Normal organ dose reduction by fluence map prediction plan adjustment

Abstract Objective. Anatomical changes in target volumes and surrounding organs-at-risk (OARs) commonly occur during radiation therapy (RT). Relying solely on the initial treatment plan can lead to suboptimal dose delivery and increased risk to healthy tissues. This study investigates a fluence map prediction-based method (FM_PD) for rapid plan adaptation. It enables online adaptive radiation therapy (OART) to better account for structural changes throughout treatment and assess its potential for improved normal tissue sparing. Approach. The PTVs and corresponding dose distribution were converted into 2D projection matrices during training. A 2D Dense-U-Net model incorporating a PTV-specific loss function (PTV_loss) was trained on a dataset of 93 IMRT plans for hypopharyngeal carcinoma. Nine re-planning scenarios (time intervals: 32 - 47 days) were used for testing to simulate an OART setting. Predicted fluence maps (FMs) were applied to the daily CTs to calculate updated dose distributions. These doses were compared to the original (non-adapted) plans to evaluate the dosimetric impact on OARs. Main results. FM_PD significantly reduced the dose to normal tissues while maintaining tumor coverage. The D2 of the PTV decreased by 1.13±5.85%, moreover, However, substantial dose decreases were observed in critical structures: Dmax to the lens, optic nerves, and brainstem decreased by 18.67±19.04%, 19.17±19.57%, and 14.54%, respectively. The total body Dmean decreased by 25.65±15.44%. In cases where the PTV was adjacent to lung tissue, the Dmean dropped significantly by 46.40±36.89%. Significance. FM_PD offers a rapid and effective approach for adapting RT plans in response to anatomical changes, significantly reducing doses to healthy tissues. Compared to maintaining the initial plan, FM_PD is a recommended strategy for cross-fraction adaptation scenarios in clinical OART practice.