ESTRO 2021 Abstract Book

S1515

ESTRO 2021

Conclusion R 2 values of some correlations were less than 0.9, which slightly affects accuracy of organ dose assessment. This was seen for organs that were partially inside the scan length such as prostate for male phantoms. However, majority of organs were above 0.92. The use of DCCs would simplify estimation of organ doses for individual patients with a good level of accuracy. This would assist in overcoming the time and computation tools required for software packages based on Monte Carlos simulation, or physics measurements that are limited with certain phantom sizes.

Digital Poster: Radiobiological and predictive modelling, and radiomics

PO-1790 Alpha-Beta ratio estimation and radiotherapeutic equivalence of chemotherapy in oesophageal cancer K.S. Chufal 1 , I. Ahmad 1 , A.A. Miller 2 , R. Bajpai 3 , R.L. Chowdhary 1 , P. Umesh 1 , K. Bhatia 1 , M. Gairola 1 1 Rajiv Gandhi Cancer Institute & Research Centre, Radiation Oncology, New Delhi, India; 2 Illawarra Cancer Care Centre, Radiation Oncology, Wollongong, Australia; 3 Keele University, School of Medicine, Staffordshire, United Kingdom Purpose or Objective This analysis's objectives were: (a) To estimate the α/β ratio of oesophageal squamous cell carcinoma, and; (b) To determine the radiotherapeutic dose equivalence of chemotherapy used in treating oesophageal carcinoma. Materials and Methods We performed a multivariable binary logistic regression (with bootstrapping) on individual patient data to predict pathologic complete response (pCR) following NA-CRT in SPSS v27. The radiotherapy variables entered into the model were: Total Dose (Gy), Total Dose multiplied by Dose per Fraction (Gy 2 ) and Duration of RT (days). Patient and chemotherapy variables entered were: Age (yrs) and Delivered Intensity of Cisplatin (mg/m 2 per week). The logit model used to estimate the probability of pCR (P) after various graded exposures of the contributing variables, and the estimates derived from the coefficients were:

The representation of dose-fractionation in this form assumes a linear-quadratic relationship between dose and effect, and additional terms included in the model test their influence on affecting the outcome. While the coefficients a 1 and a 2 estimated by logistic regression are not a direct estimate of α or β , the ratio of a 1 /a 2 is preserved, which is an estimate of α/β . The formulae listed above for Gray equivalent of chemotherapy assume that over the course of 5 ½ weeks, the ideal delivered dose would be six cycles of Cisplatin (35mg/m 2 ), and ideal intensity (without any treatment delays) would be 38mg/m 2 per week. Results A total of 82 patients were included in this analysis. The median (± SD) age, total dose, total dose multiplied by dose per fraction, duration of RT and Cisplatin intensity were 54 yrs (± 9.0), 50.4 Gy (± 2.3), 90.72 Gy 2 (± 3.91), 38 days (± 3.1) and 27.5 mg/m 2 per week (± 5.5), respectively.