Potential Dose Escalation For Cystic Tumors In Gamma Knife Radiosurgery For Brain Metastasis

Keywords: brain metastasis, cyst, gamma knife, dose escalation, radiosurgery

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     Background: Many brain metastases have an easily visualized cystic component at the time of radiosurgery planning.   Since prescription dose is determined based on size of lesion at planning, lesions with large cystic components will get less dose than those without cysts. 
     It is possible that these fluid filled cystic components can be disregarded when considering prescription dose, as the excess volume is not at risk for increased toxicity.
     : We examined 55 metastatic brain lesions with cystic components from 37 patients treated consecutively on the Leksell Gamma Knife at UAB from September 2008 to June 2009.
      Treatment planning software was used to identify both the tumor and the cystic component and respective volumes were calculated. Potential additional dose was then determined by using the new “tumor minus cystic component” volume to calculate dose based on standardized data.
     The average volume of metastatic lesions and cystic components were 8.3cc (0.23 to 26.3) and 1.6cc (0.01 to 8), respectively. The percentage of cystic component ranged between 0.3 to 69.7% of the lesion (mean 18.8% and SD=11.85), suggesting high variability in the proportion of cystic component within tumors. The mean dose originally prescribed was 17.4 Gy, and 17.7 Gy was prescribed after subtracting cystic volume. Potential additional dose ranged between 0 to 2 Gy, and directly correlated with both absolute and relative cystic volume and is inversely proportionate to the original prescription dose as determined by tumor volume (R2= 0.85).
     This is a retrospective study.
     This analysis demonstrates the dosimetric feasibility of delivering higher radiation doses to tumors with larger cystic components. 
     Although potential dose increase for patients with relatively small cystic components is low, we have shown that the potential to increase dose exists for patients with larger cystic components and may lead to improved local control at no additional toxicity.


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