G. Weinstein

Prediction of Intrafraction Prostate Motion: Accuracy of Pre- and Post-Treatment Imaging and Intermittent Imaging

PURPOSE To evaluate whether pre- and post-treatment imaging (immediately before and after a radiation therapy treatment fraction) and intermittent imaging (at intervals during a treatment fraction) are accurate predictors of prostate motion during the delivery of radiation. METHODS AND MATERIALS The Calypso 4D Localization System was used to continuously track the prostate during radiation delivery in 35 prostate cancer patients, for a total of 1,157 fractions (28-45 per patient). Predictions of prostate motion away from isocenter were modeled for a pre- and post-treatment imaging schedule and for multiple intermittent intrafraction imaging schedules and compared with the actual continuous tracking data. The endpoint was drift of the prostate beyond a certain radial displacement for a duration of more than 30 s, 1 min, and 2 min. Results were used to evaluate the sensitivity and specificity of these models as an evaluation of intrafraction prostate motion. RESULTS The sensitivity of pre- and post-treatment imaging in determining 30 s of intrafraction prostate motion greater than 3, 5, or 7 mm for all fractions was low, with values of 53%, 49%, and 39%, respectively. The specificity of pre- and post-treatment imaging was high for all displacements. The sensitivity of intermittent imaging improved with increasing sampling rate. CONCLUSIONS These results suggest that pre- and post-treatment imaging is not a sensitive method of assessing intrafraction prostate motion, and that intermittent imaging is sufficiently sensitive only at a high sampling rate. These findings support the value of continuous, real-time tracking in prostate cancer radiation therapy.

SU‐GG‐J‐17: Technical Aspects of a Simple and Effective Method to Perform Stereotactic Body Radiation Therapy (SBRT) in Lung Cancer Patients without 4D‐Ct and Gating

Purpose: In recent years, there has been considerable debate about the necessity of using respiratory gating and 4D ‐CT to perform SBRT, a fast‐growing practice with high risk to patients. Here, we describe the technical aspects of a method to perform SBRT in lung patients without the use of respiratory gating and 4D‐CT. Methods and Materials: After two years of slow dose escalation, we launched our SBRT program in 4/2009 and have enrolled 10 patients (11 lesions) so far. Each patient underwent a CT‐guided placement of a Visicoil marker. Patients were immobilized with a homegrown method utilizing a Vac‐loc and Aquaplast body cast; then underwent a fluoroscopic exam with a C‐Arm to determine tumor motion. Normal tissuesdose limits were set using RTOG/ M.D. Anderson guidelines on lungSBRT. About two‐third of lesions (7/11) were treated with 50 Gy in 4–5 fractions, the remainder with >5 fractions due to increased risk of complications. Treatments were carried out at a BrainLab Novalis system with ExacTrac image guidance. Prior to treatment, a series of x‐rays were taken with the patient in “breath‐in and hold” and “breath‐out and hold” positions. The mid‐point of tumor motion was established and used to determine the treatment isocenter. Results: Average target dimensions were 19cc (GTV) and 73cc (PTV). The lung DVH (V‐20), based on whole lung‐GTV, ranged from 3.1% to 11% with an average of 7.8%. Tumor motion observed at the treatment machine using ExacTrac x rays matched to within 3 mm of the fluoroscopic value. Patient follow‐up (9/10) showed no evidence of Grade III toxicity and no progression of disease in any patient. Conclusions: Respiratory gating and 4D‐CT are not essential components of SBRT. As pointed out by recent ASTRO/ACR guidelines, the truly critical aspect is rigurous QA of each process.