The core advantage of proton therapy - the precise lethality brought by the "Bragg Peak" - also poses its main challenge: Range Uncertainty. The final depth (range) of the proton beam traveling inside the body may deviate from the planned calculated value due to slight changes in tissue density and composition, posing a risk of accidental injury to normal tissues or insufficient coverage of the target area. CT based Image Guided Radiotherapy (CT-IGRT) is a key technology to overcome this challenge, mainly playing a role through the following three aspects:

1. Accurate "map" navigation: direct conversion from electron density to proton blocking ability

The fundamental reason for range uncertainty is that there is an error of approximately 3-5% when converting the initial CT scan based "electron density" in the treatment planning system (TPS) into "Stopping Power Ratio (SPR)". Advanced CT image-guided technology, such as Dual Energy CT (DECT), can more accurately analyze the composition of tissues (such as fat, water, calcium) by obtaining CT data under different energy spectra, thereby significantly reducing the calculation error of SPR to about 1%. This is equivalent to providing a more accurate 'in vivo map' for proton beams, reducing systematic errors in range prediction from the source.

2. Daily updates on "road conditions": correcting anatomical changes between graded treatments

The patient's anatomical structure is not fixed. As treatment progresses, weight gain or loss, tumor shrinkage, edema of normal tissues, or changes in the filling status of hollow organs (such as the stomach and bladder) can significantly alter the tissue density and thickness along the proton pathway. Cone beam CT (CBCT) or indoor CT scan before treatment can quickly register and compare the actual anatomy of the day with the planned CT. Once these changes are discovered that may lead to range deviation, physicists can adjust the treatment plan in a timely manner (such as recalculating or adjusting the range compensator) to ensure that the proton beam irradiated on that day can accurately reach the predetermined depth and cover the target area.

3. Adaptive "route" re planning: responding to significant changes in treatment

For cases with particularly drastic changes, such as rapid weight loss in head and neck cancer patients or rapid tumor shrinkage in children, only minor adjustments may not be sufficient. Based on daily CT image-guided acquisition of continuous anatomical data, doctors can initiate the Adaptive Radiotherapy (ART) process, which involves reacquiring high-quality planning grade CT scans and developing a new treatment plan that matches the current anatomy. This is equivalent to "re planning the route" for patients, fundamentally eliminating the range uncertainty accumulated due to long-term and significant anatomical changes.

Conclusion:

In summary, CT image guidance eliminates the range uncertainty of proton therapy layer by layer through three mechanisms: improving the initial SPR calculation accuracy, correcting anatomical changes between stages, and initiating adaptive replanning with greater force, which is the technical guarantee for its precise and safe application.