Quantitative Analysis of Vertebral Bone Density Change following Spine Radiosurgery

Quantitative Analysis of Vertebral Bone Density Change following Spine Radiosurgery

Proceedings of the 52nd Annual ASTRO Meeting and designed MLC shapes for opposed anterior and posterior treatment fields. Treatment dose was calculate...

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Proceedings of the 52nd Annual ASTRO Meeting and designed MLC shapes for opposed anterior and posterior treatment fields. Treatment dose was calculated with both PBC and AAA using heterogeneity correction (Modified Batho for PBC), and dose was normalized so that 98% of the CTV received the prescription dose of 44 Gy. We quantified the change in the dose indices that published reports have indicated to be predictive of normal tissue complications. For lung, these included the mean dose and the percentage of volume receiving greater than 5 Gy, 13 Gy, and 20 Gy (V5Gy, V13Gy, and V20Gy). For heart, dose indices were mean dose, V38Gy, and V42Gy. We also evaluated changes in the mean and maximum dose to the esophagus and spinal cord. Results: Required MUs to achieve equal dose coverage increased by 2.6 ± 2.5 (1.7%) when AAA was used. For AAA, the V5Gy and V13Gy of the lungs were greater than PBC by 8.3 ± 3.6% and 1.4 ± 0.6%, respectively (p = 0.04 and p = 0.05); with the change in V5Gy being larger for 6MV photons (10.1%) than for 15MV photons (6.1%). The mean heart dose increased by 66 ± 19 cGy (p = 0.01), and the mean cord dose increased by 33 ± 12 cGy (p = 0.02). Changes to all other normal tissue dose metrics were insignificant at alpha = 0.05. Although the minimum CTV dose decreased slightly (36 ± 39 cGy) and the maximum CTV dose increased slightly (52 ± 50 cGy), no dose metrics for CTV changed significantly. Conclusions: There is little difference between PBC and AAA for most dose metrics that are predictive of thoracic normal tissue complications. However due to discrepancies in the low dose regions of the lung, the dose calculation algorithm should be taken into consideration when clinical treatments are assessed using V5Gy of lung. The dose calculation algorithm could also be a confounding factor in studies correlating V5Gy with treatment outcome. Author Disclosure: J.D. Adamson, None; S. Yoo, None; Q. Wu, None; F. Yin, None.

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Photon Scatter vs. Neutron Capture Gamma at the Entrance of a Long Maze Treatment Room: Can Photon Scatter be Neglected?

D. Fondevila1 F. Saule2 1

Vidt Centro Medico, Buenos Aires, Argentina, 2Autoridad Regulatoria Nuclear, Buenos Aires, Argentina

Purpose/Objective(s): The design of a treatment room for Linear Accelerators with energies above 10MV requires a careful assessment of the characteristics of the maze and door to achieve an adequate protection of the personnel staff due to the various components of radiation generated within the room. Two distinct photon components can be identified, namely, photon scatter and neutron capture gamma-rays (NCG) differing substantially in their energy spectrums. The main document for shielding design to date, the NCRP 151, states that ‘‘since the average energy of NCG from concrete is 3.6 MeV (ˇ1/4 ), a maze and door that provide sufficient shielding for the NCG will also be adequate for the scattered photons. For mazes in high-energy accelerator rooms, (ˇ1/4 ) the photon field is dominated by NCG and the scattered photon component can be ignored.’’ This work shows, by theoretical calculation and experimental verification that for long maze treatment rooms with no curves this statement is incorrect. Furthermore it discusses implications for the design of door-less bunkers. Materials/Methods: Measurements of the total photon dose equivalent at the entrance of an existing treatment room with a 10 meter long maze were taken for different irradiation conditions. The NCRP 151 formalism was used to calculate the photon dose equivalent for each radiation component and the corresponding values were compared. The experimental energy components were discriminated through the analysis of total transmission factors. Results: The total photon dose equivalent was measured for a 15MV beam giving a value of 12.8 mSv/h averaged over the 4 main gantry positions. The measured total photon transmission factor for a 10 mm lead door was 0.07. Taking into account the energies involved this means that the high energy component was only 10% of the total photon dose component. The calculated dose equivalent rate for the high energy components (NCG and head leakage through maze wall) showed good agreement with the experimental estimation (1.1mSv/h calculated vs. 1.3 mSv/h measured), whereas calculated values of the photon scatter component underestimate the measured values by a factor of 3. Conclusions: The data presented in this work show that, in long maze treatment rooms with no curves, the photon scatter component can not only not be neglected but it can represent the dominant component. It can be seen that the existing methods for calculating the photon scatter component underestimate the actual experimental values. This situation gets worse as the maze length grows due to the usual modeling deficiencies for this component. In the actual case studied it could be seen that a door-less bunker would have been possible were it not for the presence of the photon scatter component. Author Disclosure: D. Fondevila, None; F. Saule, None.

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Quantitative Analysis of Vertebral Bone Density Change following Spine Radiosurgery

C. Altunbas, Q. Diot, B. Kavanagh, C. Chen, M. Miften University of Colorado, School of Medicine, Aurora, CO Purpose/Objective(s): In this preliminary study, changes in vertebral bone density after spinal stereotactic radiosurgery (SRS) were quantified. Bone density variation within the treatment volume was evaluated as a function of radiation dose and followup time after treatment with respect to pre-treatment bone density. Materials/Methods: The bone density evaluation was based on Hounsfield unit (HU) analysis of 8 patients’ follow-up (F/U) CT scans acquired 3 and 12 months after the spinal SRS. Each patient’s CTV covered the whole vertebral body, which contained the gross disease. Single fraction equivalent dose (SFED), determined by the method of Park (IJROBP 2008), varied from 16 Gy to 24 Gy, and the target location varied between T6 and L1 in each patient. Bone density analysis was performed within each CTV, which was further divided into bins based on the delivered dose. To analyze the rate of bone density change as a function of F/U time, a skewness test was performed on bone density volume histograms. Furthermore, biasing of image data due to CT scanner dependent variations was monitored in a reference volume outside the therapeutic radiation field. The scan-to-scan fluctuation in HU in each respective patient’s reference volume was observed to be within 2%.

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I. J. Radiation Oncology d Biology d Physics

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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 registration methods. The average bone density within the treatment volume increased by 3% (±1.9%) and 8.4% (±2.2%) after 3 and 12 months following the treatment, respectively. Only 1 out 8 patients had a decrease in bone density in time. Bone density change as a function of dose delivered appeared to be independent of dose within the range of SFED delivered. Based on the binary skewness 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 of increase 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 treated volume. Also, the rate of increase in bone density is more pronounced in relatively lower density regions of the treated vertebral body. The change in bone density seems to be independent of delivered dose above the minimum threshold delivered in the current series. These findings are difficult to reconcile with reports of insufficiency fracture after spine SRS in patients, but the predominantly sclerotic pattern of metastases in this series could be explanatory. Future studies will compare the pattern of changes seen in patients without fracture to those who experience fracture to look for patterns that might predict high risk 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 published data. A range of risk estimates can be obtained by combining the complication rates from multiple institutions. We report on a preliminary analysis of rectal toxicity risk estimates extracted from the literature given a patient’s clinical and dosimetric information. Materials/Methods: Twelve patients treated for prostate cancer at our institution were randomly selected. A thorough literature search yielded twelve reports on varying grades of rectal toxicity observed within two years after the end of radiotherapy that can be used to make estimates of normal tissue complication probabilities (NTCP) from a variety of dosimetric variables. NTCP estimates were made by applying methods used in the original publication to a patients’ treatment planning dose distributions. For publications that analyzed toxicity profiles as a function of rectal wall dose distributions, the rectal wall was autogenerated from the outer rectal contour assuming a uniform 3 mm wall thickness. The estimates were separated according to a grade of toxicity. Data from published reports providing more than one risk estimate for a given grade of toxicity were averaged. The analysis of variance (ANOVA) for each grade of toxicity was performed to test whether the variability in risk estimates between patients is more significant than the variability in estimates obtained from different published 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 rectal toxicity 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 for RTOG Grade . = 1 and Grade . = 3 toxicity, different published studies tend to predict different complication rates that are minimally affected by the variations in patients’ dosimetric information. However for Grade . = 2 rectal toxicity, NTCP estimates vary more significantly between patients than between the reports, and therefore may be clinically useful. The presented method may be used for evaluation of treatment plans. Author Disclosure: P.W. Prior, None; X.A. Li, None; V.A. Semenenko, None.

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Exposure from 131 Iodine-treated Patients 1

A. Shlomo , T. Biran2, S. Primo3, R. Ben-Yosef1, R. Ben-Yosef1, M. Levita1 1 Tel 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 exposure to family members and others in close contact with these patients. Iodine131 treatment is the common choice of therapy to thyroid cancer patients. Patients are given usually an amount of 3.70-7.40GBq. The protocol for releasing patients from the hospital after treatment is accordance to the regulatory guide .These guidelines allow for the release of a patient with an Iodine 131 activity of over 1.10 GBq (30mCi). Patient release is contingent on a dose, to family members and others who will take care of the patient, of less 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 from the treated iodine patient. Materials/Methods: Six thyroid cancer patients underwent total or near-total thyroidectomy prior to administration of iodine 131 with amounts of 3.70-5.55 GBq. To estimate iodine 131 in thyroid tissue before the therapeutic administration, a gamma camera scan was performed 24 h after administration of 0.2 MBq. External radiation doses were measured at the patient’s home by thermoluminescent dosimeters. The high sensitive TLD 700H/600H were given to each patient’s family to place at home, at the following locations: Bedroom, living room, kitchen. Radiation exposure was continuously monitored for the