Abstract Book

S1083

ESTRO 37

DVH and acute skin toxicity in a SIB approach delivering a daily dose to PTV2 equal to 2.2 Gy/fr. Additional research to corroborate this result on a larger group of patients is ongoing: if confirmed, these findings may support more effective skin-sparing planning strategies for HN patients. EP-1991 Tumour sphericity is an independent predictor for overall survival in non-small cell lung cancer. A. Davey 1 , M. Van Herk 1 , C. Faivre-Finn 1 , A. McWilliam 2 1 The University of Manchester, Division of Cancer Sciences- School of Medical Sciences- Faculty of Biology- Medicine and Health, Manchester, United Kingdom 2 The Christie NHS Foundation Trust, Christie Medical Physics and Engineering, Manchester, United Kingdom Purpose or Objective Sphericity is a shape characteristic describing how closely a volume matches a sphere. In lung cancer, this characteristic may contain information on tumour aggressiveness. This study investigates the role of sphericity as a predictor of survival for non-small cell lung cancer (NSCLC) patients, taking into account that it may also affect the mean lung dose. Material and Methods We selected 329 NSCLC patients that were treated with 55Gy in 20 fractions with curative-intent radiotherapy and had a single gross tumour volume (GTV). Surface area and volume of the GTV delineated by a radiation oncologist in free breathing CT data were extracted using in-house software. Sphericity was calculated as the ratio of the surface area of a sphere (with equivalent volume to the GTV) to the actual surface area of the GTV. Patients were grouped into high or low sphericity relative to its median value. Relationships between GTV, mean lung dose and sphericity were investigated to assess the role of sphericity as predictive for overall survival. We applied a multivariate Cox regression, including common patient and tumour characteristics. Results

Results of the multivariate analysis are displayed in Figure 2A, demonstrating the significance of sphericity (p=0.014, HR=1.411) and tumour volume (p<0.001, HR=1.004) for overall survival. Mean lung dose is not significant in the multivariate analysis. After adjusting survival curves for mean lung dose and GTV (Figure 2B), a significant split remains with more spherical tumours displaying better survival, log-rank p<0.001. Conclusion Reduced sphericity is associated with higher mean lung dose, and this relationship must be taken into account when using this shape characteristic in predictive modelling. However, sphericity remains a significant factor influencing patient survival in multivariate analysis. EP-1992 LASSO-based multivariable NTCP model for late rectal bleeding in prostate cancer patients after IMRT C.C. Huang 1,2,3 , P.J. Chao 1,4 , S.S. Guo 1,4 , C.J. Wang 1 , H.L. Luo 5 , Y.L. Su 6 , T.F. Lee 1,4,7 1 Kaohsiung Chang Gung Memorial Hospital, Department of Radiation Oncology, Kaohsiung, Taiwan 2 Chang Gung University, College of Medicine- Graduate Institute of Clinical Medical Sciences, Taoyuan, Taiwan 3 Meiho University, Department of Nursing, Pingtung, Taiwan 4 National Kaohsiung University of Applied Sciences, Medical Physics and Informatics Laboratory of Electronics Engineering, Kaohsiung, Taiwan 5 Kaohsiung Chang Gung Memorial Hospital, Department of Urology, Kaohsiung, Taiwan 6 Kaohsiung Chang Gung Memorial Hospital, Department of Internal Medicine- Division of Hematology-Oncology, Kaohsiung, Taiwan 7 Kaohsiung Medical University, Graduate Institute of Clinical Medicine, Kaohsiung, Taiwan Purpose or Objective To develop a multivariable normal tissue complication probability (NTCP) model to predict moderate to severe late rectal bleeding in prostate cancer patients after intensity-modulated radiation therapy (IMRT). Material and Methods A total of 68 patients with localized prostate cancer treated by IMRT from 2008 to 2011 were enrolled. The median follow-up time was 56 months. According to the criteria of D’Amico risk classifications, there were 9, 20 and 39 patients in low, intermediate and high-risk groups, respectively. The median prescribed radiation dose was 72 Gy. Forty-two patients were combined with androgen deprivation therapy. Fifteen patients had suffered from grade 2 or more (grade 2+) late rectal bleeding. The numbers of predictors for a multivariable logistic regression NTCP model were determined by the least absolute shrinkage and selection operator (LASSO). Results The most important predictors for late rectal bleeding ranked by LASSO were platelet count, risk group and the relative volume of rectum receiving at least 65 Gy (V 65 ).

No correlation was found between sphericity and tumour volume. A two-sample t-test demonstrated a significant difference (p<0.001) of 3.1Gy in mean lung dose between low and high sphericity groups split on the median (0.68) (Figure 1A), where more spherical tumours have a lower mean lung dose. The relationship between lung dose and volume for the two groups is shown in Figure 1B. Despite that the dose for a given volume is higher on average in the low sphericity group, the rate at which the mean lung dose increases with GTV volume is unaffected by sphericity.