3DCRT for posterior fossa: Sparing of surrounding organs at risk

Abstract

Introduction: Radiotherapy of the posterior fossa is routinely delivered using 3DCRT parallel-opposed lateral fields. However high incidence of sensorineural hearing loss, hypothalamic– pituitary dysfunction, thyroid and gonadal dysfunction during radiotherapy makes the need for treatment plan which provides adequate coverage of the target volume while sparing of the cochlea and other surrounding organs at risk (OARs) at same time inevitable.
Aim of the work: To compare the coverage of posterior fossa and the dose to surrounding OARs including non-posterior fossa brain, pituitary, cochlea, eyes, optic nerves, optic chiasm, cervical spinal cord, thyroid gland, pharynx, parotid glands and mandible using three different 3DCRT plans.
Methods: Ten patients underwent CT simulation for treatment planning of posterior fossa boost. The CT data were transferred to Precise Elekta treatment planning system where posterior fossa, non-posterior fossa brain, pituitary, cochlea, parotid glands, cervical spinal cord, thyroid gland, pharynx, mandible, eyes, lenses, optic nerves and optic chiasm were all contoured. For each patient, three plans were carried out; two parallel opposed open lateral photon fields, a pair of wedged posterior oblique fields, and a pair of wedged posterior oblique fields and an open vertex field. For all plans, the dose distributions and dose volume histogram parameters (DVPs) for the PTV and OARs were compared and analyzed statistically using excel sheet 2003 and SPSS spreadsheet (SPSS base 18). 
Results: Posterior fossa dose coverage and its homogeneity were adequate and comparable for the three plans. A part from high mean dose received by cochlea, plan 1 shows the best sparing for other OARs. 
Conclusion: 3DCRT using parallel opposed fields is recommended for posterior fossa irradiation boost as it minimizes the exit dose to all structures other than the cochlea, however its mean dose was within the tolerance.

KEYWORDS Medulloblastoma; Post fossa boost irradiation; 3DCRT; Cochlea sparing