A detailed Monte Carlo model of a proton therapy treatment head was commissioned to simulate the delivery of small field proton treatments. Small fields are challenging in the planning and quality assurance process because of aperture scattering and dosimetric disequilibrium. Four patients with small fields used in all or parts of their treatment course were studied, including two stereotactic patients and two fractionated patients. For the two stereotactic patients the overall difference of the dose covering at least 95% of the gross tumor volume between the Monte Carlo calculations and the delivery was -0.2% and -1.6%, respectively. For the two fractionated patients the overall difference of the dose covering at least 95% of the clinical target volume was -3.0% and 1.0%, respectively. We have thus confirmed that our current planning and delivery process for small proton fields is accurate enough to treat small lesions in the patient. Furthermore, we studied the impact of field size corrections and identified limitations of the pencil beam algorithm for predicting hot and cold spots and range degradation in the target due scattering in heterogeneities. For the four cases studied in this paper, these limitations appear to impact individual field calculations, but do not have a significant impact on the prescribed dose over multiple fields.