S510 I. J. Radiation Oncology d Biology d Physics Volume 78, Number 3, Supplement, 2010

was 26 Gy and 33 Gy for single and 3 fractionated treatment, respectively. Patients are trained to reduce their respiratoryvolume using as control a computerized spirometer (Elekta ABC) in order to minimize the ITV. SBRT was delivered bya 6 MV Elekta Synergy and dynamic mMLC by multiple arcs. Cone Beam CT was acquired before each treatment forcorrections of setup and isocenter position.

Results: With median follow-up of 15.8 months (3-84), 51% of treated patients are still alive (19% NED). One hundred fiftylesions were evaluable for local control, defined as no tumor re-growth in the treated site. The LC rate was 94% and 91% at1 and 2 years, respectively. Kaplan Meier OS was 90% and 79% at 1 and 2 years, respectively. Median OS was 14 months.No statistically significant differences of OS were observed between NSCLC and colorectal patients. Out of 16 lesionsrecurred in the treated site, 8 were re-treated by SBRT without severe toxicity. Toxicity for all treated patients was mild(CTCAE v.4).

Conclusions: In our experience, patients presenting lung confined oligometastases treated by SBRT can reach high rates (over90%) of local control without significant toxicity. The OS rates shows that this category of patients can be cured by SBRT inselected cases, or inducted in a chronic state of their disease. No significant differences were evaluated between different typesof primary tumors.

Author Disclosure: S. Masciullo, None; L. Masi, None; C. Menichelli, None; I. Bonucci, None; R. Doro, None; F. Casamassima,None.

2684 Role of Adaptive Radiotherapy during Concomitant Chemoradiotherapy for Small Cell Lung Cancer

A. A. Yavuz, B. S. Ozdemir, E. Dundar, B. Pehlivan, I. Karakus, N. Tuncel, M. N. Yavuz

Akdeniz University Department of Radiation Oncology, Antalya, Turkey

Purpose/Objective(s): Tumor shrinkage is frequently observed during conventionally fractionated (chemo-) radiother-apy for small-cell lung cancer (SCLC). As the tumor reduces in size during treatment, the margin added aroundthe gross tumor volume (GTV) effectively becomes larger, which can result in the excessive irradiation of normallung tissue. The specific goals of this study are to evaluate GTV changes during the course of radiation therapy(RT) and examine its potential use in adaptive radiotherapy for tumor dose escalation or normal tissue sparing inpatients with SCLC.

Materials/Methods: A total of 8 SCLC patients eligible for chemoradiotherapy underwent computed tomography (CT) after 13and 23 fractions (at a nominal dose of 23.4 and 41.4 Gy, respectively). The GTV was delineated on the repeat CT scans and twotreatment plans were generated with or without adaptation to tumor shrinkage during RT for each patient. A dosimetric andvolumetric analysis was performed.

Results: Average GTV reduction observed over 13 fractions was 58% (range, 13 to 96%; p\0.001), and 23 fractions was 66%(range, 22 to 99%; p\0.001). Compared to plan without adaptation, adaptive radiotherapy resulted in mean lung dose relativedecreases of 15%, mean lung volume receiving $ 20Gy (V20) relative decreases of 6%, mean lung volume receiving $ 5 Gy(V5) relative decreases of 10%, mean medulla spinalis dose relative decreases of 33%, mean esophagus volume receiving$ 50Gy (V50) relative decreases of 17%, mean heart volume receiving $ 42 Gy (V42) relative decreases of 18%. The ben-efits of adaptive therapy are the greatest for tumor volumes $ 30 cm3 and are directly dependent on GTV reduction duringtreatment.

Conclusions: Adaptation of the treatment plan twice during conventionally fractionated (chemo-) RT of SCLC achieved a signif-icant benefit in terms of normal tissue sparing and significantly decreases tumor volume and dose to normal tissues, allowing for thepossibility of dose escalation.

Author Disclosure: A.A. Yavuz, None; B.S. Ozdemir, None; E. Dundar, None; B. Pehlivan, None; I. Karakus, None; N. Tuncel,None; M.N. Yavuz, None.

2685 Identification of Residual Metabolic-active Areas within Lung Tumors using a Pre-radiotherapy

FDG-PET-CT Scan

H. Aerts1, A. Dekker1, C. Lavoie2, S. Green2, J. Michielsen1, D. De Ruysscher1, D. Vines2, A. Sun2

1MAASTRO, Maastricht, Netherlands, 2PMH, Toronto, ON, Canada

Purpose/Objective(s): It was recently described that on the basis of one pre-treatment 18F-deoxyglucose (FDG)-PET-CTscan, radio-resistant areas within lung tumors could be defined. To validate these results, we assessed the overlap-fraction(OF) of the high FDG-uptake areas pre-radiotherapy with the residual metabolic-active areas post-radiotherapy in an indepen-dent dataset.

Materials/Methods: Data from 19 patients at Princess Margaret Hospital with lung cancer were analyzed. All patients underwenttwo FDG-PET-CT scans, one pre-radiotherapy (pre-RT) and one approximately three months post-RT. Sixteen patients showedresidual FDG-uptake after RT in the primary tumor. Residual FDG-uptake was defined as the FDG-uptake within the primarytumor higher than the uptake within the aortic arch. Seven of these 16 patients were excluded due to large deformations betweenthe scans (n = 3) and because the residual disease was not clearly distinguishable from the surrounding-tissue (n = 4). Therefore, 9patients were included for further analysis. Auto-delineations based on several threshold-levels relative to SUVmax wereperformed on the pre- and post-RT FDG-PET-CT scans within the primary-tumor. The volumes and OFs of these pre- andpost-RT delineations were calculated.

Results: The mean volume of the residual metabolic-active areas was 20% ± 18% (mean ± SD) compared to the mean volumeof the gross tumor volume (GTV) pre-RT. The residual metabolic-active areas largely corresponded with the pre-RT GTV(OF = 88% ± 15%). The pre-RT-scan threshold delineations of 34%, 40% and 50% of the SUVmax had a large OF withthe residual region, 81% ± 21%, 71% ± 21% and 57% ± 24%, respectively. The mean volumes of the 34%, 40%, and 50%uptake areas pre-RT were 58 ± 47%, 43 ± 31% and 26 ± 20% of the GTV pre-RT.