William Adrian Hall MD, Presenter: Nothing to Disclose

Anil Sethi PhD, Abstract Co-Author: Nothing to Disclose

Suneel Nagda, Abstract Co-Author: Nothing to Disclose

Purpose/Objective: Patient setup errors can significantly alter target dose in radiation therapy. Image guided radiation therapy (IGRT) has the potential to reduce setup errors that may be critical in Stereotactic Body Radiation Therapy (SBRT) due to small target margins and large dose per fraction. We quantify setup errors in stage I non-small cell lung cancer (NSCLC) patients undergoing SBRT and evaluate their impact on target dose. Materials/Methods: Twenty-four stage I NSCLC and 4 patients with solitary metastasis to the lung were treated with SBRT and included in this IRB-approved study. Target dose ranged from 30-60 Gy (median dose=50 Gy) in 5-10 fractions (median=5 fractions). 210 setup fractions were evaluated (not all used for treatment). Prior to treatment, each patient was aligned to the isocenter using infrared (IR) markers placed on the patient?s chest. This procedure is equivalent to patient setup in a non-IGRT environment. Next, orthogonal kilo-voltage x-rays were taken and compared to digitally reconstructed radiographs (DRRs) obtained from treatment planning CT. Novalis Exactrac? 5.5(BrainLAB, Germany) was used to robotically guide patient to the treatment isocenter. The correction shifts (IR markers isocenter vs. X-ray isocenter) in the lateral, longitudinal, and vertical direction were recorded. These shifts represent daily setup errors that would result if the patients were treated without IGRT. Finally, we calculated overall mean/daily setup error for each patient and evaluated impact on the target dose. Results/Discussion: For our study group, target sizes were, GTVmean=42 cc (range 2-400cc) and PTVmean = 67 cc (range 9-580cc). I. Shift data: Mean correction shifts in the lateral, longitudinal, and vertical directions were 1.6ñ4.6mm, 1.2ñ5.4mm, and -2.1ñ5.1mm respectively for all patients, averaged over their treatment regimen. The average overall correction shift vector was 7.8ñ4.7 mm. II. Impact on Target Dose: Dose impact of not accounting for setup errors as measured by D95 (dose covering 95% of volume) showed a cumulative average 14.1(ñ19.5)% lower GTV D95 and 25.3(ñ22.8)% lower PTV D95. The maximum dose deviation was as much as 75.4% lower PTV D95 and 66.4% lower GTV D95. Similar results were observed for target Dmin. Daily target dose also showed large dose deficits. For a typical 5 fraction treatment, D95 decrease ranged between 20.5-86.9%. Conclusions: Patient setup errors can be quantified using IGRT. If not addressed, these errors can lead to unacceptably large dose reduction in SBRT. We would recommend consideration of custom target margins based on individualized IGRT derived setup data for SBRT procedures.

Hall, W, Sethi, A, Nagda, S, Importance of Image-guided Patient Setup in Stereotactic Body Radiation Therapy (SBRT) of Early Stage NSCLC.  Radiological Society of North America 2009 Scientific Assembly and Annual Meeting, November 29 - December 4, 2009 ,Chicago IL. http://archive.rsna.org/2009/8502214.html