Planning-Based Dosimetric Comparison of Three-Dimensional Conformal Radiotherapy (Flattened Versus Unflattened Beams), Intensity-Modulated Radiotherapy, and Volumetric Modulated Arc Therapy for Pituitary Adenomas on Halcyon.

This study was undertaken to perform a dosimetric comparison of four distinct radiotherapy planning strategies for pituitary adenomas on the Varian Halcyon platform (Varian Medical Systems, Palo Alto, CA, USA): three-dimensional conformal radiotherapy (3DCRT) using electronically flattened beams, 3DCRT using true flattening filter-free (FFF) beams, intensity-modulated radiotherapy (IMRT), and volumetric modulated arc therapy (VMAT). The study additionally evaluated the feasibility and dosimetric implications of reproducing conventional forward-planned beam characteristics on an inherently FFF platform using dynamic multileaf collimator (MLC)-based electronic fluence shaping. Planning data from 15 patients were utilized. Four plans were generated per patient: 3DCRT with electronically flattened beams, 3DCRT with true FFF beams, IMRT, and VMAT. A prescription dose of 54 Gy in 30 fractions was applied with ≥95% planning target volume (PTV) coverage. Plan evaluation included D2%, D50%, D98%, conformity index (CI), homogeneity index (HI), gradient index (GI), and doses to optic nerves, chiasm, lenses, cochleae, brain, and brainstem. All techniques achieved clinically acceptable coverage (V95% > 99%). IMRT and VMAT showed superior conformity compared with 3DCRT (p < 0.0001), with VMAT achieving the lowest HI. IMRT provided the lowest mean doses to lenses. VMAT achieved the lowest maximum doses to optic nerves, eyes, brain, and brainstem, and the lowest mean doses to the chiasm and brainstem. True 3DCRT FFF required fewer monitor units (MUs) and had sharper gradients, whereas electronically flattened plans were more uniform. Inverse-planned techniques provide superior organ-at-risk (OAR) sparing in pituitary adenomas. VMAT offers superior normal tissue dose constraints. Their complementary strengths suggest that technique selection should be guided by anatomical considerations and proximity of critical structures, emphasizing individualized planning rather than reliance on a single modality. Significant dosimetric differences exist between electronically flattened and true 3DCRT FFF delivery, underscoring the importance of beam model selection in forward-planned radiotherapy for pituitary adenomas.