Abstract Book

S1072

ESTRO 37

Material and Methods CT scans of 5 patients were used in this study. The CTV was defined as the 0.5 cm tissue around the applicator (then subtracting the applicator). Total dose was 30Gy delivered in 5 fractions. In HDR-BRT, dose was prescribed at a distance of 0.5 cm from the surface applicator. To take into account set-up uncertainties in external VMAT irradiation, a PTV was obtained from CTV by an expansion of 3 mm. Two VMAT plans were generated using a full arc rotation. The first plan was optimized with an anatomy- based optimization module (AB_VMAT) using a 1mm MLC beam margin to enhance dose heterogeneity and dose fallout. The second plan was generated with a full- inverse planning module (FI_VMAT). CTV and PTV coverage in terms of D99%, mean dose (Dmean) and D1% were used for plan comparison. Conformity indexes (CI100 and CI50), gradient index (GI) and homogeneity index (HI) were calculated. For rectum and bladder, mean dose and doses to 2cc volume were used for evaluation. To account for various dose heterogeneity distributions we calculated the equivalent uniform dose (EUD) using the Niemerko model. A Kruskal-Wallis analysis of variance (ANOVA) followed by Dunn’s-type multiple comparisons between any two of the three techniques was performed. The treatment time was also compared between the three techniques. Results VMAT plans provided targets coverage comparable with HDR (p<0.05). Dose distributions were more heterogeneous with HDR: Dmean were 144.2% for CTV in HDR and 118.0 and 109.1% for PTV in AB_VMAT and FI_VMAT, respectively. The mean values of EUD for CTV were: HDR 40.9 Gy; AB_VMAT 39.0 Gy; FI_VMAT 33.3 Gy. GI indexes were 2.81, 3.37 and 4.15 for HDR, AB_VMAT and FI_VMAT, respectively. Dmean and maximal doses to 2cc for rectum and bladder were lower in HDR-BRT plans compared to AB_VMAT and FI:VMAT (rectum Dmean: 39.8% vs 47.4% and 53.0%; bladder Dmean: 35.4% vs 40.4% and 46.1%,; rectum D2cc: 104.8% vs 108.0% and 116.2%; bladder D2cc: 86.64% vs 94.0% and 100.0%). The mean treatment time for AB_VMAT was approximately 2.5 min on a conventional linac. Conclusion HDR-BRT provided superior dose distribution with respect to VMAT. AB_VMAT plans tend to better mimic the HDR dose distribution, providing a fast, safe and reasonable alternative. AB_VMAT plans showed a successful capability of highly conformal dose distribution, EUD values similar to HDR, steep dose-gradient outside PTV and minimal treatment time. EP-1972 Assessment of multiple treatment planning techniques for intracranial Stereotactic Radiosurgery M. Chu 1 , C.R. Davies 1 , M.G.J. Williams 1 , N.S. Iqbal 2,3 , M.C. Smyth 1 , A.J. Bryant 1 , J.R. Powell 4 , A.E. Millin 1 1 Velindre Cancer Centre, Medical Physics, Cardiff, United Kingdom 2 Cardiff University, Division of Cancer and Genetics, Cardiff, United Kingdom 3 Cardiff University, Cardiff University Brain Research Imaging Centre, Cardiff, United Kingdom 4 Velindre Cancer Centre, Clinical Oncology, Cardiff, United Kingdom Purpose or Objective Brain metastases occur in 20-40% of cancer patients with common primary sites including lung, breast, kidney and melanoma [1]. Locally, Stereotactic Radiosurgery (SRS) is used to treat patients with 1-3 brain metastases and WHO performance status 0-2. The current clinical protocol involves a forward planned, dynamic conformal arc technique using the iPlan Treatment Planning System (TPS) (BrainLAB, Germany). This work investigates whether an inverse planned VMAT technique using the RayStation TPS (RaySearch Laboratories, Sweden) can

evaluate also the heart irradiation. All patients were simulated and treated using the Active Breath Coordinator (Elekta), a spirometer enabling a temporary controlled interruption of patient breathing at the end of inspiration phase. HMRT plans were inversely optimized by combining two open fields with six-eights subfields in two tangential beam. Open fields were setup to include the whole breast with a 2 cm flash region and to carry the 80% of beams weight. HMRT plans were compared with conventional wedged-field tangential plans (WF), field-in-field forward planned tangential plans (FiF) and volumetric modulated arc therapy (VMAT) plans. Dosimetric differences among the plans were evaluated using a Kruskal-Wallis one-way analysis of variance. The Bonferroni correction was applied for pairwise comparisons; an adjusted value of p<0.0083 was considered as statistically significant. Delivery efficiency was analysed based on number of monitor units (MUs) and estimated delivery time. Results No significant differences were found among the four techniques for PTVbr an PTVcav coverage in terms of mean dose, minimal dose (D98%), target coverage (D95% and V95%). The HMRT technique showed statistically significant reduction of healthy tissue irradiation with respect to WF and FiF techniques: V 55Gy : 89cc vs 155cc and 143cc; V 60Gy : 18cc vs 40cc and 34cc. No significant differences were found in lung and heart irradiation between the three tangential technique. VMAT supplied the best conformity dose distribution for both PTVs but the worst results for contralateral breast irradiation, ipsilateral lung and heart in the low dose region (V 5Gy ). With respect to other three techniques, HMRT reported the minimal time for planning optimization and the minimal number of monitor unit delivery. Conclusion HMRT plans resulted in superior target dose conformity and homogeneity with respect to WF and FiF techniques. In addition, due to fast planning time HMRT can be applied for all patients, minimizing the impact on human or departmental resources. EP-1971 Can SBRT-VMAT be an alternative to HDR brachytherapy for vaginal cuff boost? A dosimetric study S. Cilla 1 , G. Mattiucci 2 , F. Deodato 3 , L. Azario 4 , A. Ianiro 1 , R. Frakulli 5 , A. Galuppi 5 , A. Arcelli 5 , S. Ciabatti 5 , S. Cammelli 5 , E. Galofaro 5 , G. Frezza 6 , V. Valentini 2 , A. Morganti 5 , G. Macchia 3 1 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Medical Physics Unit, Campobasso, Italy 2 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Radiation Oncology Department, Roma, Italy 3 Fondazione di Ricerca e Cura "Giovanni Paolo II"- Università Cattolica del Sacro Cuore, Radiation Oncology Unit, Campobasso, Italy 4 Policlinico Universitario "A. Gemelli"- Università Cattolica del Sacro Cuore, Medical Physics Department, Roma, Italy 5 Department of Experimental- Diagnostic and Specialty Medicine - DIMES- University of Bologna- S.Orsola- Malpighi Hospital, Radiation Oncology Center, Bologna, Italy 6 Bellaria Hospital, Radiation Oncology Unit, Bologna, Italy Purpose or Objective To evaluate the role of stereotactic body radiotherapy (SBRT) using VMAT technique as an alternative to high- dose rate brachytherapy (HDR-BRT) in the treatment of vaginal cuff in postoperative endometrial cancer.