Breast Cancer

PV QA 4 - Poster Viewing Q&A 4

TU_8_3402 - Radiation Therapy Planning and Phantom Dosimetry of a Novel Breast Tissue Expander

Tuesday, October 23
2:45 PM - 4:15 PM
Location: Innovation Hub, Exhibit Hall 3

Radiation Therapy Planning and Phantom Dosimetry of a Novel Breast Tissue Expander
B. Farnia1, E. L. Bossart1, C. Takita1, L. M. Freedman2, and A. Ahamad1; 1Department of Radiation Oncology, University of Miami / Sylvester Comprehensive Cancer Center, Miami, FL, 2Department of Radiation Oncology, University of Miami /Sylvester C. Cancer Center, Deerfield Satellite, Miami, FL

Purpose/Objective(s): Postmastectomy radiotherapy is often delivered to the chest wall with a skin expander in place. A novel remote-controlled, carbon-dioxide filled tissue expander has recently been introduced that offers an advantage over traditional saline expanders in that patients do not need to receive serial saline injections in a physician office. Instead, they can perform expansion on their own using a remote controlled device that releases CO2 from a reservoir located inside the implant. Radiotherapy does not affect the expander per se, however the effect of the air and metal components on radiotherapy dose distribution is not well known. To-date, there is a single published study that reported large mismatches between the planned and delivered dose. The purpose of this study is to perform a preliminary estimate of the effects of this device on radiotherapy planning, plan quality, and dose delivered.

Materials/Methods: A customized phantom model representing a reconstructed breast and chest wall was fabricated in-house, placed on a CT simulator table, and scanned. The clinical target volume, air, metal and CT artifacts were contoured and a 3D plan was generated using medial and lateral tangential megavoltage beams of 266cGy per fraction. Corrections for the metal artifacts were performed. The phantom was irradiated with OSLDs located posterior to the expander at the chest wall interface, and at 0.5cm and 1.0 cm depth to the chest wall. Doses were recorded and compared to the expected values from the treatment planning system (TPS).

Results: Multiple iterations of planning consistently yielded hot and cold spots. Among dosimeters located at the chest wall interface, the mean dose reading was 265.8 cGy (range: 229.6 - 290.2 cGy, standard deviation: 23.86 cGy). The dose reading at 0.5cm depth and 1 cm depth were 283.9 cGy and 229.8 cGy respectively. These doses were all within the range of doses expected from the TPS.
Layer 1: chest wall interface OSLD Measurement (cGy) % of prescribed dose (266 cGy)
Middle 256.9 97%
Middle Superior 282.5 106%
Middle Inferior 269.6 101%
Middle Patient Left 290.2 109%
Middle Patient Right 229.6 86%
Layer 2: 0.5 cm depth
Middle 283.9 107%
Layer 3: 1.0 cm depth
Middle 229.8 86%

Conclusion: The results of this study identify marked variation in dose distribution when using the novel expander. Although measurements were within range predicted by the TPS, plans were not uniform, with many regions receiving higher or lower doses than desired. Regions that receive higher than goal doses may be at increased risk of radiation-induced toxicity, including rib fracture, chest wall pain and necrosis, while those receiving under-dosage may be at increased risk of tumor recurrence. Additional studies are needed to determine the safety and efficacy of this novel expander.

Author Disclosure: B. Farnia: None. E.L. Bossart: None. C. Takita: Employee; University of Miami Miller School of Medicine. Professor,Residency Program Director; University of Miami Miller School of Medicine. L.M. Freedman: None.

Benjamin Farnia, MD, BA

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