The international guidelines for nodal CTV delineation in the PORT setting have not changed significantly in the last decade3,13, regardless of the development of treatment techniques. As diminishing radiation-related toxicities have attracted more attention recently, some doctors have attempted to reduce the intensity of radiotherapy in clinical practice. One possible method is delivering a lower dose to the nodal CTV. By comparing treatment outcomes from the two centers, we found that dose de-escalation to HRL from 60 Gy to 50 Gy significantly increased the risk of regional recurrence. In contrast, a dose of 50 Gy for LRL did not compromise RC of uninvolved nodal levels. Then, dose reduction to the HRL was deprecated, and the LRL was worth considering.
Whether decreasing the dose to the HRL is acceptable has not been validated. The dose required for sterilizing occult disease at the HRL is defined by the volume of microscopic disease and its inherent radiosensitivity14. Considering the lower radiosensitivity of neck dissection areas and occult tumor load, the empirical dose used for HRL is 56–60 Gy. However, this prescription was extrapolated from conventional 2D or three-dimensional (3D) radiotherapy experience15. As IMRT enables target volumes to receive higher doses, it may be unsuitable. Meanwhile, regional failures are rare in local-regional recurrence6,16,17. Moreover, owing to the microscopic spread of HRL, a lower dose may be required than that for the primary tumor bed. In center 2, the radiology department adopted a prescription with a lower dose to the HRL, and most patients were considered to have a low risk of regional recurrence in the HRL, i.e., those with stage T1–2N1 tumors and close margin or stage T3–4N0 tumors. Therefore, we analyzed the regional recurrence of dose reduction from 60 Gy to 50 Gy in the HRL using real-world data to investigate whether it was feasible.
It is known that the patterns of regional recurrence could be used to evaluate the appropriateness of PORT: in-field recurrence indicates that nodal target volumes require a higher prophylactic dose, while out-field recurrence is due to the inaccuracy of target delineation18. In our study, all regional recurrence events occurred in the field, indicating that CTV delineation is reasonable. However, reduction in the radiation dose delivered to the HRL led to a significant difference in the rate of HRC. A randomized phase III study showed that patients receiving a dose of ≤ 54 Gy had a higher locoregional failure rate than those receiving > 57.6 Gy on first interim analysis19,20. However, the rate of local recurrence was not observed, which may constitute the main cause of locoregional failure. Heterogeneity was also observed among the patients in our study. Nearly half of the patients were treated with chemoradiation in center 1, although the rate of ENE was equal to that in center 2. As more patients were in the late T stage in center 1, more positive therapeutic strategies may have been preferred in clinical practice. Concurrent chemotherapy could help in local-regional control and even increase the OS rate in head and neck squamous cell carcinoma cases, which may contribute to better HRC in center 1. Therefore, less intensive treatment regimens for HRLs and suitable methods for selecting appropriate patient populations require further study.
Moreover, reduction in the dose delivered to LRLs has been recommended in many clinical practices. Our data revealed good RC of LRL with a dose of 50 Gy, including in patients with stage T3–4N0 tumors. As the extent of neck dissection varied, PORT for LRL with and without neck dissection was discussed.
For non-operated LRL (cLRL), 50–54 Gy is typically administered21. However, the higher sensitivity of improved diagnostic imaging provides an opportunity for treatment deintensification. Better image quality of CT and MRI helps in the detection of smaller nodal metastasis22,23. When positron emission tomography with Fluor-18-fluorodeoxyglucose (FDG-PET) was introduced, the sensitivity and accuracy of detecting tumor deposits improved. It provides unprecedented accuracy for the staging of neck tumors with a detection threshold between 5 and 10 mm24,25. More microscopic diseases can be diagnosed by combining information from this advanced imaging modality, which means that LRLs contain fewer and smaller subclinical tumor deposits. As a lower dose is required to sterilize a lower number of tumor cells, the maximum size of occult metastasis affects the dose–response relationship and RC. A linear relationship between dose and tumor control was observed when the maximum diameter of occult metastasis was within 3–10 mm. This model suggests that improving every 1 mm of the detection threshold of diagnostic imaging can theoretically reduce the elective dose by 1–2 Gy26. Fletcher recommended a postoperative dose of 45–50 Gy in 2-Gy fractions, which achieved high rates of control of surgically undisturbed lymph node metastases27. Even when using a dose as low as 36 Gy, the tumor control probability of a subclinical disease of 5 mm was 85%28. Some investigators have also found that dose de-escalation in LRL to 40 Gy is feasible, resulting in less toxicity without significant differences in disease control or survival29,30,31. In our study, only one patient experienced recurrence of cLRL, even after receiving 60 Gy. These results imply that doses of 50 Gy or even lower could be delivered to cLRL.
The delivered radiation dose can be a contentious issue for dissected pathological LRL (pLRL). Including pLRL as a prophylactic dose (50 Gy in 25 fractions or 54 Gy in 30 fractions) has been widely implemented in the UK. Some pLRL may even be included in an intermediate dose of 56–57 Gy, owing to the high possibility of occult lymph node metastases3,32. Our data revealed that 50 Gy was sufficient for pLRL. Furthermore, some researchers have been working on omitting PORT from pLRL in head and neck squamous cell carcinoma cases, which achieved reasonable 5-year rates of RC (93%) and unirradiated neck control (97%)33. Subsequently, a prospective DIREKHT trial attempted to omit contralateral pLRL irradiation in a predefined low-risk patient population with head and neck cancer34. However, regional failures are rare. A retrospective series of patients with oral cavity cancer experienced no regional recurrence after excluding pLRL from postoperative CTV35. However, established studies lack recurrence data for SC in such situations. Therefore, the optimal dose for pLRL requires further study.
Our study also noted that dose-reduction treatment maintained the overall rates of OS, RC, PFS, and DMFS, although it decreased the HRC rate. This may be owing to the higher number of patients with stage II disease in center 2 (27.4% vs. 1.6%), which had patients with better survival outcomes. Moreover, de-escalation of doses to nodal levels could reduce radiation-related toxicities. Lower doses to the swallowing structures decrease the prevalence of dysphagia in patients, which improves patients’ quality of life (QoL), as swallowing dysfunction has a stronger negative impact on QoL than xerostomia36,37.
Many studies have focused on definitive radiation, but only a small proportion of them have focused on PORT. To the best of our knowledge, no studies on reducing the HRL dose for PORT have been published. However, in clinical applications, a low risk of regional failure would make radiologists and patients choose a less intense radiotherapy strategy to alleviate toxicities. Our study aimed to determine the possibility of dose de-escalation delivered to the HRL during PORT. However, this study has some limitations. The first is the accuracy of the recurrence site when reconstructed using the original planning scan. Furthermore, after surgery, the nodal levels’ structure may be different from that before surgery. Second, as a retrospective design based on real-world data and characterized by different periods from two centers, selection biases and imbalances existed in the inherent variables. No treatment-related adverse events were observed. The final limitations were the relatively small sample size and implicit heterogeneity among patients.