S476 I. J. Radiation Oncology d Biology d Physics Volume 75, Number 3, Supplement, 2009

2653 Intraluminal Brachytherapy for Endobronchial Carcinoma: Clinical Results and CT-based Approach of

Dosimetric Evaluation

H. Kawamura1, T. Ebara1, H. Katoh1, T. Tamaki1, H. Ishikawa1, T. Nonaka2, Y. Nakayama2, T. Takahashi1, T. Nakano1

1Gunma University Graduate School of Medicine, Maebashi, Japan, 2Kanagawa Cancer Center, Yokohama, Japan

Purpose/Objective(s): We have treated endobronchial carcinoma with intraluminal brachytherapy (ILBT) since 1997. The firstchoice of the treatment for endobronchial carcinoma is surgery, and ILBT is considered as a palliative treatment. However, it isnot rare that patients have poor pulmonary function or multiple lesions. In those patients, ILBT combined with external beam ra-diation therapy (EBRT) might be useful as a curative treatment because of its less invasiveness. One issue of endobronchial ILBT isthe uncertainty in dosimetric assessment when using X-ray films to calculate the dose distribution. Thus, we have performed CT-based dosimetric evaluation and evaluated its efficacy.

Materials/Methods: Fifteen lesions of 12 endobronchial carcinoma patients treated with ILBT in 2000 to 2008 were retrospec-tively reviewed. Ten lesions were treated with combination of ILBT and EBRT. ILBT using high dose rate 192 iridium thin wiresystem was performed at a dose of 5 Gy/fraction. Dose prescribed points were mucosal surface (10mm for lesions in trachea, 7mmfor lesions in main bronchus, 5mm for lesions in lobular and segmental bronchus and 3mm for lesions in sub-segmental bronchuslesions, all from the center of the source). CT based dosimetry was performed in the latest 1 patient. CT was scanned using the samecouch of fluoroscopy in the treatment room and the dose distribution was calculated with PLATO (Nucletron).

Results: The patient age at the beginning of radiation therapy ranged from 57 to 82 years. Seven lesions were roentgenographicallyoccult endobronchial carcinomas, and three lesions were cancers of trachea. Seven patients had multiple cancers. ILBT were per-formed 1–5 times in 1–3 weeks (median total dose was 20 Gy). The dose of EBRT was 0–61 Gy in 0–34 fractions (median totaldose was 40 Gy). Median follow-up time was 33 months. Two-year survival rate and local control rate were 90.9% and 68.1%,respectively. Local recurrences were observed in 3 lesions. Two patients died because of lung cancer (1) and other cancer (1). Tox-icities greater than Grade 2 were not observed except Grade 3 dyspnea in 1 patient. In CT based dosimetric assessment, 95% of theclinical target volume receives more than 90% of the prescribed dose.

Conclusions: ILBT combined with EBRT might be a treatment option in inoperable endobronchial carcinoma patients with tol-erable toxicity. CT based dosimetry is a promising tool for sophisticated endobronchial ILBT.

Author Disclosure: H. Kawamura, None; T. Ebara, None; H. Katoh, None; T. Tamaki, None; H. Ishikawa, None; T. Nonaka, None;Y. Nakayama, None; T. Takahashi, None; T. Nakano, None.

2654 Intensity Modulated Radiation Therapy (IMRT) Benchmarks for Thoracic Malignancies

M. B. Palmer, S. Vedam, R. Komaki, J. W. Welsh

UT MD Anderson Cancer Center, Houston, TX

Purpose/Objective(s): The purpose of this study is to define benchmarks and limitations for non–small cell lung cancer (NSCLC)lesions treated with Intensity Modulated Radiation Therapy (IMRT).

Materials/Methods: A retrospective dosimetric planning evaluation study was performed on 97 patients treated with IMRT forNSCLC (Stage I–IV) between 2007 and 2008 to doses of 45–70 Gy to the Planning Target Volume (PTV). The patients were di-vided into two groups based on their prescription doses, 45–50 Gy and 60–70 Gy. The patients were also divided into left or rightbased on their tumor location and subdivided into upper lobe, middle lobe, or lower lobe regions. They were also identified asanterior, middle and posterior tumors. Additional structures were created for plan evaluation: Ipsilateral Lung - PTV, TotalLung (TL) - PTV, PTV - TL, and External Skin - Total Lung. Statistical comparisons were made using Pearson Correlation Co-efficient, conformality index (CI), and volumetric data of the planning and normal tissue structures.

Results: Benchmarks for achievable dose constraints can be seen from the data. The Lung/PTV ratio is the most important param-eter that defines achievable lung doses. The Lung/PTV correlates with the % of the ipsilateral lung that can be spared (IpsaL - PTV)(r = 0.6, p value\0.001), which strongly correlates to the ipsilateral lung mean dose (IpsaLMD) (r = -0.79, p value\0.001). TheIpsaLMD increases with decreasing Lung/PTV. Difficulties meeting constraints for the 60–70 Gy group resulted in weaker cor-relations for patients with Lung/PTV # 5 [PTV $ 600cc, IpsaLMD$31 Gy, ContraLMD$7 Gy, TLMD$18 Gy]. The leftlung had higher IpsaLMD by 3 Gy (32.5 Gy vs. 29.5 Gy). The IpsaLMD is the strongest contributor for the TLMD (r =-0.66, p value \ 0.001). Lower lobe and posterior tumors have higher average TLMDs (18 Gy) than the other regions (16.5Gy). Posterior tumors have a higher average ContraLMD (8 Gy vs. 6.5 Gy). The ContraLMD (5 Gy–9 Gy) is a variable factornot evaluated in depth in this study, but is related to the nodal involvement in the mediastinum, beam angles, and the degreeof IMRT optimization.

Conclusions: We have attempted to define benchmarks and limitations for thoracic IMRT dosimetric plan evaluation. Such classsolution based benchmarks could improve the efficiency and quality of IMRT treatment planning for NSCLC.

Author Disclosure: M.B. Palmer, None; S. Vedam, None; R. Komaki, None; J.W. Welsh, None.

2655 Quantification of Tumor Volume Aliasing on Helical CT Scans for Lung Stereotactic Body Radiation

Therapy (SBRT) Patients

B. Wang, K. E. Kokeny, P. Rassiah-Szegedi, H. Zhao, Y. Huang, Y. J. Hitchcock, S. Joshi, B. J. Salter

University of Utah, Salt Lake City, UT

Purpose/Objective(s): SBRT of lung is a new and promising method of treatment, delivering very high doses of radiation and,thus, requiring extreme accuracy in targeting. Recently, much attention has been paid to improving the accuracy of SBRT deliverythrough the use of in-room image guidance. Equally important, however, is an accurate understanding of tumor shape and volumeat time of initial treatment planning. Here we quantify the magnitude of errors in understanding of tumor geometry introduced when