Simultaneous Integrated Boost Gamma Knife Planning For Recurrent Malignant Gliomas

Keywords: glioblastoma, Dose, dose planning, gamma knife, radiotherapy

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     Simultaneous Integrated Boost (SIB) techniques have been have developed for fractionated radiation therapy. SIB planning differentially delivers high doses to both the gross target volume (GTV) and lower doses to the surrounding margin region of peri-tumoral edema (PTV-m).
     We report the dosimetric and operational feasibility of a SIB technique developed for Gamma Knife radiosurgery.
     5 lesions in 3 patients underwent treatment planning for recurrent glioma after prior radiotherapy (mean 54 Gy) or for brain metastasis.
     SIB Gamma Knife planning was performed using Gamma Plan 8.2 according to a technique developed by the authors to differentially treat nested target volumes: a higher dose to the gross target volume (GTV) defined by the contrast enhancing lesion as seen on the T1 contrast-enhanced SPGR sequences and a lower, volume-appropriate dose to the typically larger planning target volume (PTV-m) consisting of a margin of the T2 or FLAIR abnormal region or a metastasis surgical resection cavity. All pts were treated on the Gamma Knife Perfexion.
     Average age was 56 yrs. Mean GTV volume = 4.7 cc (.15-14); mean prescribed dose = 16.5 Gy (12-27) @ 70% (65-80). Mean coverage at prescription = 81% (70-94) and mean treatment volume = 5.1 cc (9.8-10.1). Mean PTV-m volume = 31.3 cc (7-57.9); prescribed dose = 8 Gy for re-irradiation and 17 Gy for non-re-irradiation treatment. Mean PTV-m coverage = 98% (95-100) and mean PTV-m treatment volume = 52.4 cc (9.1-86). Mean number of shots = 35 (22-54) and mean on-beam time = 78.0 minutes (33.3-158.6). Median post-treatment follow up after treatments is 16 weeks (6-28) with 1 pt deceased and 2 pts alive without evidence of complications at last follow up.
     This is a retrospective study.
     A SIB treatment planning technique analogous to the SIB technique of fractionated radiotherapy is feasible for treatment with Gamma Knife.  
     So far it has proven most useful for metastasis resection cavities where the cavity boundary and a possible residual nodule receive differential dose, as well as recurrent gliomas where areas of enhancement and white matter infiltration receive differential doses. Details of the planning methodology will be presented.


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