Comparison of conformal and intensity-modulated radiotherapy of the posterior cranial fossa with the aim of reducing cochlea dose

Radiotherapy followed by cisplatin chemotherapy of paediatric standard-risk medulloblastoma is associated with a high incidence of deafness. Cochlea-sparing radiotherapy has been suggested when delivering the posterior fossa (PF) boost in the second phase of the treatment. However, the possible dosimetric undercoverage of planning target volume (PTV), leading to a reduced probability of cure, has not been quantified. This information must be known and has been addressed in this study for different cochlea-sparing techniques. Methods: For 5 patients’ CT-MRI registered datasets, the PF was grown in 3D by 3 mm to give the PTV. A 3 mm margin was used around each cochlea to give the cochlea Planning organ-at-Risk Volume (PRV). Calculated using a collapsedcone convolution algorithm on the Pinnacle3 3D treatment planning system, a lateral conformal parallel pair (PP) was compared to 3 cochlea-sparing techniques: conformal posterior wedge pair, conformal 4-field non-coplanar (4F) and inverse-planned 4-field non-coplanar IMRT (4F-IMRT). Results: Compared to PP, cochlea-sparing techniques approximately halved cochlea PRV dose (P<0.001), but underdosed PTV at the 95% isodose level by up to 3% of its large volume (P<0.05) and increased pituitary, hypothalamus and parotid mean doses. Also, they at least halved the volume of non-target brain receiving >75% isodoses at the expense of increased volumes receiving lower doses. Conclusions: IMRT improves on the normal-tissue high-dose sparing achieved by the 4F technique and also minimizes the volume of brain receiving moderate doses. However, the cochlea-sparing techniques lead to some PTV underdosage and increase the volumes of normal brain receiving ‘low’ doses. This trades potential reduction of deafness incidence against potential loss of local control (cure) and risk of other late sequelae and merits an appropriate clinical trial.

Comparison of conformal and intensity-modulated radiotherapy of the posterior cranial fossa with the aim of reducing cochlea dose Keywords:

IMRT , conformal radiotherapy , medulloblastoma