Dosimetric Evaluation of CBCT-Guided Setup errors and Adaptive Radiotherapy Replanning in Proton Therapy for Lung Cancer

Lung cancer remains the most common cause of cancer-related death worldwide, making the precision of its treatment a matter of life and death. For decades, doctors have used beams of energy to destroy tumors, but a newer form of radiation called proton therapy has emerged as a powerful tool. Unlike traditional radiation, which passes through the body and can damage healthy tissue on its way out, proton beams are designed to stop exactly where the tumor is, depositing their energy with surgical precision. This "Bragg peak" effect means that the tissue behind the tumor receives almost no radiation at all. However, this precision comes with a catch: because the beam stops so abruptly, even a tiny shift in the patient's position or a slight change in the tumor's size can cause the dose to miss the target or, worse, hit a vital organ like the spinal cord or heart. To manage this, hospitals use cone-beam computed tomography, a type of 3D X-ray taken right before treatment, to check the patient's position. But a critical question remains: if the tumor shrinks significantly during the weeks of treatment, does the original plan still work, or must the doctors redraw the map?

A team of researchers at Anhui Provincial Hospital in China set out to answer these questions by looking back at the treatment records of 46 patients with lung cancer who received proton therapy between April 2024 and January 2026. They wanted to understand two specific things: how much the patients' daily positioning errors affected the radiation dose, and whether changing the treatment plan mid-course to match a shrinking tumor could improve the outcome. The study was divided into two parts. First, the researchers analyzed the daily movements of 29 patients who followed their original treatment plans without any changes. They used the 3D X-rays taken before every session to measure how far the patients shifted from their intended position in three directions: left-right, up-down, and front-back. They then used computer simulations to see what would happen to the radiation dose if these small shifts occurred every day.

The results of the simulation revealed that while most daily shifts were small, they were not harmless. The researchers found that the average movement in the up-down direction was the most variable, with some shifts reaching nearly 3 centimeters. When they applied these average shifts to the treatment plans, the computer showed that the radiation dose hitting the tumor dropped significantly. In simple terms, the tumor received less of the intended "kill shot" than the doctors had planned. More concerning was the effect on the spinal cord, a critical structure running down the back. For patients whose tumors were located very close to the spinal cord, even a small shift caused the spinal cord to receive a higher dose of radiation than intended. This happened because the beam, designed to stop at the tumor, ended up stopping inside the spinal cord instead. The study confirmed that for these specific cases, the margin for error is incredibly thin, and the precision of the daily setup is vital to prevent unintended damage to the nervous system.

The second part of the study focused on 17 patients whose tumors shrank noticeably during treatment. In these cases, the doctors did not stick to the original plan. Instead, when the 3D X-rays showed the tumor had reduced in size by more than 20 percent, they paused, took a new 3D scan, and created a new treatment plan tailored to the smaller tumor. The researchers then compared the total radiation dose these patients actually received against what they would have received if the doctors had stuck to the original, unchanging plan. The findings were clear: adapting the plan to the changing anatomy made a significant difference. By updating the plan, the doctors were able to lower the amount of radiation hitting the healthy lung tissue. Specifically, the volume of the lung receiving a low dose of radiation dropped, and the volume receiving a higher, more dangerous dose also decreased. The average dose to the entire lung was reduced, meaning the healthy tissue was spared more radiation than it would have been otherwise.

Interestingly, while the benefits to the lungs were statistically significant, the changes in dose to the heart and spinal cord were less dramatic in this specific group, though the trend was still toward lower exposure. The researchers noted that the tumors in these patients shrank by an average of 35 cubic centimeters, a substantial reduction that would have left a large gap between the tumor and the edge of the radiation beam if the original plan had been used. By closing that gap with a new plan, the doctors ensured the beam stopped exactly where it needed to, protecting the healthy tissue behind it. The study suggests that for lung cancer patients undergoing proton therapy, relying solely on the initial plan is risky. The daily positioning errors can compromise the dose to the tumor and endanger nearby organs, while the natural shrinking of the tumor during treatment creates a need for a fresh map.

The researchers concluded that the combination of daily 3D imaging checks and the willingness to redraw the treatment plan when the tumor changes offers the best path forward. The daily checks catch the small, daily wobbles in position, while the adaptive replanning catches the big, structural changes in the body. This approach allows the proton beam to remain a precise scalpel rather than a blunt instrument. While the study was limited by its relatively small number of patients and the fact that it looked back at past records rather than testing a new protocol in real-time, the data provides a strong argument for a more flexible approach to treatment. It shows that in the high-stakes environment of lung cancer therapy, the ability to see the patient's changing anatomy and adjust the plan accordingly is not just a technical upgrade, but a necessary step to ensure the radiation hits the cancer and leaves the rest of the body alone.