Modulated Arc Therapy (mARC) versus intensity-modulated radiation therapy in craniospinal irradiation: Evaluation of CT-slice-defined field junction thickness on dosimetric quality

One of the main technical challenges in craniospinal irradiation (CSI) is the risk of overdosing or underdosing in field overlap regions due to the need to match multiple treatment fields along the Planning Target Volume (PTV). To the authors’ knowledge, the technical and dosimetric feasibility of modulated arc therapy (mARC) has not yet been evaluated for CSI. Unlike other arc techniques, mARC delivers dose in discrete arcs. This study compared mARC with intensity-modulated radiation therapy (IMRT) and analyzed the impact of overlap region thickness, defined by the number of computed tomography (CT) slices, on plan quality. Treatment plans were generated for 10 patients using both mARC and IMRT on a Siemens Artiste linear accelerator. Optimization pseudo-structures were created with 4, 5, 6, and 7 tomographic slices at the craniothoracic and thoracolumbar junctions, resulting in eight plans per patient. The prescribed dose was 36 Gy in 20 fractions. Plans were evaluated by assessing PTV dosimetric parameters, doses to organs at risk (OARs), monitor units (MU), and treatment time. Statistical analysis employed two-way repeated-measures analysis of variance (ANOVA). The treatment technique was the main determinant of dosimetric outcomes. IMRT yielded lower mean dose values to certain OARs (kidneys, heart, lungs, and liver) and higher values for several PTV dose metrics. In contrast, mARC required fewer MUs and shorter treatment times and showed superior conformity and homogeneity indices. The number of CT slices did not produce statistically significant differences, and no significant technique–slice interactions were observed. In CSI, the planning technique has a greater impact on dosimetric outcomes than the thickness of the overlapping field region. mARC improves efficiency in treatment time and MUs and provides improved conformity and homogeneity compared to IMRT. The number of tomographic slices has no impact on dosimetric quality.