Efficacy of Online Adaptive Radiotherapy Using Surface Guidance in Treatment of Tracheal Adenoid Cystic Carcinoma

Primary tracheal tumors are rare, with adenoid cystic carcinoma (ACC) of the trachea being the second most common malignancy of the trachea. Radical surgical resection is found to have better survival outcomes in tracheal ACC. However, with higher submucosal spread rates in tracheal ACC and the inability to achieve clear margins, complete resection is not usually achievable. In these cases, the use of a 60-70 Gy radiation dose is deemed to be sufficient for definitive treatment with or without concurrent chemotherapy. We report a case of an unresectable ACC treated with online daily adaptive cone beam computed tomography (CBCT) radiotherapy on Ethos™ (Varian Medical Systems, Palo Alto, CA). She was planned to receive 59.4 Gy in 33 fractions in two phases. For daily treatment delivery, the patient was set up on the couch using the surface-guided radiotherapy (SGRT) system of AlignRT™ (Vision RT Ltd., London, UK) and translated to the treatment isocenter. A CBCT scan was acquired, followed by rigid registration with the planning scan and PET CT. Organs at risk (OAR) and primary targets were auto-generated by the AI in a two-step process, reviewed, and edited by the radiation oncologist. Adapted and scheduled plans were compared regarding planning target volume (PTV) coverage and dose to OAR. Better PTV coverage was seen in 26 of 33 fractions with the adapted plan. On the days with lesser coverage, adapted plans demonstrated improvement in the hotspot reduction and reduction in hard dose constraints of the esophagus and lungs. Hence, adapted plans were selected for all treatment days. Our results highlight the superior target coverage and improved OAR-sparing plans in daily online adaptive radiotherapy (o-ART) compared to image-guided radiotherapy (IGRT) plans. The system's ability to adapt to daily anatomical changes, improved target coverage, and better sparing of OARs make it an encouraging option for malignancies requiring motion management.