Abstract
Radiosurgery is an established option in trigeminal neuralgia. Irrespective of devices used, the pain relief with radiosurgery occurs after a lag time of about 6-8wks. Recent introduction of FFF beams with dose rates around 2000mu/mt results in abbreviated beam-on time (BOT) - 12mts & overall treatment time (OTT) - 22-28 mts. Purpose of this study is to analyse the pain response lag time in trigeminal radiosurgery using unflat beams (@dose rate 2000) and flat beams (@dose rate 600). Twenty consecutive patients aged between 33-88 yrs with Trigeminal neuralgia (Typical -17, Atypical-3) underwent double reinforced mask immobilisation, CT (0.725mm post contrast axial) and MR simulation (0.75mm 3D space, 3-D TOF sequences). Following rigid fusion, a single isocenter was positioned at either RGZ or the DREZ along the symptomatic trigeminal nerve to deliver doses of 75-80 gy prescribed to 90% isodose line. All patients underwent treatment on a C-arm linac with 5mm stereotactic cones, 9 arcs with 3D-CBCT image guidance for positional accuracy. Sub-mm and sub-degree precision all through treatment was achieved using 6-DOF robotics & intra-fraction imaging after every 4000 mu delivery. 10 consecutive pts who underwent treatment with Unflat beams were compared with retrospective cohort of 10 patients who underwent treatment with flat beams. The pain outcome of patient, Lag time for response, pain recurrence, and treatment-related complications were evaluated. Cox regression analyses was used to evaluate the time to response and the duration of pain relief in relation to the type of beams used. Using the Barrow Neurological Index (BNI) pain score, 9 cases (90%) and 8 cases (80%) of unflat and flat beams Radiosurgery achieved treatment success (BNI pain score I–IIIb) within first 2months. However the Median time to initial response after the Radiosurgery was significantly shorter in the unflat beam cohort (mean 2.5 weeks) than in the Flat beam cohort (mean 6.4 weeks) (p=0.044). There were 5 pts in BNI 1 & 2 within first 1 month post radiosurgery for unflat beams. Frequency of facial numbness was lower in the Unflat beams compared to the Flat beams (0%, 10%). Faster treatments are associated with less patient mobility and increased delivery accuracy. Prolonged duration of radiosurgery amplifies the impact of Increased submm mobility of the nerve in the prepontine cistern, factor never considered, & which might negatively affect the amount of clinically relevant fibers receiving the minimal radiation dose necessary to block excessive sensory information. To the best of our knowledge, this is the first study reported in literature which looked into the impact of the abbreviated treatment times on pain outcomes using unflat beams for trigeminal radiosurgery. Larger study is initiated to study the effects of dose rates on pain outcomes.
Published Version
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