Conformity In Gamma Knife Model C Vs Perfexion: Manual Plugging Vs Sector Plugging In The Planning Of Complex Targets

Keywords: gamma knife, dose planning, technique, radiosurgery, robotics

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     Gamma Knife (GK) “Perfexion” (PFX) has been object of a few recent reports comparing its capabilities with previous GK devices in regard to efficiency in treatment planning and radiation protection. Increased comfort for patient and operators with treatment time reduction, allow to increase the number of shots to obtain better conformability. Compared to previous GK models more complex treatment volumes are supposedly made possible by the PFX "dynamic shaping" of dose distribution.
     Our study compared the approach of experienced operators (team with 5000 GK treatments over a 15yrs period) to planning and the resulting planning performances respectively in GK model-C and PFX, focusing on prescription dose conformity, sparing of risk organs and normal brain low doses.
     Treatments of complex targets created in LGK 3C were re-planned in Perfexion planning software and conversely complex targets treated in Perfexion were re-planned by 3C software. The same operator performed all treatment planning, both in model-C and in PFX. Each plan was evaluated for conformity index, dose to the risk organs and distribution of low doses.
     Perfexion planning shows high conformity index and shorter time of treatment. Significant use of manual plugs in 3C planning could not literally be transformed in sector plugging in PFX: loss of flexibility in coronal and sagittal planes was evident, due to the rigidity of source-block by sector geometry. The use of blocked sectors in PFX tends to concentrate very low doses in more restricted areas versus the “smoother” spread in model-C.
     This is a retrospective study.
     Treatment planning in model-C compared to PFX requires a different “approach” in order to obtain isodose conformity. Experienced operators routinely employing collimators plugs, shaped in personalized patterns in each patient, will encounter a limitation due to the sectors design in shaping lower isodoses, particularly in the Z axis. In PFX operation the same conformity index and the same sparing of risk organs was attained by means of a larger number of shots and more shots with sectors “off”.
     Both latter factors may increase risk to normal brain by concentrating low doses in restricted areas. Clinical implications need to be evaluated in additional studies.


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