Evaluation of the interfractional biological effective dose (BED) variation in MammoSite high dose rate brachytherapy

Abstract

The objective of this work is to evaluate the interfractional biological effective dose (BED) variation in MammoSite high dose rate (HDR) brachytherapy. Dose distributions of 19 patients who received 34 Gy in 10 fractions were evaluated. A method was employed to account for nonuniform dose distribution in the BED calculation. Furthermore, a range of alpha/beta values was utilized for specific clinical end points: fibrosis, telangiectasia, erythema, desquamation and breast carcinoma. Two scenarios were simulated to calculate the BED value using: i) the same dose distribution of fraction 1 over fractions 2-10 (constant case, CC), and ii) the actual delivered dose distribution for each fraction 1-10 (interfractiondose variation case, IVC). Although the average BED difference (IVC - CC) was < 0.7 Gy for all clinical endpoints, the range of difference for fibrosis and telangiectasia reached -11% to +3% and -9% to +9% for one of the patients, respectively. By disregarding high inhomogeneity in HDR brachytherapy, the conventional BED calculation tends to overestimate the BED for fibrosis by 16% on average, while it underestimates the BED for erythema (7.6%) and desquamation (10.2%). In conclusion, the BED calculation accounting for the nonuniform dose distribution provides a more clinically relevant description of the clinical delivered dose. Though the average BED difference was clinically insignificant, the maximum difference of BED for late effects can differ by a single fractional dose (10%) for a specific patient due to the interfraction dose variation in MammoSite treatment.

Figure 1

Group average differential dose volume histogram (dDVH) for 19 MammoSite patients. The vertical line represents the prescribed dose of 34 Gy and the size of dose bin is 0.1 Gy. The group average dDVH is computed by averaging fractional target volume corresponding to each dose bin for 19 patients.

Figure 2

Two‐dimensional CT image to show a typical MammoSite balloon with isodose distribution for patient 19. The volume of PTV was the same with PTV_EVAL for this patient because the balloon was located in the middle of breast and the volume of skin $+5\hspace{0.17em}\text{mm}$ and lung/pectoralis muscle did not interfered with the volume of balloon $+1\hspace{0.17em}\text{cm}$. Eight possible dwell positions were used along the straight MammoSite balloon catheter.

Figure 3

Comparison of BED values over various α/β ratios between CC (solid lines) and IVC (dash lines) for 19 MammoSite patients. The BED value was computed over 9 fractions for patients 4 and 6, and over 10 fractions for the remaining 17 patients.

Figure 4

Comparison of BED values computed by uniform dose distribution using the C‐method (Eq. (3)) (black circle) and nonuniform dose distribution using the H‐method (Eq. (5)) (box graph) for 19 MammoSite patients. Because the BED value is different for each patient in the H‐method, the calculated BED values are shown as a box graph in which each parallel bar represents minimum, 25, 50, 75 percentile and maximum value for 19 patients.