Adensity assignment method for dose monitoring in head-and-neck radiotherapy

Background and purposeDuring head-and-neck (H&N) radiotherapy, the parotid glands (PGs) may be overdosed; thus, atool is required to monitor the delivered dose. This study aimed to assess the dose accuracy of apatient-specific density assignment method (DAM) for dose calculation to monitor the dose to PGs during treatment.Patients and methodsForty patients with H&N cancer received an intensity modulated radiation therapy (IMRT), among whom 15 had weekly CTs. Dose distributions were calculated either on the CTs (CTref), on one-class CTs (1C-CT, water), or on three-class CTs (3C-CT, water-air-bone). The inter- and intra-patient DAM uncertainties were evaluated by the difference between doses calculated on CTref and 1C-CTs or 3C-CTs. PG mean dose (D-mean) and spinal cord maximum dose (D-2%) were considered. The cumulated dose to the PGs was estimated by the mean D-mean of the weekly CTs.ResultsThe mean (maximum) inter-patient DAM dose uncertainties for the PGs (in cGy) were 23 (75) using 1C-CTs and 12 (50) using 3C-CTs (p0.001). For the spinal cord, these uncertainties were 118 (245) and 15 (67; p0.001). The mean (maximum) DAM dose uncertainty between cumulated doses calculated on CTs and 3C-CTs was 7cGy (45cGy) for the PGs. Considering the difference between the planned and cumulated doses, 53% of the ipsilateral and 80% of the contralateral PGs were overdosed by +3.6Gy (up to 8.2Gy) and +1.9Gy (up to 5.2Gy), respectively.ConclusionThe uncertainty of the three-class DAM appears to be clinically non-significant (<0.5Gy) compared with the PG overdose (up to 8.2Gy). This DAM could therefore be used to monitor PG doses and trigger replanning.