Onboard cone-beam CT-based replan evaluation for head and neck proton therapy

Area of Science:

• Medical Physics
• Radiation Oncology
• Image-Guided Therapy

Background:

• Quality assurance computed tomography (QACT) is standard for evaluating replan needs in proton therapy.
• QACT may inaccurately suggest replanning due to setup errors, not anatomical changes.
• Deforming the treatment planning CT (TPCT) to pretreatment CBCT (TPCTdir) may mitigate this issue.

Purpose of the Study:

• To investigate the performance of replan evaluation using deformed TPCT (TPCTdir) in proton head and neck (H&N) therapy.
• To compare the specificity of TPCTdir-based replan evaluation against traditional QACT.

Main Methods:

• Twenty-eight H&N datasets with pretreatment CBCT and QACT were analyzed.
• Dosimetric parameters (ΔD95, ΔDmean, ΔD1) for clinical target volumes (CTVs) were calculated on TPCTdir, deformed QACT (QACTdir), and QACT.
• Receiver operating characteristic (ROC) curves were generated to evaluate replan triggers based on ΔD95, using QACTdir as the reference.

Main Results:

• Significant differences in body contour changes were observed between no-replan (95.8 ± 83.8 cc) and replan (305.0 ± 235.0 cc) groups (p < 0.01).
• Dosimetric parameter changes were comparable across evaluations (TPCTdir, QACTdir, QACT).
• A threshold of CTV ΔD95 > 2.6% in TPCTdir yielded a specificity of 98%, outperforming QACT's 94% specificity.

Conclusions:

• Replan evaluation based on TPCTdir demonstrates superior specificity compared to QACT.
• The proposed TPCTdir method effectively distinguishes between setup errors and true anatomical changes requiring replanning.