Posterior fossa boost in medulloblastoma: an analysis of dose to surrounding structures using 3-dimensional (conformal) radiotherapy.

PURPOSE: The purpose of this study was to compare three different techniques of delivering the posterior fossa boost in patients with medulloblastoma. METHODS AND MATERIALS: Five patients underwent CT simulation for treatment planning of the posterior fossa boost. For each slice, the posterior fossa was contoured in addition to the cochlea, non-posterior fossa brain, pituitary gland, mandible, parotid glands, thyroid gland, pharynx, and cervical spinal cord. For each patient, plans for three techniques of delivering the posterior fossa boost were compared. Technique A utilized parallel-opposed lateral fields using bony landmarks (2-dimensional radiotherapy); the other two techniques were planned using 3-dimensional radiotherapy. Technique B utilized a pair of coplanar wedged posterior oblique beams, whereas Technique C utilized a pair of posterior oblique fields and a vertex field. Dose-volume histograms (DVH) were obtained for each of the organs contoured and for each technique and patient. The maximum, minimum, and mean dose to each organ were determined using the DVH program in our treatment planning system. RESULTS: In 3 of 5 patients, the planning target volume (PTV) was not included in the treatment field with Technique A. The cochlea received 100%, 50%, and 42% of the prescribed posterior fossa dose using Techniques A, B, and C respectively. The mean dose to the non-posterior fossa brain was highest with Technique C, intermediate with Technique A, and lowest for Technique B. The mean dose to the non-posterior fossa brain with Technique B was comparable to the mean non-posterior fossa brain dose delivered using parallel-opposed lateral fields based on the CT definition of the PTV. Likewise, mean dose to the pituitary gland was lowest for Technique B. Both Techniques B and C were associated with a higher mean dose to the thyroid gland, mandible, parotid glands, and pharynx. CONCLUSIONS: The use of Technique B minimized the radiotherapy dose to the cochlea, pituitary gland, and non-posterior fossa brain. Contrary to what one may expect, conformal radiotherapy using Technique B did not deliver a higher dose to the non-posterior fossa brain over standard parallel-opposed lateral fields. Other advantages of conformal techniques B and C over 2-dimensional radiotherapy are the inclusion of the PTV in all patients and a lower mean dose to the pituitary gland. The main disadvantage of conformal Techniques B and C employed in our patients is a higher mean dose to the thyroid gland and other tissues in the neck.