Introduction: The work is a comparative study between two modalities of radiation therapy, the aim of which is to compare 3D conformal radiation therapy (3D-CRT) and intensity modulated radiation therapy (IMRT) in treating posterior fossa boost in children with high risk medulloblastoma; dosimetrically evaluating and comparing both techniques as regard target coverage and doses to organs at risk (OAR).Methods: 20 patients with high risk medulloblastoma were treated by 3D-CRT technique. A dosimetric comparison was done by performing three plans for the posterior fossa boost, 3D-CRT, advanced 3D-CRT and IMRT plans, for the same patient using Eclipse planning system (version 8.6). Results: IMRT had a better conformity index compared to other two plans (p-value of 0.000). As for the dose homogeneity it was also better in the IMRT plans, yet it hasn't reached the statistical significant value. Doses received by the brainstem and spinal cord were significantly less in the IMRT plans (p-value < 0.05). The doses reaching the cochlea in the IMRT plans were significantly less when compared with the 3D-CRT plans, yet it did not reach a significant difference when compared with the advanced 3D-CRT plans. Conclusions: IMRT technique was clearly able to improve conformity and homogeneity index and reduce dose to the brainstem and spinal cord. As for the cochlea, IMRT and advanced 3D-CRT plans could deliver doses within its tolerance.