Predicting Grade 3 Acute Diarrhea During Radiation Therapy for Rectal Cancer Using a Cutoff-Dose Logistic Regression NTCP Model

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• Purpose/Objective(s)
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Purpose/Objective(s) 

Determination of the dose-response relationship of small bowel irradiation and the development of acute diarrhea will provide direction for strategies to avoid this morbid side effect in future research. This NTCP model determines the correlation of the absolute volume receiving at least a particular dose (“cutoff dose”) and the toxicity probability/incidence rate.

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Materials/Methods 

Following IRB approval the records were reviewed for 152 consecutive patients treated with RT in curative fashion for rectal cancer. All patients had treatment planning CT scans with small bowel contrast prior to therapy. The small bowel was outlined on each slice of the treatment planning CT scan and a small bowel DVH was calculated for the intended initial pelvic treatment to 45 Gy at 1.8 Gy per fraction. The maximum grade of acute diarrhea was scored for each patient using Common Terminology Criteria. We fitted logistic regression models to this dataset for varying cutoff-dose levels from 5 to 45 Gy in increments of 5 Gy.

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Results 

There were 33 patients with grade 3 acute diarrhea. The model with the highest significance was found for a cutoff dose of 15 Gy. The range of small bowel volumes between the minimal and maximal volume was divided into 6 bins for histogram construction. The 3 highest volume histogram bins had very few patients and were combined for a total of 4 bins overall (<123 cc, 123 to 246 cc, 246 to 368 cc, and >368 cc). This was used to construct a plot showing the predicted probability of grade 3 acute diarrhea for small bowel volumes receiving doses equal to or above the cutoff dose, with the small bowel volume on the x-axis and the incidence of diarrhea on the y-axis (Figure). The correlations found for our data were highly significant (p < 0.001) with an area under the receiver operating characteristic curve of 0.71.

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Conclusions 

The absolute volume of small bowel planned to receive at least 15 Gy of 45 Gy at the isocenter, was highly significant for the prediction of grade 3 acute diarrhea in patients treated for rectal cancer.

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