Equivalent dose and biologically effective dose for radiation oncology
EQD2 Calculator is a free, browser-based tool for radiation oncologists. It computes the equivalent dose in 2 Gy fractions (EQD2) and the biologically effective dose (BED) using the linear-quadratic model, sums cumulative EQD2 across multiple radiation courses for retreatment, and applies an optional tissue-recovery correction.
EQD2 Calculator was built to fill a specific gap in radiation-oncology education and reference work: a single, reliable tool that calculates biologically equivalent dose, adds multiple courses together, and applies recovery corrections for retreatment — quickly and transparently. These steps are often spread across spreadsheets, references, and manual arithmetic; the purpose of this tool is to bring them together in one place that is fast to use and easy to verify. EQD2 Calculator is an informational and educational aid, not a clinical or treatment-planning system, and every output is meant to be independently verified.
EQD2 and BED are derived from the linear-quadratic model of radiation response. BED (biologically effective dose) quantifies the biological effect of a fractionation schedule. EQD2 (equivalent dose in 2 Gy fractions) expresses that effect as the total dose that, if delivered in 2 Gy fractions, would produce the same biological effect — making schedules with different fraction sizes directly comparable.
Core formulas
BED = D × (1 + d / (α/β))
EQD2 = D × (d + α/β) / (2 + α/β)
Here D is total dose, d is dose per fraction, and α/β is the tissue-specific ratio from the linear-quadratic model — commonly taken as about 10 Gy for tumours and early-responding tissue and about 3 Gy for late-responding normal tissue.
The tool covers the calculations a clinician most often needs when comparing radiation schedules:
When a site is re-irradiated, some normal tissues may recover part of their radiation tolerance in the interval between courses. To account for this, EQD2 Calculator includes an optional tissue-recovery correction that discounts the biologically effective contribution of a prior course by a chosen percentage (0–50%), so the cumulative EQD2 reflects that assumed recovery.
recovery-adjusted EQD2 = EQD2 × (1 − recovery% / 100)
The recovery percentage is a modeling choice the clinician makes, not a value the tool supplies. EQD2 Calculator applies the percentage the clinician selects, per course, and shows the adjusted cumulative dose alongside the unadjusted result. As with every output, the adjusted value is an informational aid that must be independently verified.
Deciding how much of an initially delivered dose can be discounted — “forgiven” — when accounting for cumulative normal-tissue exposure is a central and largely unresolved problem in reirradiation. Standard practice converts both courses to a common biological framework (BED or EQD2) and sums them, but that summation implicitly assumes a recovery of occult radiation injury whose magnitude is poorly characterized in humans. The degree of recovery is known to depend on the organ irradiated, the dose delivered in the first course, and, to a lesser degree, the interval between courses — none of which simple addition captures.1
Most quantitative estimates of long-term normal-tissue recovery come from experimental models rather than prospective human data, leaving the clinician to weigh durable tumour control against potentially catastrophic late toxicity with incomplete information.1 Rigorous composite dose accumulation and deformable image registration can make these assumptions explicit, but cannot resolve the underlying biological uncertainty.2 The site-specific tolerance estimates and expert recommendations in the recent literature — spanning the head and neck,3 thorax,4 brain,5 liver,6 and abdomen and pelvis,7,8 together with modality-specific guidance for proton reirradiation9 — therefore remain provisional, resting largely on retrospective series and pooled expert opinion rather than prospective validation.
This is why EQD2 Calculator makes the recovery assumption an explicit, clinician-controlled input rather than a built-in default: the tool surfaces the assumption for inspection; it does not resolve the biology.
The calculator is designed for the settings where biologically equivalent dose comparisons matter most and have to be made quickly: retreatment planning, hypofractionated and stereotactic regimens, and palliative radiotherapy.
EQD2 Calculator runs entirely in the browser on a deterministic calculation engine. It does not use a large language model to perform the arithmetic. The engine carries a built-in self-test suite that checks the formulas on every load, including the identity check that EQD2 equals the total dose when the dose per fraction is 2 Gy. Every value displayed can be traced to an input and a published formula.
The tool is an informational and educational aid for licensed healthcare professionals and authorized researchers. It is not a medical device and is not a validated clinical treatment-planning system. All outputs must be independently verified before any clinical reliance.
EQD2 Calculator was developed in 2026 by Daegan Sit, MD, FRCPC, a radiation oncologist at BC Cancer – Vancouver Centre whose practice spans head and neck cancer, breast cancer, and thyroid malignancies.