Semester of Graduation

Spring 2018

Degree

Master of Science (MS)

Department

Medical Physics

Document Type

Thesis

Abstract

Purpose: The delivery of post-mastectomy radiotherapy (PMRT) with bolus electron conformal therapy (BECT) for patients with left-sided breast cancer can reduce second cancer complication probability (SCCP) compared to modern rotational intensity modulated x-ray (IMXT) techniques such as volumetric modulated arc therapy (VMAT) or Tomotherapy. However, rotational IMXT yields superior levels of dose homogeneity compared to BECT. This study investigates the use of intensity modulated (IM) BECT (IM-BECT) to improve dose homogeneity in the chest wall (CW) region of the PMRT planning target volume (PTV) abutted to parallel opposed IMXT tangents for the remaining PTV for a clinically-representative set of patients, and quantitatively compares treatment planning metrics of BECT+IMXT to IM-BECT+IMXT.

Methods: Nine left-sided PMRT patients previously treated with VMAT at the Mary Bird Perkins Cancer Center were included in this study. PTVs included the CW and regional lymph nodes. BECT+IMXT and IM-BECT+IMXT plans were constructed with a commercial IMXT treatment planning system, a research version of a commercial BECT planning system (modified to support IM-BECT planning), and a treatment planning strategy developed in this study. The resulting plans were compared based on PTV dose homogeneity index (DHI) and conformity index (CI), tumor control probability (TCP), dose to organs at risk (OAR), normal tissue complication probability (NTCP), and SCCP. Statistical significance of differences between BECT+IMXT and IM-BECT+IMXT plans were tested using the two-way Wilcoxon Signed Rank test (p<0.05).

Results: The IM-BECT+IMXT treatment plans provided significantly lower volumes receiving 107% and 110% prescribed dose (i.e. V107% and V110%) in the electron subtarget (e-Target), the volume of the PMRT PTV treated with electrons. At a prescribed dose of 50 Gy, the average V107% and V110% decreased from 10.6% and 2.9% with BECT+IMXT to 2.2% and 0.2%, respectively, with IM-BECT+IMXT. The IM-BECT+IMXT treatment plans also provided significantly lower maximum dose to the lung (Lung-Dmax). The average Lung-Dmax decreased from 45.7 Gy with BECT+IMXT to 44.8 Gy with IM-BECT+IMXT. Both BECT and IM-BECT plans produced potentially clinically acceptable PMRT plans with no less than 99.0% TCP.

Conclusion: Both BECT+IMXT and IM-BECT+IMXT provided acceptable PMRT treatment plans for the patients included in this study. However, IM-BECT+IMXT showed a statistically significant advantage in terms of e­­­-Target dose homogeneity.

Date

1-9-2018

Committee Chair

Carver, Robert

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