ESTRO 37 Abstract book

S1084

ESTRO 37

studies dealing with the quantitative relationship between the skin dose and the risk of developing acute or late toxicity are very rare. The aim of this work was to assess the correlation between individually recovered planning skin dose-volume/surface data and acute toxicities after Radiochemotherapy for HN cancer patients. Material and Methods The study involved 32 HN consecutive patients (oropharynx: 14, ipopharynx: 8, nasopharynx: 5, others: 5) previously treated with Helical Tomotherapy (HT) with radical intent (SIB technique: 54/66Gy to PTV1/PTV2 in 30fr). All patients received neo-adjuvant and/or conco- mitant chemotherapy (no patients received Cetuximab) and daily image-guidance. After recovery of CT planning data, a superficial body layer with a thickness of 2 mm (SL2) was automatically delineated with a cranial-caudal extension corresponding to the high-dose PTV2 (i.e.: receiving 66Gy). CTCAE v4.0 acute skin toxicities were reported for all patients. Absolute dose-volume histograms (DVH) of SL2 were calculated and average values for patients who developed severe/ moderate (G3/G2) and mild/none (G1/G0) acute toxicities were assessed: DVH differences between the two groups (G3/G2 vs G1/G0) were analyzed by two-tails t-test to define the most discriminative regions of SL2 DVH. Results Average DVHs are shown in Figure 1. Fifty-eight % of patients experienced G2/G3 toxicity (rate of G3: 27%) against 42% for G1/G0. Differences in skin DVHs were significant in the range 53-63 Gy (Figure 2), suggesting that the fraction of SL2 receiving around 2 Gy/fr is highly correlated with the risk of skin acute toxicity. In the plateau of the DVH region with highly significant differences, p-values ranged between 0.005 and 0.01: of note, the mean values of V60 were 4 cc and 11 cc for the two groups, corresponding to a skin surface of 20 cm 2 and 55 cm 2 respectively. V60 was found to be highly discriminative (AUC:0.84, 95%CI: 0.64-0.95) with a best cut-off value equal to 3.3 cc: the rate of G2/G3 toxicity was equal to 82% and 11% for V60 ≥ 3.3 cc and < 3.3 cc respectively (p=0.003).

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

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.

Conclusion Despite the limited number of patients, current results quantified for the first time the relationship between SL2 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

Results of the multivariate analysis are displayed in Figure 2A, demonstrating the significance of sphericity