Effect of Acuros XB algorithm on monitor units for stereotactic body radiotherapy planning of lung cancer.

Stereotactic body radiotherapy (SBRT) is a curative regimen that uses hypofractionated radiation-absorbed dose to achieve a high degree of local control in early stage non-small cell lung cancer (NSCLC). In the presence of heterogeneities, the dose calculation for the lungs becomes challenging. We have evaluated the dosimetric effect of the recently introduced advanced dose-calculation algorithm, Acuros XB (AXB), for SBRT of NSCLC. A total of 97 patients with early-stage lung cancer who underwent SBRT at our cancer center during last 4 years were included. Initial clinical plans were created in Aria Eclipse version 8.9 or prior, using 6 to 10 fields with 6-MV beams, and dose was calculated using the anisotropic analytic algorithm (AAA) as implemented in Eclipse treatment planning system. The clinical plans were recalculated in Aria Eclipse 11.0.21 using both AAA and AXB algorithms. Both sets of plans were normalized to the same prescription point at the center of mass of the target. A secondary monitor unit (MU) calculation was performed using commercial program RadCalc for all of the fields. For the planning target volumes ranging from 19 to 375cm(3), a comparison of MUs was performed for both set of algorithms on field and plan basis. In total, variation of MUs for 677 treatment fields was investigated in terms of equivalent depth and the equivalent square of the field. Overall, MUs required by AXB to deliver the prescribed dose are on an average 2% higher than AAA. Using a 2-tailed paired t-test, the MUs from the 2 algorithms were found to be significantly different (p < 0.001). The secondary independent MU calculator RadCalc underestimates the required MUs (on an average by 4% to 5%) in the lung relative to either of the 2 dose algorithms.

Validation of the Eclipse AAA algorithm at extended SSD.

The accuracy of dose calculations at extended SSD is of significant importance in the dosimetric planning of total body irradiation (TBI). In a first step toward the implementation of electronic, multi-leaf collimator compensation for dose inhomogeneities and surface contour in TBI, we have evaluated the ability of the Eclipse AAA to accurately predict dose distributions in water at extended SSD. For this purpose, we use the Eclipse AAA algorithm, commissioned with machine-specific beam data for a 6 MV photon beam, at standard SSD (100 cm). The model was then used for dose distribution calculations at extended SSD (179.5 cm). Two sets of measurements were acquired for a 6 MV beam (from a Varian linear accelerator) in a water tank at extended SSD: i) open beam for 5 x 5, 10 x 10, 20 x 20 and 40 x 40 cm2 field sizes (defined at 179.5 cm SSD), and ii) identical field sizes but with a 1.3 cm thick acrylic spoiler placed 10 cm above the water surface. Dose profiles were acquired at 5 cm, 10 cm and 20 cm depths. Dose distributions for the two setups were calculated using the AAA algorithm in Eclipse. Confidence limits for comparisons between measured and calculated absolute depth dose curves and normalized dose profiles were determined as suggested by Venselaar et al. The confidence limits were within 2% and 2 mm for both setups. Extended SSD calculations were also performed using Eclipse AAA, commissioned with Varian Golden beam data at standard SSD. No significant difference between the custom commissioned and Golden Eclipse AAA was observed. In conclusion, Eclipse AAA commissioned at standard SSD can be used to accurately predict dose distributions in water at extended SSD for 6 MV open beams.