Dosimetric evaluation of lung tumor immobilization using breath hold at deep inspiration

Purpose:To examine the dosimetric benefit of self-gated radiotherapy at dee p-inspiration breath hold (DIBH) in the treatment of patients with non-smal l-cell lung cancer (NSCLC). The relative contributions of tumor immobilizat ion at breath hold (BH) and increased lung volume at deep inspiration (DI) in sparing high-dose lung irradiation (greater than or equal to 20 Gy) were examined. Methods and Materials:Ten consecutive patients undergoing radiotherapy for Stage I-IIIB NSCLC who met the screening criteria were entered on this stud y. Patients were instructed to BH at DI without the use of external monitor s or breath-holding devices (self-gating), Computed tomography (CT) scans o f the thorax were performed during free breathing (FB) and DIBH. Fluoroscop y screened for reproducible tumor position throughout DIBH, and determined the maximum superior-inferior (SI) tumor motion during both FB and DIBH. Ma rgins used to define the planning target volume (PTV) from the clinical tar get volume included 1 cm for setup error and organ motion, plus an addition al SI margin for tumor motion, as determined from fluoroscopy. Three confor mal treatment plans were then generated for each patient, one from the FB s can with FB PTV margins, a second from the DIBH scan with FB PTV margins, a nd a third from the DIBH scan with DIBH PTV margins, The percent of total l ung volume receiving greater than or equal to 20 Gy (using a prescription d ose of 70.9 Gy to isocenter) was determined for each plan. Results:Self-gating at DIBH was possible for 8 of the 10 patients; 2 patien ts were excluded, because they were not able to perform a reproducible DIBH , For these 8 patients, the median BH time was 23 (range, 19-52) s. The mea n percent of total lung volume receiving greater than or equal to 20 Gy und er FB conditions (FB scan with FB PTV margins) was 12.8%. With increased lu ng volume alone (DIBH scan with FB PTV margins), this was reduced to 11.0%, tending toward a significant: decrease in lung irradiation over FB (p = 0. 086). With both increased lung volume and tumor immobilization (DIBH scan w ith DIBH PTV margins), the mean percent lung volume receiving greater than or equal to 20 Gy was further reduced to 8.8%, a significant decrease in lu ng irradiation compared to FB (p = 0.011). Furthermore, at DIBH, the additi onal benefit provided by tumor immobilization (i.e., using DIBH instead of FB PTV margins) was also significant (p = 0.006). The relative contribution s of tumor immobilization and increased lung volume toward reducing the per cent total lung volume receiving greater than or equal to 20 Gy were patien t specific; however, all 8 of the patients analyzed showed a dosimetric ben efit with this DIBH technique. Conclusion:Compared to FB conditions, at DIBH the mean reduction in percent lung volume receiving greater than or equal to 20 Gy was 14.3% with the in crease in lung volume alone, 22.1% with tumor immobilization alone, and 32. 5% with the combined effect. The dosimetric benefit seen at DIBH was patien t specific, and due to both the increased lung volume seen at DI and the PT V margin reduction seen with tumor immobilization. (C) 2001 Elsevier Scienc e Inc.