The Clinical Application of the "Timmerman Tables" and the LQ Approach to Isoeffectiveness In the Re-Treatment Setting
Abstract
Purpose
Organ at risk dose limits as a function of varying fractionation scheme are conveniently given in the well-known “Timmerman tables”. Application of these limits in the re-treatment environment using the LQ approach (EQD2) can lead to overestimation of available dose for some OARs. In this work we describe a method to use the given limits and the EQD2 method in an isoeffective manner.
Methods
The physical dose limits for each OAR were converted to EQD2 and graphed as a function of fraction number. An LQ uncertainty boundary was delimited for physical dose above approximately 6Gy/fx. The average EQD2 for fractionation not exceeding the uncertainty boundary was delimited. If individual EQD2 values correspond to this average, the standard EQD2 method can be used directly to determine available dose. If individual EQD2 values appreciably exceed the average value do not use directly. Convert previous OAR dose to EQD2 applying repair if applicable and then convert to physical dose for the current fractionation number. Subtract this physical dose from the current fractionation physical dose limit to determine available physical dose.
Results
The 5-fraction rectum physical max dose limit from Timmerman is 55Gy or 154Gy EQD2 assuming an α/β of 3Gy for late effects. The average EQD2 above the LQ uncertainty boundary is only 105Gy. Applying 0-50% repair to the average EQD2 (eg. previous dose) and the above method results in EQD2 values of 116-94Gy. An average EQD2 limit of 81Gy is found for HyTEC (4-5 fx).
Conclusion
When moving from high to low fractionation schemes, use the methods described to avoid overestimation of available dose. Moving from low to high fractionation schemes when the initial course is described by HyTEC limits, use EQD2 method. If the initial course is described by Timmerman limits, consider alternative re-treatment methods such as PLDR.