What is the cost of cochlear avoidance in pediatric medulloblastoma? evaluation of 3D conformal planning techniques for the posterior fossa boost

Abstract

Purpose/Objective: There is an increasing tendency to employ complex radiotherapy techniques for the posterior fossa (PF) boost with the aim of avoiding organs at risk (OR). The current favorite is cochlear avoidance to reduce ototoxicity from the combination of radiotherapy and adjuvant cisplatin. We assessed the consequences of cochlear avoidance on dose coverage of planning target volume (PTV) and dose distribution of non-PTV brain. We discuss the potential impact on tumor control and long term neurocognitive outcome. Materials/Methods: For 6 fused CT and MRI pediatric datasets the PF (tumor bed, cerebellum, brainstem and leptomeningeal extensions) was outlined as the clinical target volume and grown in 3D by 5mm to give the PTV. Non-PTV brain and both cochleas were localized as ORs. The planning organ at risk volume (PRV) for the OR included a 3D margin of 5mm. 3D treatment plans were generated on ADAC Pinnacle3 using 6MV photons. We compared 2 techniques using conformal blocking; a conventional lateral parallel opposed pair (PP) and a posterior wedged pair with the aim of reducing the cochlear dose to <50% (WP). The DVH of the PTV and PRV were compared using a 2 tailed paired t test. Results: WP results in significant underdosage of PTV at the 90% and 95% isodoses (p<0.01). While a smaller volume of brain receives doses of 25Gy (p<0.001), a larger volume of brain receives doses of 11Gy (p<0.001) due to exit into the temporal lobes. IMRT techniques (data not shown) produce similar results with more widespread distribution of lower doses in non-PTV brain. Conclusions: The conformal WP technique reduces the cochlear doses to <16Gy from the PF boost, which is translated into a reduction from 55.8Gy to 39.4Gy with the CSA dose included. The clinical impact of this dose reduction on hearing in patients treated with cisplatin is not clear. The consequence of cochlear avoidance is significant PTV underdosage, which may have an impact on tumor control. The reduction in volume of normal brain receiving high doses with the WP technique is accompanied by a marked increase in volume of normal brain receiving lower dose (11Gy). The total dose is 34.4Gy (11Gy + 23.4Gy from CSA) and is in the range associated with an increased risk of neurocognitive impairment. Increasing complexity of PF treatment delivery in pediatric medulloblastoma trades reduction of OR toxicity against a potential worsening of tumor control and higher risk of cognitive dysfunction. Tabled 1TechniquePTV underdosageNon-PTV brain dosesmean volume of PTV receiving <90% of dosemean volume of PTV receiving <95% of dosemean volume receiving 11Gy meanvolume receiving 25Gycm3%cm3%cm3%cm3%PP000.30.1225.421.4181.717.3WP8.32.811.73.9304.128.9104.39.9 Open table in a new tab