SU-E-T-700: Single Isocenter Volumetric Modulated Arc Therapy (VMAT) and Intensity Modulated Radiotherapy (IMRT) for Intact Left Breast Cancer Patients with Internal Mammary (IM) and Supraclavicular (S/C) Nodes - a Treatment Planning Study

Authors


Abstract

Purpose:

To investigate treatment plan quality and feasibility of VMAT for treatment delivery of intact left breast cancer patients with involved internal mammary (IM) and supraclavicular (S/C) nodes.

Methods:

Ten patients were selected for this retrospective study. All PTVs (Intact left breast, IM & S/C nodes) were contoured by a single radiation oncologist. Four 200° partial arcs with 2cm overlapping jaws (two clockwise and two counterclockwise; gantry angle 300°–140°) and ten fixed-field equally spaced coplanar modulated fields were used to optimize single isocenter VMAT (RapidArc) and IMRT treatment plans, respectively, using Eclipse TPS. Prescribed dose to PTV was 46Gy in 23 fractions with prescription dose covering 95% of the PTV.

Results:

Four of the 10 patients had PTV > 1200cc. The mean PTV volume was 1200.17±571.51cc (findings are reported as mean ± 1 SD). All plans were judged clinically acceptable while satisfying dose-volume constraints, with VMAT showing significantly better conformity than IMRT. The mean Paddick conformity index (PCI) was 0.88±0.03 for VMAT vs. 0.82±0.06 for IMRT plans (p=0.001). However, maximum doses were within ±1.0% of 111% of the prescription dose for both plans. The heart mean dose was 5.00±0.99Gy for VMAT vs. 7.09±1.70Gy for IMRT (p<0.001), heart volume receiving >30Gy was 0.18±0.29% for VMAT vs. 0.68±1.24% for IMRT (p=0.19), and V5Gy and V7Gy were 31.67±11.56% and 14.93±7.41% for VMAT vs. 67.15±17.66% and 36.55±16.59% for IMRT plans, respectively (p<0.001). The mean total lung dose was 8.95±0.52Gy vs. 8.48±0.81Gy (p=0.06) and V20Gy was 10.65±2.00% vs. 8.12±1.37% (p=0.055) for VMAT and IMRT plans, respectively. The right breast mean dose was 2.83±0.46Gy for VMAT vs. 4.06±0.61Gy for IMRT plans (p<0.001).

Conclusion:

VMAT plans achieved intended dose coverage to the intact left breast, IM & S/C nodes at ≥ 95% with significantly lower dose to critical structures, especially, to heart and contralateral breast compared to IMRT plans.

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