Practical Clinical Implementation of the Special Physics Consultation Process in the Re-irradiation Environment

Abstract

Purpose The purpose of this work is to present a practical, structured process allowing for consistent, safe radiation therapy delivery in the re-treatment environment. Methods and materials A process for reirradiation is described with documentation in the form of a special physics consultation. Data acquisition associated with previous treatment is described from highest to lowest quality. Methods are presented for conversion to equieffective dose, as well as our departmental assumptions for tissue repair. The generation of organ-at-risk available physical dose for use in treatment planning is discussed. Results using our methods are compared with published values after conversion to biologically effective dose. Utilization of pulsed-low-dose-rate delivery is described, and data for reirradiation using these methods over the previous 5 years are presented. Results Between 2015 and 2019, the number of patients in our department requiring equieffective dose calculation has doubled. We have developed guidelines for estimation of sublethal damage repair as a function of time between treatment courses ranging from 0% for <6 months to 50% for >1 year. These guidelines were developed based on available spinal cord data because we found that 84% of organs at risk involved nerve-like tissues. The average percent repair used increased from 32% to 37% over this time period. When comparing the results obtained using our methods with published values, 99% of patients had a cumulative biologically effective dose below the limits established for acceptable myelopathy rates. Pulsed-low-dose-rate use over this period tripled with an average prescription dose of 49 Gy. Conclusions The methods described result in safe, effective treatment in the reirradiation setting. Further correlation with patient outcomes and side effects is warranted.

Automated planning of stereotactic spine re-irradiation using cumulative dose limits

“…While re-irradiation has become more prevalent in recent years, only a few studies have investigated dedicated approaches for optimizing re-irradiation plans by accounting for the spatial dose distribution of the previous treatment [10] , [11] , [12] , [13] , [14] , [15] . Existing solutions typically conservatively rely on rigid registration or the maximum OAR dose, are not fully streamlined for clinical application, or are only implemented in research software.…”

Brain Re-Irradiation Robustly Accounting for Previously Delivered Dose

“…In particular, there are almost no planning software tools for use specifically in the reRT setting [8]. Instead, a 'post hoc' approach to the evaluation of cumulative doses to organs at risk (OARs) is often used, based on maximum OAR doses in the two (or more) treatment plans: a typical approach might include extracting the point maximum OAR dose from the original irradiation (origRT) and the reRT treatment plans, manually converting the two values to an equivalent dose in 2 Gy fractions (EQD2) or biological equivalent dose (BED), and summing the two [9]. While this provides a simplified radiobiological representation of the combined dose, allowing the clinician to assess whether cumulative doses are acceptable, it disregards any spatial information (i.e., the location of the maximum dose in the origRT is unlikely to coincide with that in the reRT), nor does this approach work for any type of volumetric dose constraints, and it does not directly guide plan optimisation.…”