  Imbalanced token distributions naturally exist in text documents, leading
neural language models to overfit on frequent tokens. The token imbalance may
dampen the robustness of radiology report generators, as complex medical terms
appear less frequently but reflect more medical information. In this study, we
demonstrate how current state-of-the-art models fail to generate infrequent
tokens on two standard benchmark datasets (IU X-RAY and MIMIC-CXR) of radiology
report generation. % However, no prior study has proposed methods to adapt
infrequent tokens for text generators feeding with medical images. To solve the
challenge, we propose the \textbf{T}oken \textbf{Im}balance Adapt\textbf{er}
(\textit{TIMER}), aiming to improve generation robustness on infrequent tokens.
The model automatically leverages token imbalance by an unlikelihood loss and
dynamically optimizes generation processes to augment infrequent tokens. We
compare our approach with multiple state-of-the-art methods on the two
benchmarks. Experiments demonstrate the effectiveness of our approach in
enhancing model robustness overall and infrequent tokens. Our ablation analysis
shows that our reinforcement learning method has a major effect in adapting
token imbalance for radiology report generation.
