CT-Guided Online Adaptive Stereotactic Body Radiotherapy (SBRT) for Retroperitoneal Sarcoma After Previous Radiotherapy: A Case Report.

CT-guided online adaptive radiotherapy (OART) is a novel and robust treatment technique in radiotherapy. Thanks to its excellent accommodation of inter-fraction variations, OART is characterized by superior accuracy compared to other contemporary treatment techniques in radiotherapy such as image-guided radiotherapy (IGRT). Planning target volume (PTV), which takes into account interfraction movements, could therefore be reduced while utilizing OART with a consequent dose reduction for adjacent healthy tissue. Herein we report our successful experience in treating a patient with retroperitoneal sarcoma after previous radiotherapy and surgery. The tumor was in close proximity to the spinal canal and abutted a large bowel segment and the last portions of the duodenum. Radiotherapy regimen consisted of 30 Gy in five fractions. Treatment implementation and delivery were feasible and the treatment was given without any interruptions. After a follow-up period of nine months so far, the patient reported no radiation-related adverse effects. Furthermore, her post-treatment magnetic resonance imaging (MRI) scans demonstrate good radiographic response.  Our case highlights the use of OART to treat a challenging case in radiotherapy as it was given in the setting of re-irradiation to a mesenchymal tumor, which is considered exquisitely radio-resistant.