It is increasingly common for radiotherapy departments to encounter high density objects in patients being planned for radiotherapy. Many cases, such as artificial hip prostheses, are well documented. In our cancer center we have recently come across a new type of implant--a McGhan Style 133 Tissue Expander--implanted in patients who have undergone a mastectomy and will in future have breast reconstruction. This type of implant contains a small rare earth magnet encased in a titanium body, which induces significant perturbations in the dose distribution. These perturbations have been measured using a p-type semiconductor diode. Attenuation of up to 30% of local dose has been observed for a single beam. However, in more realistic clinical situations using tangential parallel opposed beams, it is estimated that part of the planning target volume maybe be underdosed by approximately 10%. Comparisons have been made between measured attenuation and that calculated by a treatment planning system, which demonstrates inadequate modeling of the dose perturbation caused by the implant in this case.

Stereotactic radiotherapy was an effective treatment modality for TIO in our patient. Fractionated stereotactic radiation therapy represents an alternative to surgery for patients with TIO who are not surgical candidates or who decline surgery.

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The authors take pleasure in acknowledging the assistance of many members of the Wilmington and San Francisco staffs of the Shell Development Company. They are particularly indebted to A. J. Johnson for his guidance and advice. THERMALLY CRACKED GASOLINES LITERATURE CITED Conventional thermally cracked gasolines are generally un-(1) Am. Soc. Testing Materials, A.S.T.M. D 873.suitable for the production of high-quality aviation blending (2) Ellis and Jones, Analyst, 61, 812 (1936). stocks unless they have been subjected to a severe treatment to

The importance of metacognitive language exposure to early educational achievement is widely recognized in the development literature. However, few studies have explored parents' metacognitive language, while accounting for family SES and stress within the parent-child relationship. This is a preliminary descriptive study to explore metacognitive language to preschoolers during a collaborative task, and explored family SES (parent education and occupation) and stress within the parent-child relationship as predictors. Using partial correlations controlling for child age and language ability, parent education, occupation, and stress associated with parent-child dysfunctional interaction were found to significantly correlate with parents' metacognitive questions. A hierarchical regression (controlling for child age and language ability) indicated that only parent education levels uniquely predicted questions overall.