One of the issues that a planner is often facing in HDR-brachytherapy, is the selective existence of high dose volumes around some few dominating dwell positions. This effect can be eliminated by limiting the free modulation of the dwell times. HIPO, a state of the art inverse treatment plan optimization algorithm, offers this possibility. The quality of 12 clinical HDR brachytherapy implants for prostate utilizing HIPO and modulation restriction has been compared to alternative plans with HIPO and free modulation. All common dose-volume indices for the prostate and the organs at risk have been considered together with COIN and EI. The radiobiology based EUD has been evaluated for all volumes of interest and used for comparison as well. Our results demonstrate that HIPO with a modulation restriction value of 0.1-0.2 delivers high quality plans which are practically equivalent to those achieved with free modulation. In the comparison, all the dosimetric, conformity based and EUD parameters produced compatible results. The modulation restricted clinical plans demonstrated a lower total dwell time by a mean of 1.4% that was proved to be statistically significant (p=0.002).

Purpose: The dosimetric accuracy of the recently released Acuros XB advanced dose calculation algorithm (Varian Medical Systems, Palo Alto, CA) is investigated for single radiation fields incident on homogeneous and heterogeneous geometries, as well as for two arc (VMAT) cases and compared against the analytical anisotropic algorithm (AAA), the collapsed cone convolution superposition algorithm (CCCS) and Monte Carlo (MC) calculations for the same geometries. Methods and Materials: Small open fields ranging from 1 × 1 cm 2 to 5 × 5 cm 2 were used for part of this study. The fields were incident on phantoms containing lung, air, and bone inhomogeneities. The dosimetric accuracy of Acuros XB, AAA and CCCS in the presence of the inhomogeneities was compared against BEAMnrc/DOSXYZnrc calculations that were considered as the benchmark. Furthermore, two clinical cases of arc deliveries were used to test the accuracy of the dose calculation algorithms against MC. Results: Open field tests in a homogeneous phantom showed good agreement between all dose calculation algorithms and MC. The dose agreement was +/−1.5% for all field sizes and energies. Dose calculation in heterogenous phantoms showed that the agreement between Acuros XB and CCCS was within 2% in the case of lung and bone. AAA calculations showed deviation of approximately 5%. In the case of the air heterogeneity, the differences were larger for all calculations algorithms. The calculation in the patient CT for a lung and bone (paraspinal targets) showed that all dose calculation algorithms predicted the dose in the middle of the target accurately; however, small differences (2%-5%) were observed at the low dose region. Overall, when compared to MC, the Acuros XB and CCCS had better agreement than AAA. Conclusions: The Acuros XB calculation algorithm in the newest version of the Eclipse treatment planning system is an improvement over the existing AAA algorithm. The results are comparable to CCCS and MC calculations especially for both stylized and clinical cases. Dose discrepancies were observed for extreme cases in the presence of air inhomogeneities.

RTOG 0617, a randomized phase III cooperative group trial using 2 x 2 factorial design, with radiation dose as one factor and cetuximab as the other factor. Materials/Methods: Patients enrolled in RTOG 0617 with retrievable RT dosimetry were eligible for this study. Circulating immune cells, including rapidly circulating ones in the heart, lung and blood vessels, and slowly circulating ones in the lymphatic system and blood reservoirs (a portion of veins/capillaries), were considered as a surrogate for OARIS. The effective dose to the immune cells (EDIC) was modeled with assumptions that in each fraction, radiation dose was uniformly delivered to all cells for rapidly circulating ones, and only to those in the irradiated volume for slowly circulating ones. EDIC was thus calculated as a function of the number of RT fractions and doses to the lung, heart, and the whole body (integral dose). Associations between EDIC and local tumor control, denoted as local progression-free survival (LPFS), and OS were assessed using Cox regression model. Results: The analysis included 464 patients (261 from 60-Gy arm and 203 patients from 74-Gy arm). EDIC was significantly higher in the 74-Gy group (6.9 AE 2.1 Gy) than in the 60-Gy group (5.7 AE 1.7 Gy) (P < 0.0001). EDIC was associated with LPFS (P < 0.0001) and OS (P < 0.0001) in the whole group of all 464 patients, and within the 60-Gy (P Z 0.007 for LPFS and P < 0.0001 for OS) and 74-Gy groups (P Z 0.04 for LPSF and P Z 0.003 for OS). The 2-year OS rates were 72, 52, 49, and 15% for EDIC in the range of <4.0, 4.0-6.5, 6.5-9.5, and >9.5 Gy, respectively (P < 0.0001). Conclusion: Radiation dose to OARIS was significantly associated with local tumor control and overall survival for patients with stage-III NSCLC, suggesting that radiation-induced immune toxicity could be an important contributing factor to tumor progression and death. In-depth study of OARIS is warranted and will be reported at the meeting.Purpose/Objective(s): Recent data demonstrate an association between radiation (RT) dose to the heart and Grade 3+ cardiac events (CEs) in patients with locally advanced non-small cell lung cancer. Those with baseline coronary artery disease (CAD) were at particularly increased risk. A majority of patients, however, do not have overt pretreatment CAD and we sought to determine which combinations of pretreatment cardiac risk and RT dose are associated with an elevated CE rate.

Purpose: Craniospinal axis irradiation (CSI) is a method of treating various central nervous system malignancies. The large target volume typically includes entire spinal cord and whole brain. Dosimetric comparison was performed between tomotherapy, volumetric modulated arc therapy (VMAT), and 3D conformal radiation therapy (3D-CRT) for CSI. Methods and Materials: Five (n = 5) CSI patients were planned using 3D-CRT, VMAT, and tomotherapy (normalized such that 95% of PTV received at least 23.4 Gy in 13 fractions). Plans were compared using PTV conformity number (CN) and homogeneity index (HI), normal tissue (NT) dose statistics, integral dose, and treatment time. Results: On average, tomotherapy plans showed higher CN (0.932 vs. 0.860 and 0.672 for SmartArc and 3D-CRT). In terms of HI, VMAT plans consistently showed better dose homogeneity (1.07 vs. 1.15 and 1.13 for tomotherapy and 3D-CRT). SmartArc delivered lower maximum dose for majority of NT, but higher mean dose. 3D-CRT plans delivered higher maximum dose but lower mean dose to NT. Conclusions: SmartArc treatments achieved better PTV homogeneity and reduced maximum dose to NT. Tomotherapy showed better target conformity, but 3D-CRT was shown to reduce mean dose to NT. Integral doses were similar between treatment modalities, but tomotherapy treatment times were much longer.