Ray-Trace is Inferior to Monte Carlo Dose Calculation Algorithm for ...

Proceedings of the 53rd Annual ASTRO Meeting to determine: (1) the accuracy of kilovoltage (KV) X-ray matching to bony anatomy for tumor positioning; (2) the optimal planning target volume (PTV) margin; (3) the magnitude of intrafraction motion; and (4) whether treatment time or patient characteristics correlate with intrafraction motion. Materials/Methods: 43 patients with NSCLC underwent 4DCT simulation with images acquired from 8 phases of the respiratory cycle. A gross tumor volume (GTV) was contoured on the free breathing CT, and an internal GTV (IGTV) encompassing the motion envelope of the GTV was created. Patients were initially positioned with orthogonal KV matching to bony anatomy followed by conebeam CT (CBCT), wherein the CBCT-visualized tumor was matched to the IGTV from the simulation dataset. Intrafraction motion was assessed on an immediate post treatment CBCT, which was matched to the IGTV. 807 total shift events were analyzed. Results: The average CBCT-based shifts after initial KV matching to bony anatomy in the vertical, longitudinal, and lateral axes were: 2.5 mm (n = 145; s = 3.5), 2.0 mm (n = 145; s = 2.8), and 1.7 mm (n = 145; s = 1.7). The mean total shift vector was 4.4 mm (n = 435; s = 4.0). There were P88 CBCT-based shifts greater than 3 mm (37 vertical, 31 longitudinal, and 20 lateral shifts). The population systematic error ( ) and random error (s) were 0.33 and 0.28 vertically, 0.24 and 0.32 longitudinally, and 0.16 and 0.18 laterally. Applying the Van Herk margin formula, the optimal PTV for KV positioning was 10.2 mm vertically, 8.3 mm longitudinally, and 5.3 mm laterally. The average post-treatment CBCT-based shifts were: 1.4 mm (n = 124; s = 1.7) vertically; 1.5 mm (n = 124; s = 2.0) longitudinally, and 0.9 mm (n = 124; s = 1.1) laterally. The mean total vector shift was 2.8 mm (n = 372; s = 2.3). 39 shifts were greater than 3mm (17 vertical, 20 longitudinal, and 2 lateral shifts). There was no correlation between intra-fraction motion and patient age, BMI, PFTs, treatment time, or tumor size. Conclusions: This study shows that KV matching of the tumor to the IGTV required CBCT-based shifts greater than 3 mm in approximately 20% of cases, and intrafraction motion was greater than 3mm in approximately 10% of cases. Since the typical IGTV-PTV expansion employed in RTOG protocols is 3 - 5 mm, these data suggest that (1) orthogonal KV matching to bony anatomy is inadequate for accurate tumor positioning; (2) intrafraction motion can result in tumor motion outside the PTV; and (3) further work is required to find determinants of inter- and intra-fraction motion that may help guide the individualized application of PTV margins. Author Disclosure: M.N. Corradetti: None. K. Teo: None. N. Anderson: None. L. Bonnar Millar: None. C. Barlow: None. R. Rengan: None.

3359

Ray-Trace is Inferior to Monte Carlo Dose Calculation Algorithm for Intracranial Robotic Radiosurgical Treatment: A Comparative Dosimetry Study

S. A. Hawksworth1, M. M. Qureshi1, H. XIang2, M. Truong2, L. Chin2, J. Holsapple2, J. Willins2, L. A. Kachnic2, G. Russo2 Boston University School of Medicine, Boston, MA, 2Boston Medical Center, Boston, MA

1

Purpose/Objective(s): Ray trace (effective path length method; EPL) is inferior to the Monte Carlo (MC) dose calculation algorithm when treating tumors at tissue interfaces of varying densities, such as lung tumors. We hypothesized that the anatomic complexity of the skull base would introduce similar uncertainties, due to the multiple tissue interfaces in this location (e.g. bone, brain, and air). We set out to determine whether differences between the EPL and the MC dose calculations were more pronounced for lesions of the skull base when compared to those within the substance of the brain and what dosimetric and anatomic characteristics influence these differences. Materials/Methods: A retrospective, comparative dosimetric study of 65 CyberKnifeÔ treatment plans was performed. All treatments were planned with the EPL dose calculation. The delivered plans were re-calculated using the MC algorithm, with identical beam orientation and number of monitor units, in MultiPlan v3.5.2 (Accuray, Inc.). Patients were designated as either having a lesion at the skull base (SB), adjacent to bone and air, or a control (CTRL) lesion, completely surrounded by brain tissue. Comparison of changes in the treatment plan characteristics between the two groups was performed when plans were recalculated with MC. Results: Skull base and control lesions demonstrated a statistically significant drop in the target coverage (SB: 96.4% vs. 93.7%, P = 0.02; CTRL: 97.5% vs. 94.7%, P = 0.03) when the dose calculation was performed with MC. The degree and magnitude of these changes were similar for SB and CTRL plans. Recognizing this similar behavior we combined all 65 patients into one analysis. The mean target coverage decreased to a similar degree and magnitude as the individual groups; critical structure dose variations were insignificant. We identified 11 treatment plans with a .5% (range 6.25 to 23.25%) decrease in target coverage from EPL to MC dose calculation. Comparing these outliers with the remaining 54 treatment plans we found that they were more likely to have smaller target volumes (mean 1 cc vs. 6.9 cc; P\0.0001), and a trend towards a lower beam number per treatment plan (mean 95 vs. 121; P = 0.091). Conclusions: We demonstrate that the EPL dose calculation significantly overestimates the degree of radiation therapy dose coverage for all intracranial targets, regardless of location. Smaller intracranial targets (\1cc) treated with fewer beams (\100) had the greatest degree of variation in target coverage. As such we recommend the use of Monte Carlo dose calculation for all intracranial CyberKnifeÔ radiosurgery treatments. Author Disclosure: S.A. Hawksworth: None. M.M. Qureshi: None. H. Xiang: None. M. Truong: None. L. Chin: None. J. Holsapple: None. J. Willins: None. L.A. Kachnic: None. G. Russo: None.

3360

Comparison of RapidArc Planning Strategies for Stereotactic Body Radiation Therapy (SBRT) for the Peripheral Lung

A. M. Weyh1, D. Lack1, G. Dyson2, A. Konski1,3 1

Karmanos Cancer Center, Detroit, MI, 2Karmanos Cancer Institute, Detroit, MI, 3Wayne State University, Detroit, MI

Purpose/Objective(s): The peripheral lung is very heterogeneous, with large variations in tissue densities, causing SBRT planning to be difficult. An ideal plan has a highly conformal and uniform dose distribution that spares normal tissues. Volumetric arc techniques, such as RapidArc (RA) have been used for SBRT treatments and have the benefit of fast delivery. A variety of planning strategies can be used with RA, such as varying gantry and pedestal angles and the number of arcs, using optimization and

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