S714 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 3, Supplement, 2010

Results: A total of 12 F/U CT scans were fused to the respective patient’s simulation CT scan and treatment plan using rigid reg-istration methods. The average bone density within the treatment volume increased by 3% (±1.9%) and 8.4% (±2.2%) after 3 and12 months following the treatment, respectively. Only 1 out 8 patients had a decrease in bone density in time. Bone density changeas a function of dose delivered appeared to be independent of dose within the range of SFED delivered. Based on the binary skew-ness test, at F/U times of 3 and 12 month, the less dense regions within the pre-treatment CTV had a higher rate and magnitude ofincrease in bone density than the more dense regions within the pre-treatment CTV. This trend was observed for 7 out of 8 patients,and in both 3 and 12 month F/U time points.

Conclusions: Our preliminary data suggests that spinal SRS can lead to increase in vertebral bone density within the treatedvolume. Also, the rate of increase in bone density is more pronounced in relatively lower density regions of the treated vertebralbody. The change in bone density seems to be independent of delivered dose above the minimum threshold delivered in thecurrent series. These findings are difficult to reconcile with reports of insufficiency fracture after spine SRS in patients, butthe predominantly sclerotic pattern of metastases in this series could be explanatory. Future studies will compare the patternof changes seen in patients without fracture to those who experience fracture to look for patterns that might predict highrisk of injury.

Author Disclosure: C. Altunbas, None; Q. Diot, None; B. Kavanagh, None; C. Chen, None; M. Miften, None.

3139 Risk Estimates for Rectal Toxicity in Individual Patients Based on a Meta-analysis of Published Data

P. W. Prior, X. A. Li, V. A. Semenenko

Medical College of Wisconsin, Milwaukee, WI

Purpose/Objective(s): Meta-analysis of radiotherapy induced toxicity is complicated due to the heterogeneity present in the pub-lished data. A range of risk estimates can be obtained by combining the complication rates from multiple institutions. We report ona preliminary analysis of rectal toxicity risk estimates extracted from the literature given a patient’s clinical and dosimetric infor-mation.

Materials/Methods: Twelve patients treated for prostate cancer at our institution were randomly selected. A thorough lit-erature search yielded twelve reports on varying grades of rectal toxicity observed within two years after the end of radio-therapy that can be used to make estimates of normal tissue complication probabilities (NTCP) from a variety of dosimetricvariables. NTCP estimates were made by applying methods used in the original publication to a patients’ treatment planningdose distributions. For publications that analyzed toxicity profiles as a function of rectal wall dose distributions, the rectalwall was autogenerated from the outer rectal contour assuming a uniform 3 mm wall thickness. The estimates were sep-arated according to a grade of toxicity. Data from published reports providing more than one risk estimate for a given gradeof toxicity were averaged. The analysis of variance (ANOVA) for each grade of toxicity was performed to test whether thevariability in risk estimates between patients is more significant than the variability in estimates obtained from differentpublished reports.

Results: NTCP estimates for RTOG Grades . = 1, 2, and 3 rectal toxicities were made from 3, 9, and 3 published reports,respectively. Estimates for Grade . = 1 toxicity were significantly different between the published reports (p = 2 rectaltoxicity produced estimates that varied more significantly between patients (p = 1 toxicity, the variations in risk for Grade. = 3 toxicity were found to be significantly different between the three reports (p = 0.09), but not between patients (p =0.23).

Conclusions: The estimates of rectal toxicity risk demonstrate a considerable heterogeneity in the published data. Particularly forRTOG Grade . = 1 and Grade . = 3 toxicity, different published studies tend to predict different complication rates that are min-imally affected by the variations in patients’ dosimetric information. However for Grade . = 2 rectal toxicity, NTCP estimates varymore significantly between patients than between the reports, and therefore may be clinically useful. The presented method may beused for evaluation of treatment plans.

Author Disclosure: P.W. Prior, None; X.A. Li, None; V.A. Semenenko, None.

3140 Exposure from 131 Iodine-treated Patients

A. Shlomo1, T. Biran2, S. Primo3, R. Ben-Yosef1, R. Ben-Yosef1, M. Levita1

1Tel Aviv Medical Center, Tel Aviv, Israel, 2radiation Safety Division, Soreq Nrc, Yavne, Israel, 3Radiation Safety Division,Soreq Nrc, Yavne, Israel, Yavne, Israel

Purpose/Objective(s): Cancer patients treated with radioactive 131 iodine are a potential source of external and internal exposureto family members and others in close contact with these patients. Iodine131 treatment is the common choice of therapy to thyroidcancer patients. Patients are given usually an amount of 3.70-7.40GBq. The protocol for releasing patients from the hospital aftertreatment is accordance to the regulatory guide .These guidelines allow for the release of a patient with an Iodine 131 activity ofover 1.10 GBq (30mCi). Patient release is contingent on a dose, to family members and others who will take care of the patient, ofless than 5 mSv. Better understanding of the radiation exposure over the duration of the post-iodine administration time period,would be beneficial for radiation safety decision making. Six families were given TLDs to measure the radiation exposure fromthe treated iodine patient.

Materials/Methods: Six thyroid cancer patients underwent total or near-total thyroidectomy prior to administration of iodine131 with amounts of 3.70-5.55 GBq. To estimate iodine 131 in thyroid tissue before the therapeutic administration, a gammacamera scan was performed 24 h after administration of 0.2 MBq. External radiation doses were measured at the patient’shome by thermoluminescent dosimeters. The high sensitive TLD 700H/600H were given to each patient’s family to place athome, at the following locations: Bedroom, living room, kitchen. Radiation exposure was continuously monitored for the