A two-variable linear model of parotid shrinkage during IMRT for head and neck cancer

Purpose: To assess anatomical, clinical and dosimetric pre-treatment parameters, possibly predictors of parotid shrinkage during radiotherapy of head and neck cancer (HNC). Materials: Data of 174 parotids from four institutions were analysed; patients were treated with IMRT, with radical and adjuvant intent. Parotid shrinkage was evaluated by the volumetric difference (Delta V) between parotid volumes at the end and those at the start of the therapy, as assessed by CT images (MVCT for 40 patients, KVCT for 47 patients). Correlation between Delta Vcc/% and a number of dosimetric, clinical and geometrical parameters was assessed. Univariate as well as stepwise logistic multivariate (MVA) analyses were performed by considering as an end-point a Delta Vcc/% larger than the median value. Linear models of Delta V (continuous variable) based on the most predictive variables found at the MVA were developed. Results: Median Delta Vcc/% were 6.95 cc and 26%, respectively. The most predictive independent variables of Delta Vcc at MVA were the initial parotid volume (IPV, OR: 1.100; p = 0.0002) and Dmean (OR: 1.059; p = 0.038). The main independent predictors of Delta V% at MVA were age (OR: 0.968; p = 0.041) and V40 (OR: 1.0338; p = 0.013). Delta Vcc and Delta V% may be well described by the equations: Delta Vcc = 2.44 + 0.076 Dmean (Gy) + 0.279 IPV (cc) and Delta V% = 34.23 + 0.192 V40 (%) - 0.2203 age (year). The predictive power of the Delta Vcc model is higher than that of the Delta V% model. Conclusions: IPV/age and Dmean/V40 are the major dosimetric and clinical/anatomic predictors of Delta Vcc and Delta V%. Delta Vcc and Delta V% may be well described by hi-linear models including the above-mentioned variables. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 94 (2010) 206-212