A dosimeter that is configured to assist with patient-specific quality assurance (“QA”) in arc-based radiotherapy, such as stereotactic radiotherapy, is provided. The dosimeter facilitates acquiring multiple different dosimetric measurements simultaneously. For instance, the dosimeter can be configured to acquire dosimetric measurements from multiple different modalities, including gel dosimetry and film dosimetry, and is configured for simultaneous use with a diode-array QA phantom.