy fractions according to the linear quadratic model with α/β=3), total reference air kerma (TRAK) and the volume of 100% isodose in terms of rectal toxicities (acute and late). Results A total of 78 consecutive children were included. Median age was 2.9 years (range: 0.8-14.9 years) and the male/female ratio was 1.3. Most of the tumors were embryonal bladder / prostate rhabdomyosarcoma (66%) or vaginal botryoid rhabdomyosarcoma (22%). Six patients received EBRT and median total dose (brachytherapy + EBRT) was 60 Gy (range: 48.73 – 65 Gy). Median follow-up was 21.3 months (range: 0.2-162.4 months). Median D 0.5cm 3 , D 1cm 3 and D 2cm 3 were 53.4 Gy (range: 10.4-97.6 Gy), 45.7 Gy (range: 8.2-74.8 Gy), and 35.4 Gy (range: 6.4-69.4 Gy), respectively. At last follow-up, a total of 30 children (38%) experienced grade ≥ 1 acute or late rectal events: 24% had grade 1 events, 7.7% had grade 2 and 6.4% had grade 3. No grade > 3 toxicity was observed (e.g. fistula or stricture). Most frequent rectal events reported were rectal bleeding, encopresis, abdominal-pelvic pain and transit disorder. There was a significant correlation between the D 0.5cm 3 , D 1cm 3 and the probability of grade 1–3 (p=0.009 and 0.017, respectively) and grade 2–3 rectal morbidity (p=0.007 and 0.049, respectively). D 2cm 3 did not show significant correlation with rectal toxicities (p=0.057 for grade 1-3 and p=0.407 for grade 2-3). D 0.5cm 3 was significantly