BIOLOGICALLY EFFECTIVE DOSE DISTRIBUTION BASED ON THE LINEAR-QUADRATIC MODEL AND ITS CLINICAL RELEVANCE

Purpose: Radiotherapy plans based on physical dose distributions do no t necessarily entirely reflect the biological effects under various fr actionation schemes. Over the past decade, the linear-quadratic (LQ) m odel has emerged as a convenient tool to quantify biological effects f or radiotherapy. In this work, we set out to construct a mechanism to display biologically oriented dose distribution based on the LQ model. Methods and Materials: A computer program that converts a physical do se distribution calculated by a commercially available treatment plann ing system to a biologically effective dose (BED) distribution has bee n developed and verified against theoretical calculations. This softwa re accepts a user's input of biological parameters for each structure of interest (linear and quadratic dose-response and repopulation kinet ic parameters), as well as treatment scheme factors (number of fractio ns, fractional dose, and treatment time). It then presents a two-dimen sional BED display in conjunction with anatomical structures. Furtherm ore, to facilitate clinicians' intuitive comparison with conventional fractionation regimen, a conversion of BED to normalized isoeffective dose (NID) is also allowed. Results: Two sample cases serve to illustr ate the application of our tool in clinical practice. (a) For an ortho gonal wedged pair of x-ray beams treating a maxillary sinus tumor, the biological effect at the ipsilateral mandible can be quantified, thus illustrates the so-called ''double-trouble'' effects very well. (b) F or a typical four-field, evenly weighted prostate treatment using 10 M V x-rays, physical dosimetry predicts a comparable dose at the femoral necks between an alternate two-fields/day and four-fields/day setups. However, our BED display reveals an approximate 21% higher BED for th e two-fields/day scheme. This excessive dose to the femoral necks can be eliminated if the treatment is delivered with a 3:2 (anterio-poster ior/posterio-anterior (AP/PA):bilaterally opposed (BLO)) dose weightin g. With Co-60 beams, the increase of BED with alternate two-fields/day , 1:1 setup was even more pronounced (26%). Conclusion: We have demons trated the feasibility of constructing a biologically oriented dose di stribution for clinical practice of radiotherapy. The discordance betw een physical dose distributions and the biological counterparts based on the given treatment schemes was quantified. The computerized displa y of BED at nonprescription points greatly enhanced the versatility of this tool. Although the routine use of this implementation in clinica l radiotherapy should be cautiously done, depending largely on the acc uracy of the published biological parameters, it may, nevertheless, he lp the clinicians derive an optimal treatment plan with a particular f ractionation scheme or use it as a quantitative tool for outcome analy sis in clinical research.