Multi Session Treatments With A Gamma Knife Perfexion: Preliminary Data Review From Radio-biological Point Of View

Keywords: gamma knife, Fractionated radiosurgery, Perfexion, outcome, radiobiology

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     Stereotactic radio-surgery is a technique for treating lesions in a single session. If the target is too close to Organs At Risk (OAR), it is advisable to perform a conventional fractionated radiotherapy because multiple session treatments allow to take advantage of biologic differences between the lesion and normal tissue. Basic principles of radiobiology suggest that the alpha/beta ratio is important to decide the number of fractions: if the difference between the alpha/beta values for target and OARs is great the fractionation is justified; if the values are similar the advantage of a multi session treatment is not so clear.
      In San Raffaele Hospital Milan, until the end of 2005, patients with lesions too close to an OAR, have been sent to Radiotherapy Department; then following the experiences of other radio-surgery centres in Europe and Italy, some multi session treatments have been performed.
     In the period January 2006 – December 2008, 31 patients with benignant brain disease (meningioma and cavernous sinus meningioma), were selected for a 3 days treatment.
     The mean dose delivered to the 50% isodose was 6.8 Gy (6.5 – 7.0) and this value allows to use the linear quadratic model to calculate the Biological Equivalent Dose (BED) to compare treatments delivered in 1, 3 sessions or conventional radiotherapy. The BED is also useful in calculating the probability of normal tissue complications. In our case involved OARs were optical path, chiasm and brainstem.
     Considered alpha/beta 3.3 for meningiomas, 1 for optical path and chiasm, 3 for brainstem. Our results are:
     This is a retrospective study.
     Using LQ model and constrains derived from conventional RT, we had no side effects.
     Our data confirms the goodness of LQ model for radio-surgery at prescribed doses.


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