Comparative Analysis of Strut-Based High-Dose-Rate Brachytherapy Versus Intensity Modulated Proton Therapy for Delivery of Accelerated Partial Breast Irradiation

Abstract

Accelerated partial breast irradiation (APBI) for early-stage breast cancer patients has been delivered via varied modalities ranging from brachytherapy (BT) to 3-dimensional conformal external beam. As BT applicators have evolved in customizing dose conformality from interstitial catheters to balloon to strut-based devices, we undertook an investigation of strut-based HDR BT (S-BT) compared to intensity modulated proton therapy (IMPT) to determine whether IMPT could offer dose distributions comparable to S-BT. Ten consecutive patients previously treated with S-BT were analyzed. These patients underwent CT scans before and after the S-BT placement. The dose was 34 Gy in 10 twice-daily fractions prescribed to 1 cm from the S-BT applicator. For the S-BT plans, planning target volume (PTV)-Eval was a 1-cm expansion of the cavity minus S-BT applicator, lung, and chest wall as needed. All 10 cases met the planning goals of 90% of PTV-Eval receiving 90% of the prescribed dose (Rx dose), less than 10 mL PTV-Eval receiving 200% of Rx dose; and 50 mL receiving less than 150% of Rx dose. For the virtual IMPT plans, clinical target volume (CTV) was a 1-cm expansion from the cavity. Three-field IMPT was used with robust optimization (3-mm setup and 3.5% range uncertainties). Delivery time for such 3-field IMPT is less than 2 minutes. In order to perform an objective comparison, IMPT plans were normalized to the S-BT target coverage D90 to 90% of PTV-Eval volume. In addition, the cavity was excluded from the proton CTV for DVH parameter calculations. The 3-field IMPT covers a larger target compared to the S-BT (91.6±34.8 mL vs. 74.0±18.6 mL, p=0.18) due to the setup and range uncertainties. There is no significant difference in the absolute volumes of PTV over 115% for plans with protons and those with S-BT, 41.2±31.1 mL vs. 55.9±14.3 mL (P=0.19). However, the percentage volume of PTV over 115% with proton plans is significantly less than those with S-BT, 45.6%±28.7% vs. 75.5%±3.4%, p<0.01. Lung dose >5 Gy for IMPT plans is 30.3±29.1 mL, which is significantly less than that for S-BT plans with 188.0±65.4 mL, p<0.01. The maximum dose to the skin and ribs for protons vs S-BT is comparable, 35.7±4.3 Gy vs. 31.1±7.6 Gy (p=0.10) and 35.0±6.2 Gy vs. 28.8±8.1 Gy (p= 0.07), respectively. However, the proton plans are inferior compared to the S-BT plans in the V50 body integrated dose, 432.6±170.4 mL vs. 267.1±84.2 mL (p= 0.01). This comparative analysis of IMPT to S-BT for the delivery of APBI reveals a dosimetric advantage of IMPT over S-BT in offering more homogeneous target dose coverage along with improved sparing of underlying lung. Maximal dose to the skin and ribs is comparable between protons and S-BT. Moreover, IMPT is a noninvasive form of external beam in contrast to S-BT requiring invasive placement of an applicator. Additional patients along with further IMPT planning will be needed to substantiate the versatility of IMPT planning for APBI.