Optimization of radiation therapy for locally advanced adenoid cystic carcinomas with infiltration of the skull base using photon intensity-modulated radiation therapy (IMRT) and a carbon ion boost

Background: Tumor doses > 70 Gy are needed for local control in adenoid cystic carcinomas. These tumor doses cannot be delivered if the tolerance doses to neighboring organs at risk (OAR) are respected. This treatment planning study investigates the physical advantage of combined photon intensity-modulated radiation therapy (IMRT) plus carbon ion boost compared to photon IMRT alone. Patients and Methods: For nine patients, treatment plans were generated using a) photon IMRT alone (integrated boost concept), and b) sum plans consisting of a photon IMRT plan and a carbon ion boost plan. 54 Gy were prescribed to the planning target volume 1 (PTV1), the boost volume (PTV2) received 72 Gy. The tolerance doses of the delineated OAR were strictly adhered to. Plan quality of IMRT plans and sum plans was compared using adequate physical parameters. Results: Both therapy techniques Lead to highly conformal dose distributions that allow the prescription of the desired target doses. Target conformality and heterogeneity as well as target coverage for PTV1 are comparable for both techniques. The target coverage for PTV2 can be significantly improved using carbon ion beams (median 95% coverage 93.7% vs 87%; p = 0.039). Furthermore, the mean doses to the OAR can be reduced by 8.3% (median % reduction of mean doses to OAR; p = 0.00001) using carbon ions. Conclusions: The combination of photon IMRT with carbon ions improves the target coverage for the boost volume and offers better sparing of OAR close to the PTV2 (gross tumor volume) in comparison with photon IMRT alone. A clinical study has been initiated to evaluate whether these potential advantages translate into clinical benefit.