The Unity MR-Linac makes adaptive planning possible, which is especially effective for stereotactic body radiation therapy (SBRT) treatments in the liver and lung due to the excellent contrast of the MRI. Performing a patient specific quality assurance (PSQA) measurement on every liver SBRT treatment plan ensures the highest quality treatments are delivered. However, tumors in the liver and lung can be located very laterally in the body which has posed a challenge in performing the PSQA measurements in the past. This work provides a simple, yet accurate, methodology for the PSQA measurement of very lateral tumors which is applicable to other centers that have an MR-Linac system.