Radiation and Cancer Physics

PD 06 - Physics 2 - Poster Discussion - Treatment Delivery

1052 - Dosimetric Evaluation of Post-Mastectomy Irradiation Involving Air Expander Implants

Monday, October 22
11:27 AM - 11:33 AM
Location: Room 217 C/D

Dosimetric Evaluation of Post-Mastectomy Irradiation Involving Air Expander Implants
P. J. Black1, C. Chin1, B. Wolthuis2, J. A. Ascherman3, E. P. Connolly4, and C. S. Wuu1; 1Department of Radiation Oncology, Columbia University Medical Center, New York, NY, 2Norwalk Hospital, Norwalk, CT, 3Department of Surgery, Columbia University Medical Center, New York, NY, 4Dept of Radiation Oncology, Columbia University Medical Center, New York, NY

Purpose/Objective(s): AeroForm (AirXpanders, Inc., Palo Alto, CA) is a patient-controlled, carbon-dioxide filled tissue expander containing stainless steel elements that is used for breast reconstruction following mastectomy. The XPAND trial demonstrated the use of AeroForm as an effective method of tissue expansion with more convenient and expedient expansion than traditional saline expanders. In patients who require post-mastectomy radiation therapy, however, the presence of the AeroForm presents a dosimetric challenge given the material components of the device. This study evaluates the relative accuracy of the Treatment Planning System (TPS) in calculating dose by confirming dose delivered at a number of anatomic locations in a phantom system. In addition, based on phantom measurements, we compare the impact of tissue heterogeneity corrections on delivered dose treated with 3D conformal and volumetric arc therapy (VMAT) techniques.

Materials/Methods: The ATOM phantom system (Computerized Imaging Reference Systems, Norfolk, VA) incorporating an AeroForm expander placed on the anterior chest between 0.5cm bolus posteriorly and 1.5cm bolus anteriorly was used to simulate a post-mastectomy patient. External beam radiation plans were created using Eclipse (Varian Medical Systems, Palo Alto, CA). Both 3D conformal and VMAT treatment techniques were used. For each, two comparison plans were created using either the anisotropic analytical algorithm (AAA) or Acuros XB (AXB) to perform dose calculations. Dose measurements were performed using optically stimulated luminescent dosimeters (OSLD, nanoDot™, Landauer) placed at points across the chest wall, 5 anterior and 5 posterior to the tissue expander, and at single points both medial and lateral to the expander.

Results: AXB more accurately predicted dose distribution than AAA, with smaller discrepancy between TPS and measured dose using either treatment technique. Attenuation of the tangent beam in our 3D treatment plan by stainless steel elements in the implant device resulted in dose reduction to the lateral breast. This was predicted by AXB but not AAA and confirmed in our phantom system with OLSD measurements. A summary of our findings is provided in Table 1.
Technique Location Average AAA Calculated (cGy) Average Acuros Calculated (cGy) Average OSLD Dose (cGy) Average % Diff AAA Average % Diff Acuros
VMAT Anterior 213.1 210.7 196.0 -8.0% -7.0%
VMAT Posterior 208.7 203.5 195.5 -6.3% -3.8%
VMAT Lateral 219.8 216.0 203.0 -7.6% -6.0%
3D Anterior 228.7 203.5 216.7 -5.3% -6.2%
3D Posterior 208.0 207.5 193.8 -6.9% -6.6%
3D Lateral 199.6 178.4 175.9 -11.9% -1.4%

Conclusion: The Acuros XB algorithm more accurately predicts dose in the presence of an AeroForm expander using either treatment technique. Attenuation of the dose by stainless steel elements was less significant with the use of VMAT. This presents a preliminary dosimetric analysis in a phantom system, with the intent to further evaluate the effect expander volume on dose homogeneity.

Author Disclosure: P.J. Black: None. C. Chin: None. B. Wolthuis: None. J.A. Ascherman: Consultant; Air Xpanders Inc. E.P. Connolly: Employee; Celgene. Research Grant; Eisai, Merck. Advisory Board; Eisai. C. Wuu: None.

Paul Black, PhD

Disclosure:
No relationships to disclose.

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