IMPACT OF PROTON THERAPY ON PERIPHERAL BLOOD IMMUNE CELLS

Proton therapy relieves RIL and reduces the effective radiation dose to immune cells (EDIC)

RIL is an important biomarker for radiation‐induced immunotoxicity caused by exposure to radiation. RIL occurs when circulating lymphocytes pass through irradiated areas and is associated with poor survival outcomes in several solid tumors. 54 Notably, unlike typical radiation‐induced cell death via apoptosis, 55 human peripheral blood lymphocytes (HPBLs) exhibit a different mode of death during proton therapy. 56 As observed by Miszczyk J et al., photons tended to induce necrosis rather than apoptosis in HPBL, especially at relatively high doses (≥ 1.5 Gy). 56 Furthermore, photons could kill tumor cells through necrosis, which enhances immunogenicity and thereby improves abscopal effects. 56 , 57 , 58 Proton therapy can reduce the occurrence of RIL via several mechanisms. These include reducing the radiation dose, 15 minimizing the radiation field, 15 and creating a volume effect by sparing larger volumes of normal tissue during Bragg peak therapy. 59 For example, a cohort study of 143 patients with unresectable HCC showed that compared with those who did not receive proton therapy, those receiving proton therapy had a higher median absolute lymphocyte count (ALC) nadir (0.41 vs. 0.32 k/µL, P = 0.002). Furthermore, compared with patients treated with photon therapy, these patients had longer median overall survival (OS) (33.2 vs. 13.2 months, P = 0.002) and median disease‐free survival (DFS) (36.2 vs. 19.6 months, P = 0.018). These improvements may be attributed to the reduced radiation exposure at lymphocyte generation sites, such as the spleen. 60 In addition, by reducing the radiation dose at non‐immune organs, proton therapy can alleviate lymphocytopenia and ultimately improve survival outcomes as well, which has been proved in patients with stage III NSCLC. 61 , 62

The EDIC model assesses the average radiation dose absorbed by circulating lymphocytes. It performs better in reflecting the immunotoxicity caused by irradiation. The EDIC model was proposed by Jin et al. as the Jin model 63 and later modified into the Ladbury model by Ladbury et al. 64 Recently, several studies have reported that proton therapy is efficient in reducing EDIC, which can ultimately improve tumor prognosis. For locally advanced NSCLC, proton therapy resulted in a significantly lower median EDIC than photon therapy (4.0 Gy vs. 5.3 Gy, P<0.001). 65 Intensity‐modulated proton therapy (IMPT), an emerging proton therapy technique, exhibits excellent ability to minimize EDIC. For NSCLC, compared with intensity‐modulated radiotherapy, IMPT showed robustness in EDIC reduction (Jin model: 3.04 Gy vs. 4.99 Gy, P < 0.001; Ladbury model: 4.50 Gy vs. 7.60 Gy, P < 0.002), which led to an increase in the 2‐year OS rate (median 71% vs. 63%, P = 0.03). 66 Considering the complicated calculation of EDIC, which includes the mean heart dose (MHD), lung dose (MLD), liver dose (MID), integral total dose volume (ITD), mean body dose (MBD), and so on, 63 , 64 , 67 identifying the main driving factors is essential. For instance, a clinical study involving 10 patients with mediastinal Hodgkin lymphoma confirmed that the primary drivers of IMPT‐mediated EDIC reduction were integral dose reduction, accounting for 53.7%, and lung sparing, contributing 33.4%. 68 These findings, along with those of Nakamura et al., highlight the significant role of the lungs in lymphocytopenia. 62 , 68 Additionally, tumor location is an important factor influencing IMPT‐mediated EDIC reduction. For locally advanced BC, integral dose reduction was the primary driver of IMPT‐mediated EDIC reduction and was associated with liver‐sparing and lung‐sparing in patients with right‐sided and left‐sided BC, respectively. 67 In summary, personalizing the evaluation of the factors driving IMPT‐mediated EDIC reduction is indispensable.

Unique immune cell subpopulations and cell interactions

Cancer‐related inflammation plays a vital role in the initiation and progression of tumors. The key mechanisms include the intrinsic (oncogene activation) and extrinsic (inflammation or infection) pathways. Tumor‐related transcription factors such as nuclear factor‐κB (NF‐κB), signal transducer and activator of transcription 3 (STAT3), and hypoxia‐inducible factor 1α (HIF1α) play crucial roles. In addition, inflammatory mediators, such as cytokines, chemokines, and cyclooxygenase 2; myelomonocytic lineage cells (neutrophils and monocytes); and the cancer‐related inflammatory microenvironment are involved. 69 Proton irradiation induces distinct alterations in immune cell subpopulations. The ratio of circulating neutrophils to lymphocytes, normally ranging from 0.78 to 3.53. 70 , is positively associated with poor progression‐free survival (PFS) and OS in HNSCC (PFS: 39.2% vs. 75.8%, P<0.001; OS: 50.9% vs. 83.8%, P<0.001), especially in nasopharyngeal, hypopharyngeal, and laryngeal cancers. 71 Subsequently, Pham et al. conducted a detailed analysis of immune cell subpopulations. They classified leukocytes into T cells, B cells, NK cells, neutrophils, and monocytes and examined changes in each population separately. After 12 h of proton irradiation (4 fractions of 2.5 Gy), lymphocyte counts (CD4+ T cells, CD8+ T cells, B cells, and NK cells) did not decrease significantly, whereas neutrophil counts increased significantly (P = 0.03). In addition, they analyzed the effects of radiation parameters (irradiation type, irradiation volume, and dose rate) on six leukocyte subpopulations (including CD4+T cells, CD8+T cells, B cells, NK cells, neutrophils, and monocytes) using linear regression and tree‐based models, and observed that the impact of irradiation type on lymphocytes was stronger than that on myelocytes (CD8+ T cells were the most affected, followed by NK cells, B cells, and CD4+T cells). 72 Similarly, a mouse‐based study showed that early proton irradiation increased myeloid cell subpopulations. Its effect on decreasing the number of lymphocytes was relatively delayed and minor. 73 Furthermore, proton FLASH–radiation therapy (pFLASH) can up‐regulate the proportion of specific lymphocyte subsets at certain times. In a rat model of orthotopic glioma, Iturri et al. discovered that the proportion of B cells in the pFLASH group 24 hours after irradiation (257 ± 2 Gy/s) was significantly higher than that in the unirradiated group (P = 0.0054) and in the pFLASH group after 7 days of irradiation (P<0.0002). 74 Further research has found that, compared with photon therapy, proton therapy can also avoid the loss of T cell receptor diversity. 75 Proton irradiation protects lymphocyte subpopulations not only by preserving their numbers but also by enhancing interactions among specific subpopulations. For example, under the Bayesian network model, interactions between CD4+ T cells and CD8+ T cells, as well as between CD8+ T cells and B cells were strengthened after proton irradiation. 76 Because these studies mainly focused on mice rather than humans, the applicability of these results in clinical practice must be carefully evaluated.