Normal organ dose reduction by fluence map prediction plan adjustment

<i>Objective.</i>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 (FM) prediction-based method (FM_PD) for rapid plan adaptation. It enables online adaptive RT (OART) to better account for structural changes throughout treatment and assess its potential for improved normal tissue sparing.<i>Approach.</i>The planning target volumes (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 intensity-modulated RT plans for hypopharyngeal carcinoma. Nine re-planning scenarios (time intervals: 32-47 days) were used for testing to simulate an OART setting. Predicted 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.<i>Main results.</i>FM_PD significantly reduced the dose to normal tissues while maintaining tumor coverage. The D₂of the PTV decreased by 1.13 ± 5.85%, moreover, substantial dose decreases were observed in critical structures: D_maxto the lens, optic nerves, and brainstem decreased by 18.67 ± 19.04%, 19.17 ± 19.57%, and 14.54%, respectively. The total body D_meandecreased by 25.65 ± 15.44%. In cases where the PTV was adjacent to lung tissue, the D_meandropped significantly by 46.40 ± 36.89%.<i>Significance.</i>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.