Probabilities for a Probabilistic Network: 
A Case­study in Oesophageal Carcinoma 
L.C. van der Gaag, S. Renooij, C.L.M. Witteman, 
Utrecht University, Institute of Information and Computing Sciences, 
P.O. Box 80.089, 3508 TB Utrecht, The Netherlands 
{linda,silja,cilia}@cs.uu.nl 
B.M.P. Aleman, and B.G. Taal 
The Netherlands Cancer Institute, Antoni van Leeuwenhoekhuis, 
Department of Radiation Oncology and Gastroenterology, 
Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands 
{baleman,bgtaal}@nki.nl 
Abstract 
With the help of two experts in gastrointestinal oncology from the Netherlands Can­ 
cer Institute, Antoni van Leeuwenhoekhuis, a decision­support system is being devel­ 
oped for patient­specific therapy selection for oesophageal carcinoma. The kernel of 
the system is a probabilistic network that describes the characteristics of oesophageal 
carcinoma and the pathophysiological processes of invasion and metastasis. While 
the construction of the graphical structure of the network was relatively straightfor­ 
ward, probability elicitation with existing methods proved to be a major obstacle. 
We designed a new method for eliciting probabilities from experts that combines the 
ideas of transcribing probabilities as fragments of text and of using a scale with both 
numerical and verbal anchors for marking assessments. The method allowed us to 
elicit the many probabilities required for our network in little time. Using data from 
185 patients, we conducted an evaluation study to assess the quality of the proba­ 
bilities obtained. We found that for 85% of the patients, our probabilistic network 
yielded the correct outcome. 
1 Introduction 
The Netherlands Cancer Institute, Antoni van Leeuwenhoekhuis, is a specialised center for 
the treatment of cancer patients. Every year some eighty patients receive treatment for 
oesophageal carcinoma at the center. These patients are currently assigned to a therapy 
by means of a standard protocol that includes a small number of prognostic factors. Based 
1