CT densitometry for the grading of subcutaneous fibrosis after the photon-neutron therapy of malignant salivary gland tumors

Abstract

To evaluate alternative treatment regimen e.g. neutron therapy determination of treatment efficacy as well as side effects is important. Sensitivity of computed tomography (CT) in detecting changes of connective tissue after neutron therapy was examined. In the course of their follow-up period (median 45 months) 12 patients with malignant salivary gland tumors who had postoperatively received neutron (1/12) or photon/neutron therapy (11/12) were examined by means of CT densitometry on 3 representative scans in the area of radiotherapy. In 3 ROI (regions of interest) in subcutaneous fatty tissue the density at the irradiated and the non-irradiated side was determined according to Hounsfield units (HU) and the average density was calculated. The average density of both sides was compared and correlated with the clinical grade of fibrosis according to LENT SOMA. All CT measurements (216 ROI, 18 ROI per patient) showed higher density levels on the irradiated side than on the non-irradiated side. The average density on the irradiated side was -57.7 +/- 4.7 HU and on the non-irradiated side -69.4 +/- 5.8 HU (p = 0.002). In 3/12 patients a clinical fibrosis was not seen; however, the relative density measured on the irradiated and non-irradiated side deviated by up to 8%. This could have been caused by minimal changes not being noticed by either patient and examiner. In patients with determined fibrosis Grade 1 (8/12) the relative density deviation was 4 to 39%. In 1/12 patients with determined fibrosis Grade 2 the relative density deviation was 50% (Figures 1a and 1b). Fibrosis Grade 3 and 4 did not occur (Table 1). Fibrosis is correlated with an increasing value of HU of the tissue density in CT. With the described method it is possible to graduate radiation induced subcutaneous fibrosis in correlation to the clinical fibrosis grade according to LENT SOMA. In the patients we examined subcutaneous fibroses after photon/neutron therapy were moderate. Especially in characterising subclinical or slight changes of connective tissue after radiotherapy computed tomography is of value.