5 Peripheral doses in stereotactic brain treatments: Comparison Versa HD®/CyberKnife® M6™

Introduction Stereotactic radiotherapy is known to deliver a very high dose per fraction. The corresponding peripheral dose has to be taken into consideration as it increases late toxicities and the risk of second cancer. A recent study has been conducted for Cyberknife® (CK), evaluating peripheral doses according to collimator type and aperture [1] . The purpose of this study is to compare peripheral doses during brain stereotactic treatments with two linear accelerators: Versa HD® (Elekta) equipped with Exactrac® (Brainlab) versus Cyberknife® M6™ (Accuray). Methods The doses were measured with GR 200A (LiF:Mg, Cu, P) thermoluminescent dosimeters (TLD). Each TLD was individually calibrated in a 6 MV beam. The TLD responses were analysed with a PCL3 automatic reader (FIMEL). The doses were measured for several points representing organs at risk: thyroid, breast, umbilicus and gonads. The peripheral dose evaluation in Versa HD® was performed on an anthropomorphic phantom (CIRS ATOM 701-c) for 10 treatment plans, calculated with Pinnacle v9.10 (Philips) with 4 VMAT non-coplanar arcs delivering 30 Gy (in 3 fractions). The peripheral doses in CK® were in vivo measurements for 23 patients with a 3-fractions treatment (total dose between 24 and 30 Gy). Treatment plans were performed with Multiplan TPS v5.3 (Accuray) with 3 collimator types (fixed, Iris™ and MLC Incise2™). The peripheral doses were expressed as a proportion of dose per monitor unit (% cGy/UM). Results Table 1 compares the peripheral dose for both linear accelerators. The doses measured in the thyroid were 2 to 3 times higher in VersaHD® treatments than in CK®: 0.16% cGy/UM and 0.05% cGy/UM (mean for 2 collimators type), respectively. The breast, umbilicus and gonads dose measurements were not significantly different between both equipment. With CK® the thyroid doses were 2 to 3 times higher with MLC than with Iris™ and fixed collimators. Due to the fact that CK treatment plans need 2.7 times more MU, the thyroid doses measured for the entire treatment were not significantly different between 2 linear accelerators: 76.5 mGy with VersaHD® instead of 61.5 mGy with CK® (mean for 3 collimators types). This dose decreased with distance in VersaHD®: 6 mGy delivered to the gonads instead of 32 mGy with CK® (mean for 3 collimator types). Download high-res image (269KB) Download full-size image Conclusions For a same dose prescribed to the target volume, the peripheral dose delivered during treatment depends on accelerator type, collimator type and also MU number. The doses delivered to the thyroid are equivalent for both accelerator types and can reach 75 mGy. However, VersaHD® stereotactic treatments can divide by five the peripheral dose at a large distance compared to CK®. The peripheral dose should therefore be evaluated in paediatric and benign treatments and should be a criterion for the optimisation of treatment plans.