DOSIMETRIC COMPARISON AT CRIBRIFORM PLATE AND MIDDLE CRANIAL FOSSA BETWEEN DIFFERENT ISOCENTERS IN WHOLE SUBARACHNOID IRRADIATION

Abstract

Backgrounds : For whole subarachnoid irradiation in patients with CSF seeding tumor, an important factor of tumor recurrence and survival rate is adequate tumor coverage. The two common missing area are cribriform plate and middle cranial fossa. Objectives : To compare dose coverage at cribriform plate and middle cranial fossa from prophylactic cranial irradiation between using lateral orbital rim isocenter and mid-brain point isocenter technique. Materials and Methods : The study was performed in patients having undergone cranial CT simulation from January 2011 to February 2012. Two-lateral-opposed whole subarachnoid irradiation was planned on CT brain images using two different points of isocenters, i.e. at mid-brain point (point A) for plan A and at lateral orbital rim (point B) for plan B. The Varian Eclipse planning system version 8.0 was applied. Isodose distributions were assessed at cribriform plate, middle cranial fossa and eye lenses for each plan. Results : All 44 patients were enrolled in this study. The median age of patients was 38 years. Male patients were 26 (59.1% ) and female patients were 18 (40.9% ). The most common diagnosis was CNS tumor (63.64 %). The study found that plan B significantly improved the minimum doses at CTV and PTV (p-value = 0.001 and 0.002, respectively). Mean lens dose in plan B was more than in plan A by 3.6 percentage points of the prescribed dose. Conclusions : Using isocenter at lateral orbital rim for prophylactic cranial irradiation increased the dose coverage of the minimum dose at whole brain, especially in the area of cribriform plate and middle cranial fossa comparing with using mid-brain point isocenter technique.