Estimation of tissue volume irradiated by intracavitary implants

The volume of space enclosed by a specified isodose surface arising from an intracavitary implant may correlate with clinical outcome. Several investigators have proposed using the product of the three maximum orthogonal dimensions of the isodose surface as a measure of this volume. We have examined the accuracy of this proposal and compared it to a simpler model for estimating volume which requires only knowledge of the mgRaEq-hrs (total reference air kerma) and the dose level.

Orthogonal films from 204 intracavitary implants of 128 patients with carcinoma of the cervix were used to reconstruct the 137Cs-source coordinates. The source location, strength and duration data were used to calculate dose-volume histograms, yielding the volume enclosed by each dose level as well as its orthogonal dimensions: thickness, width, and height. Using bony landmarks to align films for different insertions in the same patient, similar calculations were repeated for composite implant source coordinates.

Curve-fitting techniques revealed that the volume encompassed by each isodose level could be predicted by a modified power-law function of the mgRaEq-hr/dose ratio: predicted volume = [104.8 - 8.103.(M/D) + 0.437.(M/D)2].(M/D)1.635 where M/D = mgRaEq-hr/cGy. The volume predicted by this simple model is accurate within +/- 10% in 95% of the implants when mgRaEq-hr/cGy = 0.8. Accuracy increases with increasing mgRaEq-hr/cGy. In contrast, the ratio, product of orthogonal dimensions/actual volume, varies widely from implant-to-implant, as well as differing systematically from one implant type to another. Investigation of the individual orthogonal dimensions demonstrated that width and height, but not thickness, were moderately well correlated with corresponding maximum implant dimensions. However, in all cases the dimensions were more sensitive to changes in mgRaEq-hr/cGy than to changes in implant geometry.

The product of the orthogonal dimensions is an unsatisfactory estimator of the actual irradiated volume encompassed by an isodose surface. Isodose surface volumes can be accurately estimated knowing only mgRaEq-hr. Prescribing intracavitary brachytherapy by mgRaEq-hr, or its derivative, total reference air kerma, is equivalent to requiring that an isodose surface encompass a specified volume which does not depend on the implant geometry. Constraining the mgRaEq-hr delivered therefore serves to limit the volume of tissue irradiated to high doses.