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Abstract We present ∼10–40μm SOFIA-FORCAST images of 11isolatedprotostars as part of the SOFIA Massive (SOMA) Star Formation Survey, with this morphological classification based on 37μm imaging. We develop an automated method to define source aperture size using the gradient of its background-subtracted enclosed flux and apply this to build spectral energy distributions (SEDs). We fit the SEDs with radiative transfer models, developed within the framework of turbulent core accretion (TCA) theory, to estimate key protostellar properties. Here, we release the sedcreator python package that carries out these methods. The SEDs are generally well fitted by the TCA models, from which we infer initial core massesM_cranging from 20–430M_⊙, clump mass surface densities Σ_cl∼ 0.3–1.7 g cm⁻², and current protostellar massesm_*∼ 3–50M_⊙. From a uniform analysis of the 40 sources in the full SOMA survey to date, we find that massive protostars form across a wide range of clump mass surface density environments, placing constraints on theories that predict a minimum threshold Σ_clfor massive star formation. However, the upper end of them_*−Σ_cldistribution follows trends predicted by models of internal protostellar feedback that find greater star formation efficiency in higher Σ_clconditions. We also investigate protostellar far-IR variability by comparison with IRAS data, finding no significant variation over an ∼40 yr baseline.