Quantitative Differences in Planned and Delivered Dose for Liver SBRT Using MRI Guided Delivery

Abstract

An MRI guided Cobalt-60 radiation therapy unit at our facility continuously gates delivery, when MRI images indicate that the target is within a specified boundary to ensure delivery of dose as planned. While the GTV holds a relatively stable position in the field via gating, organs-at-risk (OARs) may greatly change their position between fractions compared to the original plan. Magnetic resonance imaging during treatment allows accurate determination of delivered doses to OARs not easily delineated with cone beam or megavoltage CT due to abdominal motion and bowel gas artifact. We compare planned and delivered doses to OARs for six cases of MRI guided SBRT to the liver to determine the extent of dosimetric deviations. Gross tumor volume, PTV, and OARs were contoured on planning scans and 18 scans obtained prior to delivery of SBRT to lesions within or adjacent to the liver of six patients. Each patient was planned for five fractions. Contours were drawn on the MRI scans. Diagnostic MRI and CT scans were fused to aid in contour definition and treatment planning. Delivered doses were calculated using the daily MRI scans. Deviations from delivered to planned doses were calculated so that positive numbers reflect delivered doses being higher than planned. All deviations were normalized by planned dose. The patients included four males and two females, averaging 63.8 years of age. Mean difference between planned and delivered GTV D99% was -1.5%, 95% CI = [-3.4, 0.5%]. On average, deviations between planned and delivered maximum doses to 0.5 cm3 of OARs were 0.6% for cord [-1.9, 3.2%], 0.8% for bowel [-4.7, 6.3%], -6.6% for esophagus [-13.3, 0.1%], –5.9% for skin [-9.0, -2.7%], and 0.7% for stomach [-7.7, 9.0%]. The average deviation for the 700 cm3 of liver receiving the lowest dose was -6.5% [-12.0, -1.0%]. Kidney deviations, calculated as percent receiving 2 Gy per fraction, averaged greater than planned by 0.9% [-10.5, 12.2%]. The number of individual daily doses that exceeded the planned dose by >10% was: bowel (26.7% of the fractions), right kidney (47.1%), liver (11.1%), and stomach (6.7%). There were no large daily deviations for cord, esophagus, or skin. Maximum single fraction deviations from planned dose ranged from 5% for skin to 42% for kidney. Deviations between planned and delivered doses to OARs are small when averaged across numerous fractions, yet, mobile OARs such as bowel, kidney, and stomach, had large variation. Several individual fractions were found to deliver doses significantly greater than planned. Although these deviations average out to minimal net departure from the total planned dose across patients, the data raise concern about accumulating elevated doses across fractions for individual patients. Daily MRI guidance and scans can be useful for calculating delivered doses to OARs to ensure that cumulative doses do not exceed tolerances.