Contrast-enhanced radiotherapy (CERT) is a binary treatment modality in which the absorbed radiation dose is not only determined by the parameters of the external radiation source but also affected by the concentration of a dose-enhancing agent (DEA) in the studied object. In this work we assessed the distribution of the absorbed dose in a murine B16F10 melanoma injected with a single dose of an aqueous Bi-DTPA solution. The mice were exposed to a single fraction of X-ray irradiation for 28.5 min. In vivo measurements of DEA concentrations were done on a micro-CT scanner using the radiopacity values of malignant tissues from the obtained CT images. We found that the presence of DEA enhanced the absorbed dose more than twofold in 6% of the tumor volume; in 29% of the tumor volume the absorbed dose increased more than onefold. The tumor growth delay time calculated for our model was 0.76 days (we only accounted for the damage caused directly by radiation), whereas in our previous research study tumor growth delay was 10 days. This discrepancy may indicate that in the tumors exposed to contrast-enhanced radiotherapy growth delay results from both the damage directly caused by radiation and other antitumor mechanisms.