Is There a Role for an 18F-fluorodeoxyglucose-derived Biological Boost in Squamous Cell Anal Cancer?

Area of Science:

• Oncology
• Radiotherapy
• Nuclear Medicine

Background:

• Anal cancer treatment often involves chemoradiotherapy (CRT).
• Identifying areas of high metabolic activity using 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) can inform treatment strategies.
• Understanding the stability of these metabolically active subvolumes during CRT is crucial for optimizing radiation delivery.

Purpose of the Study:

• To assess the spatial consistency of high FDG-avidity subvolumes in anal cancer during CRT.
• To determine if baseline FDG-PET identified subvolumes remain consistent throughout treatment.
• To explore the potential of these subvolumes as targets for dose escalation.

Main Methods:

• Retrospective analysis of FDG-PET scans from 21 anal cancer patients undergoing CRT.
• Identification of FDG-avid subvolumes (primary tumors and involved nodes >2 cm) using thresholds of 34%, 40%, and 50% of maximum standardized uptake value (SUVmax) on pre-treatment scans.
• Comparison of baseline subvolumes with those from scans on day 8 or 9 of CRT using deformable registration, overlap fraction, and vector distance calculations.
• Defined 'good' correlation as an overlap fraction >0.7.

Main Results:

• High spatial consistency was observed between baseline and mid-treatment FDG-avid subvolumes.
• Median overlap fractions ranged from 0.81 to 1.00, indicating good to excellent correlation.
• Twenty of 29 volumes (69.0%) met the >0.7 overlap fraction criterion between the 50% baseline subvolume and 80% mid-treatment subvolume.
• Median vector distances were small (0.71-0.74 mm), further supporting spatial consistency.

Conclusions:

• FDG-avid subvolumes in anal cancer are spatially consistent during CRT.
• The 50% SUVmax subvolume identified on pre-treatment FDG-PET scans is a reliable indicator of metabolically active tumor regions throughout treatment.
• This consistency suggests that baseline FDG-avid subvolumes are suitable targets for dose escalation strategies in anal cancer therapy.