With the successes of stereotactic radiotherapy and systemic therapies, multiple courses of radiation therapy are increasingly frequent in the clinic. Longer survival time after initial radiotherapy increases the risk of second malignancy in the initial irradiation fields. Re-irradiation or treating disease in the proximity of previously irradiated tissues brings about the challenge of assessing potential for critical overlap, and thus to the importance of determining safe case-specific planning limits.
At our institution, we have developed a systematic approach to structure clearly the planning of re-irradiation cases. In this process, the physicist works closely with the dosimetrist and radiation oncologist to develop a re-treatment plan. The dose assessments are well-documented, and so are the potential sources of uncertainty in total dose estimation for critical organs.
Our process starts with the physician’s decision that there is a high likelihood of overlap among courses of treatments, and that the cumulative dose assessment is warranted. The physician lists the OARs of concern, corresponding a/b parameters and DVH constraints in EQD2 . Subsequently, the dosimetrist and physicist triage the cases. In some cases, a pre-planning procedure is initiated by the physicist to assess the feasibility of achieving specified dose constraints from both courses, based on what was delivered in the initial courses. The new plan is generated by the dosimetrist, and the physicist will evaluate the cumulative EQD2 of current and previous plans. For serial organs, when near maximum doses are evaluated, a simple manual calculation of ‘worst-case assumption’ may suffice to ensure that dose tolerances are not being violated. In other cases, a more complex analysis is initiated, which involves the creation of a composite dose plan, in EQD2 , based on rigid or deformable image registration. The potential sources of uncertainty invovled in the process of sum plan creation is discussed with the physian, and the decision-making process is clearly documented in the patient chart. This simplifies the peer-review process and may aid with any future re-irradiations.
The current workflow is a powerful tool for ensuring patient safety and documenting the decision-making process. This work provides a foundation for future clinical evaluative studies which investigate organ recovery time between irradiations and aim to establish clinical evidence for dose-effect of organs and endpoints.