The impact of inherent dose heterogeneity of brachytherapy on calculating iso-effective dose-fractionation regimens for nonmelanoma skin cancers.

Abstract

Purpose: Utilizing equivalent uniform biologically effective dose (EUBED) concept to calculate iso-effective dose-fractionation regimens in nonmelanoma skin cancer high-dose-rate (HDR) brachytherapy (BT) to address dose heterogeneity and comparing it with the simple form of biologically effective dose (BED) formula.

Methods and materials: Two hypothetical HDR BT treatment plans were created for surface and interstitial techniques. Then iso-effective dose-fractionation regimens were calculated (with both EUBED and BED equations) to prescribe a total EQD2₁₀ (equivalent dose in 2 Gy fractions with α/β = 10Gy) of 56, 60 and 65 Gy to the planning target volume (PTV) over a range of five to fifteen fractions. Three different treatment schedules were considered: two and three fraction per week for surface BT and two times a day for interstitial BT. If the treatment duration exceeded 1 month (T_k = 28 days), tumor repopulation was taken into account. Other radiobiological parameters used were α/β = 10Gy, α = 0.3Gy^-1, and T_p = 4 days. Finally, the dose per fraction calculated in the EUBED method was compared with the simple form of the BED formula.

Results: The BED formula, compared to the EUBED equation, may lead to less than 5% overestimation in the calculated dose per fraction. This difference is more noticeable in surface BT compared to interstitial implants, especially when prescribing lower total doses, when the total treatment duration in surface BT approaches 28 days, and when using more fractionated interstitial BT treatment regimens.

Conclusions: Based on the findings of this study, dose distribution inhomogeneity in nonmelanoma skin cancer HDR brachytherapy has minimal clinical impact on calculating iso-effective dose-fractionation regimens.