ESTRO 37 Abstract book

S1098

ESTRO 37

Purpose or Objective Delivered dose to the rectum in prostate radiotherapy differs from planned dose due to interfraction motion. By developing independent dose calculation systems based on daily megavoltage image guidance scans, we have achieved a better estimate of delivered dose to the rectum [1]. Here we have developed two Normal Tissue Complication Probability (NTCP) models for rectal bleeding to compare discriminatory ability between delivered dose and planned dose. Material and Methods The study comprised 200 prostate cancer patients treated with TomoTherapy. Cumulative incidence of rectal bleeding ≥ Grade 2 (CTCAEv4.03) at 1 year was prospectively assessed (n=15, 7.5%). By assuming equal biologically effective dose between treatment schedules of 60Gy/20 fractions (n=86) and 74Gy/37 fractions (n=114) [2], an α/β ratio of 2.123 was derived for the rectum and used to combine datasets. Dose surface maps (DSMs) of the rectal wall were pixel-corrected to equivalent dose in 37 fractions. DSMs enable preservation of geometric information during dose accumulation, overcoming a fundamental limitation associated with dose volume histograms. The dosimetric parameters investigated were Equivalent Uniform Dose (EUD), and ‘DSM-dose widths’ (a spatial surrogate for the lateral dimension of a given isodose) [1]. Univariate analyses were used to assess clinical prognostic factors (Fisher’s Exact, Mann-Whitney U) and dosimetric parameters (independent t-test) for inclusion in a multivariable prediction model. Odds ratios were calculated for independent variables. Dose parameters from both delivered and planned DSMs were tested for collinearity and removed if the variance inflation factor was > 10. Univariate associations with p > 0.15 were also removed. Results Two binomial logistic regression models were developed and optimised using delivered and planned DSM parameters respectively. Common predictors of rectal bleeding were 35, 45, 55, and 65 Gy DSM dose-widths, alongside pre-treatment status of previous pelvic or abdominal surgery, and presence of rectal bleeding at baseline. Model performance was internally validated and corrected using bootstrapping. NTCPs for risk of rectal bleeding were calculated based on variables from each model. Discriminatory ability, assessed using the area under the receiver operating characteristic curve (AUC), indicated that the delivered-DSM derived model (AUC = 0.809, CI: 0.711-0.907) was a superior predictor of rectal bleeding than the planned-DSM derived model (AUC = 0.782, CI: 0.662-0.902) (Figure 1).

LRBG3. The calibration showed the following results: concerning LRBG23 the slope was equal to 0.22 and R2 was equal to 0.38 (Fig a); the corresponding values for LRBG3 were1.12 and 0.97 (Fig b).

Conclusion A metamodel for prediction of LRB was derived from literature, including all currently available information on association between LRB and clinical/dosimetric factors. The model for LRBG3 was successfully validated on a large population proving to be a valuable tool for predicting toxicity before RT. For what concern LRBG23, the model predicted very well toxicity rate below 25% (which involved 87% of the population) while partially failing at higher probabilities. However, metamodelling is a flexible process that can be improved and enriched during time, including new studies and factors till the achievement of satisfactory calibration coefficients (slope and R2 as close as possible to 1). EP-2010 NTCP model for rectal bleeding based on delivered dose surface maps in prostate radiotherapy L.E.A. Shelley 1 , D.J. Noble 2 , M. Romanchikova 3 , K. Harrison 4 , A.M. Bates 2 , M.P.F. Sutcliffe 1 , S.J. Thomas 3 , N.G. Burnet 2 1 University of Cambridge, Department of Engineering, Cambridge, United Kingdom 2 Cambridge University Hospitals, Department of Oncology, Cambridge, United Kingdom 3 Cambridge University Hospitals, Department of Medical Physics & Clinical Engineering, Cambridge, United Kingdom 4 University of Cambridge, Cavendish Laboratory, Cambridge, United Kingdom