A method to convert biologically equivalent DVH for different fractionations in radiotherapy with a build-in script

Advancements in radiotherapy have led to diverse fractionation schemes for cancer treatment. Dosimetric assessment for moderately hypo-fractionated radiotherapy (mHFRT) plans is usually challenging due to the absence of explicit dose thresholds for these fractionations and the limitations of standard equivalent dose in 2 Gy per fraction (EQD2) conversion approach in dose volume histogram (DVH) conversion.

This study aims to develop a method for converting physical DVHs of mHFRT into equivalent DVHs for conventional-fractionated radiotherapy (CFRT).

This study presents an improved method, named EQDd (equivalent dose in d Gy fractions), for converting physical DVHs of mHFRT to equivalent DVHs of CFRT. Unlike the traditional EQD2 conversion method that uses a constant dose of 2 Gy per fraction, the EQDd method converts each mHFRT dose point with variable per-fraction doses. The conversion formula is expressed as EQDd_i=D_i*(1+d_i/(α/β))/(1+2*d_i/(d_P*α/β))(i=1,2,…m), where d_p is the prescription dose per fraction. Validation and illustration were performed by comparing the EQDd and EQD2 conversions for DVHs of an exemplary thoracic plan with 3 hypothetical prescriptions: (1) 2 Gy x 30; (2) 3 Gy x 20; (3) 4 Gy x 15, respectively. Additionally, a plugin for the Eclipse treatment planning system based on Eclipse Scripting Application Programming Interface (ESAPI) was developed to automate these conversions.

For the 2 Gy x 30 prescription, the DVHs after EQDd conversion keep the same as the original DVHs, while EQD2 underestimates the volumetric doses of organs at risk (OARs). For the mHFRT plans (3 Gy x 20 and 4 Gy x 15), the DVHs after EQDd conversion show higher dose/volume than the original DVH for all structures, reflecting the increased biological effect for larger fraction sizes. While EQD2 appears to be only suitable for evaluating hotspots in the target and nearby serial organs, its utility for assessing full DVH patterns is limited. A plugin ``DVH Convert'' based on ESAPI has also been developed, which can convert DVH of an mHFRT to CFRT by EQDd method within just a few seconds.

This study demonstrated the limitations of the EQD2 conversion method for OARs. The improved EQDd method provides more biologically reasonable conversions, which may have important applications for clinical dose evaluation.