On the interplay effect for moving targets treated with the CyberKnife static tracking system

Purpose: To assess the interplay effect amplitude between different planned MU distributions and respiratory patterns in the CyberKnife system when treating moving targets with static tracking technique. Methods: Small-and Large-Respiratory Motions (SRM and LRM) differing in amplitude and frequency were simulated in a semi-anthropomorphic dynamic thorax phantom. The interplay effect was evaluated for both respiration motions in terms of GTV coverage and conformity for three plans designed with an increasing range of MU per beam (small, medium and large). Each plan was delivered three times changing the initial beam-on phase to assess the inter-fraction variation. Dose distributions were measured using radiochromic films placed in the GTV axial and sagittal planes. Results: Generally, SRM plans gave higher GTV coverage and were less dependent on beam-on phases than LRM plans. For SRM (LRM) plans, the GTV coverage ranged from 95.2% to 99.7% (85.9% to 99.8%). Maximum GTV coverage was found for large MU plans in SRM and for small MU plans in LRM. Minimum GTV coverage was found for medium MU plans for both SRM and LRM. For SRM plans, dose conformity decreased with increasing MU range while the variation was reduced for LRM plans. Large MU plans reduced the inter-fraction variation for SRM and LRM. Conclusions: We confirmed the interplay effect between target motion and beam irradiation time for CyberKnife static tracking. Plans with large MU per beam improved the GTV coverage for small motion amplitude and the inter-fraction dose variation for large motion amplitude.IPHY

A treatment planning comparison of contemporary photon-based radiation techniques for breast cancer

Background and purpose: Adjuvant radiation therapy (RT) of the whole breast (WB) is still the standard treatment for early breast cancer. A variety of radiation techniques is currently available according to different delivery strategies. This study aims to provide a comparison of six treatment planning strategies commonly adopted for breast-conserving adjuvant RT and to use the Pareto concept in an attempt to assess the degree of plan optimization. Materials and methods: Two groups of six left- and five right-sided cases with different dose prescriptions were involved (22 patients in total). Field-in-Field (FiF), two and four Fields static-IMRT (sIMRT-2f and sIMRT-4f), Volumetric-Modulated-Arc-Therapy (VMAT), Helical Tomotherapy (HT) and Static-Angles Tomotherapy (TomoDirect™ – TD) were planned. Dose volume constraints were taken from the RTOG protocol 1005. Pareto fronts were built for a selected case to evaluate the reliability of the plan optimization process. Results: The best target dose coverage was observed for TD able to improve significantly (p < 0.01) the V95% in a range varying from 1.2% to 7.5% compared to other techniques. The V105% was significantly reduced up to 2% for HT (p < 0.05) although FiF and VMAT produced similar values. For the ipsilateral lung, V5Gy, V10Gy and Dmean were significantly lower than all other techniques (p < 0.02) for TD while the lowest value of V20Gy was observed for HT. The maximum dose to contralateral breast was significantly lowest for TD (p < 0.02) and for FiF (p < 0.05). Minor differences were observed for the heart in left-sided patients. Plans for all tested techniques were found to lie on their respective Pareto fronts. Conclusions: Overall, TD provided significantly better results in terms of target coverage and dose sparing of ipsilateral lung with respect to all other evaluated techniques. It also significantly minimized dose to contralateral breast together with FiF. Pareto front analysis confirmed the reliability of the optimization for a selected case. Keywords: Treatment planning, Whole-breast irradiation, Planning comparison, Pareto fron

Clinical evaluation of a novel optimization algorithm for cyberknife IRIS-based treatment plans

Purpose: To report on the clinical implementation of the VOLOTMoptimization algorithm for SRS/SBRT treatments and its dosimetric comparison with the previous Sequential Optimization (SO) algorithm. Materials and methods: Forty patients treated for brain, spine, prostate and lung tumors, and planned with the SO algorithm were re-planned with VOLOTM. Comparison involved target coverage, conformity, gradient and homogeneity indexes (CN, GI and HI) and specifc indicators of dose to OARs. Plans were also compared in terms of number of nodes, beams, MU and delivery time. All dose distributions designed with VOLOTM were validated via ionisation chamber measurements. Results: The following statistically signifcant dosimetric differences were observed. VOLOTM was superior in terms of target coverage for prostate (99% vs 96%) and spine (92% vs 81%), GI for brain (4.41 vs 4.76), CN for brain (0.77 vs 0.72), and for brain and urethra dose sparing. SO gave better results for GI for prostate (3.67 vs 4.05) and maximum dose to brain stem. VOLOTM showed a steeper peripheral dose fall-off for brain and lung cases; SO was superior for prostate and spine cases. Overall, the number of nodes, beams and MU were reduced with VOLOTM up to 36%, 14% and 31%, respectively. The average reduction of delivery time was 20% (ranging from 8% for brain to 30% for prostate). The mean 1D dose measurement deviation from calculation was –0.2% (range –1.3–1.7%). 2D dose measurements returned a mean distance-to-agreement of 0.8±0.8 mm and a mean dose difference of 0.6±1.0%. Conclusions: VOLOTM optimization algorithm provided clinically acceptable dose distributions, most of the time better than SO. The treatment time was signifcantly reduced up to 30%. The peripheral dose fall-off increased for large collimator sizes. The verifcation measurements proved its clinical implementation

Hypersonic film cooling

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