Evaluation of Various Resolutions for Optimization and Dose Calculation in VOLOTM Ultra of the Tomo Therapy Treatment Planning System

OBJECTIVE Helical tomotherapy can deliver a radiation dose using modulated intensity. This delivery technique requires inverse treatment planning for dose distribution. The specific parameters include the resolution of optimization and dose calculation. The highest resolution provided a high plan quality in a previous study, but that plan would increase the calculation time. This study investigated the quality of plans using various resolutions of optimization and dose calculation in VOLOTM Ultra. The exploration aimed to determine the optimal resolution to inform the treatment planning process in nasopharyngeal carcinoma (NPC). METHODS Forty-one cases of NPC who had been previously treated using helical therapy were randomly recruited between January 2022 and December 2023. Nine treatment planning scenarios were created using the VOLOTM Ultra planning system using a combination of optimization and calculated resolution. Three different levels of resolutions were chosen: high (H), medium (M) and low (L). The different resolution combinations were expressed as, e.g., H/H for high resolution optimization and dose calculation. The dosimetric parameters of the planning target volumes (PTVs), organs at risk (OARs), and plan quality index, such as dose coverage (D98%, Dose at 98% target volume), conformity index (CI) and homogeneity index (HI), were evaluated. RESULTS The PTVs revealed an increasing dose value when the dose calculation resolution was increased. In contrast, the study found an increasing dose value when the resolution was decreased. A similar trend was also found in the conformity index value in the optimization process. The OARs demonstrated similar dose levels, with the exception of the spinalcord and both parotid glands. CONCLUSIONS This study illustrates that the resolution of M/H and M/M scenarios is a specific choice of interest. These scenarios provide higher dose coverage, have a mild impact on OAR dose, a moderate impact on conformity and a shorter treatment planning time.