1. CONTACT
Catalina A. Riley
The University of Texas MD Anderson Cancer Center
School of Health Professions
Varian Halcyon Dosimetric Comparison for Multi-Arc VMAT Prostate and Head-and-Neck Cancers
Catalina Riley, Christopher Cox, Sasha Graham, Holly Havran, Bethany Kramer,
Tucker Netherton, DMP, Christine Peterson, PhD, Tyler Williamson, CMD, Laurence Court, PhD
The University of Texas MD Anderson Cancer Center School of Health Professions
INTRODUCTION
RESULTS
The Varian Halcyon linear accelerator is designed for faster treatment time and fewer treatment errors than conventional linear accelerators. Its increased dose rate, gantry speed, and leaf speed reduce treatment time. The two MLC (multi-leaf collimator) leaf banks of 1 cm leaf width could cause planning difficulties; utilizing many arcs could, however, address this challenge. While three or more arcs are uncommonly used due to increased treatment time, the Halcyon can deliver five arcs in under three minutes
Purpose: Determine if increasing the number of arcs would increase plan quality regarding organ-at-risk (OAR) sparing, target coverage, homogeneity, conformity, and hotspot volume without compromising treatment time in prostate and head-and-neck patients.
All four-arc and five-arc prostate plans were considered clinically acceptable based on MDACC guidelines. The one-arc, two-arc, and three-arc plans achieved clinical constraints 13.3, 20, and 93.3% of the time, respectively. The five-arc head-and-neck plans were all clinically acceptable and the one-arc, two-arc, three-arc, and four-arc plans met MDACC constraints 26.7, 73.3, 86.7, and 93.3% of the time, respectively. The isodose distributions below show the one-arc through five-arc plans for prostate (top) and head-and-neck (bottom). 1 2 3 4 5
The graphs below show the results of increasing the number of arcs when using the Halcyon.
Image courtesy of Varian Medical Systems, Inc.©
METHODS AND MATERIALS
Fifteen prostate and fifteen head-and-neck patients
Re-planned on the Halcyon using Eclipse v 15.6 with VMAT technique
One to five arc plans created for each patient for a total of 150 plans
Five-arc plans optimized to meet MD Anderson Cancer Center (MDACC) target coverages and constraints
DISCUSSION
CONCLUSION
Prostate MDACC Constraints
and Target Volume Coverage
Head-and-neck MDACC Constraints
Organ at Risk
Maximum Dose
Dose Volume Parameter
Mean Dose Parameter
Bladder
V60Gy < 40%
Brain
< 54Gy
V70 ≤ 20%
Brainstem
Femoral Heads
V45Gy ≤ 50%
Cochlea
< 35Gy
V50Gy ≤ 10%
Esophagus
< 34Gy or ALARA
Rectum
< 82Gy
V30Gy<80%
Lens
< 5Gy
V40Gy ≤ 60%
Mandible
< 70Gy
V69.3Gy ≤ 0% 
Parotid Gland
< 26Gy
V60Gy ≤ 40%
Spinal Cord
< 45Gy
V70Gy ≤ 20%*
Spinal CordPRV
< 50Gy
V70Gy ≤ 12%#
pCTV57
V57Gy ≥ 99%
V76Gy ≤ 15%
pCTV60
V60Gy ≥ 99%
V80Gy ≤ 5%
pCTV66
V66Gy ≥ 99%
PTV
V78Gy ≥ 98%
PTV66
V66Gy ≥ 95%
* no rectal balloon,# rectal balloon
Results showed improved OAR sparing, decreased hotspot volume, and more homogenous and conformal PTV dose distribution when increasing the number of arcs for all prostate and head-and-neck patients. On average, plans with three or more arcs achieved lower doses to critical structures on the Halcyon. However, total MUs increased for both prostate and head-and-neck patients as the number of arcs increased. Currently, the Halcyon is configured with a 1 cm MLC leaf width that is not able to conform as closely to the OAR borders, which exposes the organ to more radiation. Although the decreased resolution may cause increased OAR dose, the deficit can be resolved by increasing the number of arcs in the plan.
The Halcyon’s increased gantry speed allows for more arcs without compromising treatment time. Although some OARs did not show a significant difference in dose as arc number increased, the dose to the majority of OARs such as the rectum and parotid glands significantly decreased. Similarly, dose became more conformal and more homogeneous for both prostate and head-and-neck plans as the number of arcs increased. These results could influence how all VMAT treatments are planned on the Halcyon and ultimately, provide patients with more optimal radiotherapy treatment without compromising treatment time.
Acknowledgments: Varian Medical Systems, Inc.© for providing access to Eclipse v 15.6.
Objectives from these clinically acceptable plans were copied to the one-arc, two-arc, three-arc, and four-arc plans
OAR dose, hotspot volume, homogeneity index (HI), and conformity index (CI) compared
HI and CI defined as:
REFERENCES
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