2116 - Early Tumor Dynamics during Short-Course Fractionated SRT under Strict Imaging-to-Treatment Interval Control: Dosimetric Consequences and Implications for Adaptive Replanning

03:00pm - 04:00pm ET

Poster Hall - Exhibit Hall A

Screen: 4

Presenter(s)

Tsuyoshi Kudo, MS, RT - Department of Radiological Technology, Hiroshima City Hiroshima Citizens Hospital, Hiroshima, Japan, Hiroshima, Hiroshima

T. Kudo¹, K. Matsuura², I. Shintani¹, Y. Kitao¹, R. Ito¹, M. Nakamoto¹, A. Kawakubo¹, Y. Ando¹, Y. Adachi², and K. Nishihara¹; ¹Department of Radiological Technology, Hiroshima City Hiroshima Citizens Hospital, Hiroshima, Japan, ²Department of Radiation Oncology, Hiroshima City Hiroshima Citizens Hospital, Hiroshima, Japan

Purpose/Objective(s):

Short-course fractionated stereotactic radiotherapy (SRT) is increasingly used for brain metastases; however, early intratreatment tumor dynamics and their dosimetric impact remain poorly characterized. In this single-institution study conducted under strict imaging-to-treatment interval control (=36 hours), we investigated early tumor volume and positional changes during short-course SRT to evaluate the risk of planning target volume (PTV) coverage deterioration and the potential value of adaptive replanning.

Materials/Methods: Following institutional review board approval, we retrospectively analyzed 63 brain metastases in 53 consecutive patients treated with short-course fractionated SRT. To ensure temporal consistency, both planning and intratreatment MRI scans were obtained within 36 hours of treatment initiation or resumption. Gross tumor volume (GTV) changes were quantified, and three-dimensional centroid displacement was measured in lesions undergoing repeat imaging for adaptive replanning (n=46). Dosimetric impact was assessed by applying the initial treatment plan to the updated PTV and calculating changes in ?PTV V100%. Lesions were stratified by volumetric response as shrinkage, stable disease, or growth.

Results: Clinically meaningful tumor dynamics were observed within the short treatment interval. The median volumetric change was -8.8% (range, -73.2% to +73.8%). Three-dimensional centroid displacement correlated with initial tumor volume (Spearman ?=0.40, p=0.006) and was significantly greater in lesions with changes in peritumoral edema (p=0.009). Without adaptive replanning, PTV coverage deteriorated substantially in growing lesions. Median ?PTV V100% was +0.6% in the shrinkage group, -1.7% in the stable group, and -14.7% in the growth group (Kruskal–Wallis p<0.001). Adaptive replanning restored PTV coverage across all groups, maintaining a median ?PTV V100% of 0.0% (p<0.001 vs non-adaptive application).

Conclusion: Rapid and clinically significant changes in tumor volume and position can occur during short-course SRT, rendering static initial treatment plans vulnerable to PTV underdosage, particularly in growing lesions. These findings support the potential value of protocolized early intratreatment MRI monitoring with selective adaptive replanning to maintain optimal target coverage throughout the SRT course.