Fifty-sixth annual meeting of the American association of physicists in medicine
SU-D-16A-05: Fitting Grade>=2(2+) Rectal Complication Rates to Lyman Kutcher Burman (LKB) Model with a Database of 81 Prostate Patients Treated with IMRT
Quantec review of best parameters for the LKB NTCP model of rectal complications is based exclusively on datasets obtained with 3D conformal techniques. The report suggests that inherent differences in rectal dose distributions obtained with IMRT techniques could require modification of the parameters which provide the best fit to clinical data.
We compiled a database of 81 prostate patients who were treated with an IMRT technique to a dose of 77.4 Gy, 1.8Gy/fx, with an integrated boost to 81-83Gy in a sub-volume of a prostate which was identified on a pre-treatment MRI study. Acute grade 2+ rectal toxicities were graded according to CTCAE v4 by a physician who retrospectively reviewed patient's medical records. We used the API interface of the Eclipse Treatment Planning System (Varian, Inc) to extract dosimetric data for rectum with contents. We used “R” statistical package to perform a maximum likelihood fit of the LKB model to data. We defined the model in terms of parameters, m,n and TD_50, as recommended in the Quantec report. We converted the dose to 2Gy equivalent dose using linear-quadratic model with alpha over beta of 3Gy, also as recommended in the Quantec report.
Grade 2+ acute rectal toxicity occurred in 19% (15/81 patients). The best fit of LKB model to data was obtained for n = 0.06, m = 0.073, and TD_50 = 67.4Gy. Prediction accuracy was quantified using the Area Under the Curve method which yielded a value of 0.63, consistent with published data.
We obtained LKB model parameters which are reasonably consistent with Quantec recommendations. A low value of parameter ‘n’ indicates sensitivity of rectal complications to high doses in the rectum. Our value of TD_50 is lower than Quantec recommendations, likely due to our use of acute complications.