Dose-volume and complication in interstitial implants for breast carcinoma.

A common radiotherapeutic technique for treating breast cancer is the combination of external beam radiation with an interstitial iridium-192 boost. When smaller tumors (T1 and T2) are treated using this technique, the soft tissue complication rate is small. However, with treatment of more advanced stages of disease, where large volumes of breast tissue must be treated to high radiation doses, the incidence of complication increases. This paper investigates the dose and volume relationships for breast tissue treated by interstitial technique and correlates this to the risk of soft tissue radiation injury. A method of analysis of interstitial radiation implants suitable for intra- or inter-institutional clinical evaluations is offered. The records of 111 patients treated at Georgetown University Hospital, were retrospectively analyzed and the five who had experienced radiation-related complications were compared to 51 randomly selected patients experiencing no complications. The volumes of tissue enclosed by selected isodose surfaces were calculated and used to determine a relationship between these dose-volumes and the probability of complication. The mean volume at specified dose levels between 10 Gy and 50 Gy was significantly higher (p less than .05) for the patients developing complications than those in whom no complications were seen. Using the 20 Gy isodose surface as defining our usual treated volume, a complication probability versus dose-volume curve was developed using a linear logistic model. The curve fitted the data closely (p less than .006) suggesting that, for our cases, the calculated treatment volume (within the 20 Gy isodose surface) can be used to effectively separate patients into groups that have different probabilities of developing complications. We propose this method as a basis for specification of dose and volume which can be used for clinical risk assessment, and for intra- and inter-institutional comparison.