Prediction of optimal needle configuration in the first fraction of cervix brachytherapy.

BACKGROUND AND PURPOSE: Applying needles in the first brachytherapy (BT) fraction for patients with locally advanced cervical cancer allows for more dose conformality and OAR sparing, but is more challenging than in subsequent fractions, as pre-implant imaging with applicator in situ is lacking. We investigate whether a needle simulation, a fixed needle configuration or a multidisciplinary discussion-based configuration can predict more accurately which applicator needle positions are best suited for use in the first BT fraction. MATERIALS AND METHODS: For 20 patients we retrospectively determined the "reference" needle configuration (RC) for the first BT fraction using magnetic resonance imaging (MRI) scans with applicator in situ. We simulated a pre-MRI needle configuration (PC) using the MRI made in the fourth week of external beam radiotherapy (EBRT) without applicator in situ. We generated a fixed needle configuration (FC) from the most common RC needles. Using Dice's similarity coefficient (DSC) we compared each of these needle configurations, including the clinically applied "multidisciplinary consensus" needle configuration (MC), with RC. We considered two scenarios: allowing up to ten needles (scenario 1), and limiting the needle number (scenario 2). The analysis was repeated omitting two mid-ventral needles previously determined as non-essential to treatment planning. RESULTS: For both scenarios, the median DSC for PC and FC was higher than for MC (scenario1:DSCPC = 0,78; DSCFC = 0,75; DSCMC = 0,57; scenario 2:DSCPC = 0,74; DSCFC = 0,73; DSCMC = 0,59), while omitting mid-ventral needles resulted in no statistically significant differences in DSC. CONCLUSIONS: The PC or FC method are at least as accurate as the MC, with the FC preferred for efficiency.