3154 - EQD2 Constraints for Organs at Risk in Re-Irradiation Planning for Thoracic and Head and Neck Cancers

S. Dabaghmanesh, E. M. Gore, E. S. Paulson, and A. Tai; Department of Radiation Oncology, Medical College of Wisconsin, Milwaukee, WI

Purpose/Objective(s): The increasing demand for re-irradiation (re-RT) in radiation oncology has posed a significant challenge due to the heightened risk of severe toxicity in organs at risk (OARs) associated with cumulative doses. While the need for re-RT is evident, the lack of unified guideline and agreement on cumulative dose constraints acceptable for OARs complicates its safe practice. In this study we conducted a literature review to provide a list of composite dose constraints for thoracic and head and neck (HnN) re-RT. Materials/

Methods: A comprehensive literature search covered 30 studies on thoracic and HnN re-RT encompassing single retrospective institutional and review/consensus studies published from 2008 to 2023. These studies were from radiation treatments of various fractionation schemes such as conventional, hypofractionation, and stereotactic body radiotherapy (SBRT). Cumulative equivalent doses in 2 Gy fractions (EQD2) were collected associated with grade 3, some limited grade 4, and no grade 5 late toxicities as categorized by CTCAE v.3 or v.4, RTOG, and EORTC. The collected EQD2 constraints were reviewed based on number of patients, toxicity rates, treatment interval, follow up time, and single institutional study vs expert consensus. The number of studies including single institutional/expert consensus and number of patients in single institutional studies are shown in the Table.
Results: A set of cumulative EQD2 constraints was generated using a/ß = 3 Gy for re-RT planning in our department, detailed in the above Table. These recommended EQD2 constraints have been utilized in our clinic to create composite EQD2 dose summaries for re-RT planning. We observed a consistent trend in SBRT treatments, where the extracted EQD2 constraints tend to be significantly higher than those in conventional fractionation. This is particularly notable in organs with a/ß ratios much larger than 3 Gy, such as the carotid and bronchial tree.
Conclusion: This study consolidates composite EQD2 constraints for OARs in thoracic and HnN re-RT, derived from clinical outcomes and consensus opinions, thoroughly reviewed by our clinic’s radiation oncologists. The suggested dose limits are guidelines constrained by limitations of published data including limited information in how accumulated dose was calculated, limited patient and tumor specific information, and retrospective nature of most trials. A prospective clinical trial is underway to improve processes for dose accumulation including EQD2 conversions, methods for registering prior and current plans, and collection of patient and tumor specific information and toxicity.

Abstract 3154 – Table 1

OAR	Esophagus	Bronchial tree/ trachea	Brachia plexus	Aorta	Carotid	Brainstem	Optic nerve/ Chiasm	Mandible	Temporal lobe	Chest wall/Skin
Number of studies	10	11	5	5	6	5	4	4	2	2
Number of pts	150	110	81	77	339	302	175	264	23	38
EQD2 constraints (a/ß= 3 Gy) (Gy)	100 (0.03cc)	130 (0.03 cc)	83-95 (0.03cc)	120 (1cc)	125 (1cc)	100 (0.03cc)	75 (0.03cc)	120 (0.03cc)	105 (0.03cc)	105 (0.03cc)