High Dose Rate CT Guided Brachyablation Versus High Dose Rate Stereotactic Ablative Radiotherapy for Lung Tumors

Dosimetric comparisons of CT-guided high dose rate (HDR) brachyablation (BT) versus HDR stereotactic ablative radiotherapy (SABR) of lung tumors. HDR BT with a single virtual needle defined by an interventional radiologist on CT images were simulated for six pulmonary tumors (>1 cm and <3.0 cm in diameter) that were considered practical for CT- guided percutaneous biopsy. The virtual needle was assumed to pass through the tumor's center in all plans with a 3 mm margin of clinical target volume (CTV) accounting for the microscopic disease. The advanced collapse cone dose engine was used to achieve the dose distribution in all cases on Oncentra® Brachy v.4.5 Planning system (Elekta AB, Stockholm, Sweden). Each tumor also had simulation SABR plans for HDR delivery by intensity modulated arc technique (IMAT) with a 3mm CTV margin based on the assumption of ideal image-guided 4D gating. To emulate the BT plans, SABR plans were optimized at ∼60% isodose level to achieve higher dose in the gross tumor volume (GTV) with steeper dose gradient outside using a Monte Carlo dose engine. For both BT and SABR, a single fraction 34 Gy was prescribed following the dose constraint guidelines of the RTOG 0915 protocol. The GTV mean dose was 2.3 times higher for BT. The RTOG 0915 constraints for absolute normal lung volume receiving 7.0 and 7.4Gy (V7Gy and V7.4Gy) were well under 1500, and 1000cc for both BT and SABR. V7Gy, and V7.4 were 163.4cc and 151.6cc for BT, and 289.2 and 234.6 cc for SABR over all patients. The maximum dose (Dmax) constraints were exceeded for the airways at 20.2 Gy in one SABR plan by 2.0 Gy, for the heart at 22.0 Gy in one BT and SABRT plan by 5.7 and 6.8Gy, respectively for the same tumor. In one tumor attached to the chest wall, the Dmax was 106.8 Gy in BT and 41.3 Gy in SABR, which was much higher than the limit of 30.0 Gy. Due to the obvious limitation of BT to shape the dose distribution considering the possible needle insertion path with respect to the tumor shape, the dose conformity (CI) was much worse in BT (range: 1.15 - 1.69) than in SABR (1.08 - 1.14). Assuming a Iridium 192 source activity of 10.0 Ci, and SABR dose rate at 1400 monitor units / minute as at our center, the averaged treatment delivery time were 13.6 ± 7.9 (1 SD), and 8.2 ± 0.7 minutes for BT and SABR, respectively. The treatment time increased with tumor volume for BT while showing no trend for SABR. Both HDR BT and SABR can achieve most dose constraints while delivering high dose within 15 minutes. While BT showed lower normal tissue doses in most cases, it must be carefully practiced in tumors attached to the chest wall due to the possibly unacceptable high dose to the ribs due to its limited flexibility to shape the dose distribution.