Conclusions: The QVH tool provided a method to assess how well the prescription dose to target volumes was met in the treatment plans of this study. The tool could be used to identify a single suboptimal plan and to compare groups of plans which cannot be distinguished by means of the one-dimensional QF value. The QVH tool could, for example, be used in the quality assurance of a multi-centre trial, and for visualizing the learning curve of multiple dose level treatment planning in routine

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... cal practice. It could provide a valuable contribution to level 3 reporting of ICRU report 83. 1 Duprez et al. IJROBP 2011 Purpose/Objective: Recently, a Cyberknife (CK) with an MLC became available. A key CK feature is easy delivery of non-coplanar (n-CP) beams. Objective evidence for added value of such beams is mostly lacking, partly because a common TPS for both systems did not exist until recently. Moreover, commercial TPS require trial-and-error tweaking of plan parameters for individual patients, making the outcome of planning highly user-dependent. With the recent MLC introduction, our in-house algorithm for multi-criterial plan optimization, iCycle, can now also optimize IMRT for CK. With iCycle, plan generation is fully automatic (no per patient tweaking), allowing objective comparison of treatment strategies in a single TPS. In this study we used iCycle to compare n-CP CK and CP linac approaches for prostate cancer. Materials and Methods: For 10 patients, iCycle was used to automatically generate plans with optimized profiles and beam angles for the CK and a conventional linac, emulating HDR brachytherapy dosimetry. For the linac, the beam search space contained 72 coplanar orientations (separation 5°). For both systems, Pareto optimal IMRT plans were automatically generated, containing 1-30 optimized beam directions (60 plans per patient), delivering 4 fractions of 9.5Gy. Low rectum dose was the highest priority OAR objective for generating optimal plans within hard constraints, following achievement of a PTV V 95 of ±99%.Other OARs considered were urethra, bladder, penis, scrotum, and femoral heads. Results: Both for the CK and the linac, PTV V 95 was >99%. For both systems, plan quality did heavily depend on the number of beams included in the plan. For CK, rectum D mean , V 40Gy , V 60Gy , and D 1cc in 25 beam plans were on average 50%, 67%, 71% and 15% lower than in 11 beam plans, for equal PTV coverage. For 25 beam plans, differences between CK and linac in PTV-and peripheral zone D mean and D 98% were on average smaller than 0.3% (p>0.05), while the CK plans had 18.5±8.0%, 23.2±14.1%, 21.4±20.8%, and 3.0±3.8% lower rectum D mean , V 40Gy , V 60Gy , and D 1cc , respectively (p=0.002; for all 10 patients, all 4 rectum parameters were lowest for CK). There were no differences in urethra D mean and D 2% and in bladder D mean . Bladder D 2% was 2.5% lower for CK (p=0.002). For the femoral heads, D mean and D 2% were 20% and 10% lower in CK plans (p=0.002). In all plans, penis/scrotum D mean and D 2% were lower than 2Gy and 4Gy, respectively, as imposed by hard constraints. Delivery time of 25 beam CK plans was 18.1±0.5 min, including imaging and correction for each beam. Conclusions: Non-coplanar hypo-fractionated treatment of prostate cancer with a Cyberknife did strongly reduce rectum dose compared to coplanar linac plans. For both systems, 25 beam plans greatly improved rectum dose compared to 11 beam plans. Due to the MLC, 25 beam CK non-coplanar plans could be delivered in only 18 minutes, including image-guided beam-tumour alignment before delivery of each beam.