Neutron Radiation Therapy for Treatment of Refractory Merkel Cell Carcinoma

Fast and Furious: Fast Neutron Therapy in Cancer Treatment

Fast neutron therapy has been used for decades. In conjunction with recent advances in photonic techniques, fast neutrons are no longer of much oncologic interest, which is not unequivocally positive, given their undoubted therapeutic value. This mini-review recalls the history of medical research on fast neutrons, considers their physical and radiobiological properties alongside their benefits for cancer treatment, and discusses their place in modern radiation oncology.

Comparisons of 3-Dimensional Conformal and Intensity-Modulated Neutron Therapy for Head and Neck Cancers

PURPOSE

Neutron therapy is a high linear energy transfer modality that is useful for the treatment of radioresistant head and neck (H&N) cancers. It has been limited to 3-dimensioanal conformal-based fast-neutron therapy (3DCNT), but recent technical advances have enabled the clinical implementation of intensity-modulated neutron therapy (IMNT). This study evaluated the comparative dosimetry of IMNT and 3DCNT plans for the treatment of H&N cancers.

MATERIALS AND METHODS

Seven H&N IMNT plans were retrospectively created for patients previously treated with 3DCNT at the University of Washington (Seattle). A custom RayStation model with neutron-specific scattering kernels was used for inverse planning. Organ-at-risk (OAR) objectives from the original 3DCNT plan were initially used and were then systematically reduced to investigate the feasibility of improving a therapeutic ratio, defined as the ratio of the mean tumor to OAR dose. The IMNT and 3DCNT plan quality was evaluated using the therapeutic ratio, isodose contours, and dose volume histograms.

RESULTS

When compared with the 3DCNT plans, IMNT reduces the OAR dose for the equivalent tumor coverage. Moreover, IMNT is most advantageous for OARs in close spatial proximity to the target. For the 7 patients with H&N cancers examined, the therapeutic ratio for IMNT increased by an average of 56% when compared with the 3DCNT. The maximum OAR dose was reduced by an average of 20.5% and 20.7% for the spinal cord and temporal lobe, respectively. The mean dose to the larynx decreased by an average of 80%.

CONCLUSION

The IMNT significantly decreases the OAR doses compared with 3DCNT and provides comparable tumor coverage. Improvements in the therapeutic ratio with IMNT are especially significant for dose-limiting OARs near tumor targets. Moreover, IMNT provides superior sparing of healthy tissues and creates significant new opportunities to improve the care of patients with H&N cancers treated with neutron therapy.

A portable fast neutron irradiation system for tumor therapy

A portable neutron tube was introduced as a small-sized (weight≤14.4 kg, power consumption ≤50W and cost≤ $100,000) neutron accelerator and applied for irradiation therapy on cancer. The effect of growth-inhibiting in vitro by neutrons irradiation on HeLa cells (human cervical cancer cells) was evaluated by colony formation assays, and cell apoptosis was evaluated by Flow Cytometry. A polyethylene protection device as the neutron moderator was designed and connected to the neutron tube to shield normal tissue and organs of the test animals from scatter radiation. Hematology and blood biochemistry were investigated to evaluate the protective effect of polyethylene. U14 (mice cervical cancer cell) tumor-bearing mice were further investigated to determine the tumor suppression effect of neutron irradiation. We found that cells showed a dose-dependent relationship after fast neutrons irradiation at different dose (1.11 Gy, 2.23 Gy, 3.34 Gy and 4.45Gy). Furthermore, in vivo experiments showed that the anti-tumor effect on U14 tumor-bearing mice greatly depended on the neutron irradiation dose. A high dose of fast neutron irradiation (26.73 Gy) could have tumor growth rate only 12.31% compared to 56.07% with control group. All the blood cell counts and blood biochemistry parameters were in the standard value ranges. Immunohistochemistry examinations clearly indicated the apoptosis cells in tumor tissues by the TUNEL assay. This work provides useful evidences on cancer irradiation therapy using fast neutron in pre-clinical study. And the neutron therapy system device has great potential to be a more convenient tool in clinical application with significantly lower power consumption, irradiation toxicity and cost.

Does Neutron Radiation Therapy Potentiate an Immune Response to Merkel Cell Carcinoma?

BACKGROUND

Merkel cell carcinoma (MCC) is a rare and aggressive cutaneous malignancy. In the advanced setting, MCC is often treated with immune checkpoint inhibitors such as anti-PD-1/PD-L1 antibodies. X-ray radiation therapy (XRT) is commonly used for palliation. There is an unmet need for new treatment options in patients progressing on immunotherapy and XRT. We present 2 patients with progressive MCC who were successfully treated with high linear energy transfer neutron radiation therapy (NRT).

CLINICAL OBSERVATIONS

Patient A, an 85-year-old white male with chronic lymphocytic leukemia had progressive MCC with multiple tumors on the face despite prior XRT and ongoing treatment with pembrolizumab. The 5 most symptomatic lesions were treated with a short course of NRT (2 × 3 Gy) while continuing pembrolizumab. All irradiated facial lesions demonstrated a complete response 2 weeks after NRT. Remarkably, an additional 4 lesions located outside the NRT fields also completely resolved. Patient B, a 78-year-old white male with no immunosuppressive condition had recurrent MCC in the scalp and bilateral cervical nodes. The painful, ulcerative tumors on his scalp were progressing despite multiple courses of XRT and multiple immunotherapy regimens, including pembrolizumab. He was treated with NRT (16-18 Gy) to the scalp and had a complete response with successful palliation. While his disease subsequently progressed outside the NRT fields, the response to NRT bridged him to receive further investigational immunotherapies, and he remains disease free 3 years later.

CONCLUSION

Short courses of high linear energy transfer particle therapy deserve consideration as a promising modality for local tumor control in XRT refractory tumors. The out-of-field response suggests that NRT has potential for synergizing with immunotherapy. While more data are required to identify optimal NRT parameters, the NRT dose that potentiates an antitumor immune response appears to be well below organ-at-risk tolerance.

Dosimetric characteristics of the University of Washington Clinical Neutron Therapy System

The University of Washington (UW) Clinical Neutron Therapy System (CNTS), which generates high linear energy transfer fast neutrons through interactions of 50.5 MeV protons incident on a Be target, has depth-dose characteristics similar to 6 MV x-rays. In contrast to the fixed beam angles and primitive blocking used in early clinical trials of neutron therapy, the CNTS has a gantry with a full 360° of rotation, internal wedges, and a multi-leaf collimator (MLC). Since October of 1984, over 3178 patients have received conformal neutron therapy treatments using the UW CNTS. In this work, the physical and dosimetric characteristics of the CNTS are documented through comparisons of measurements and Monte Carlo simulations. A high resolution computed tomography scan of the model 17 ionization chamber (IC-17) has also been used to improve the accuracy of simulations of the absolute calibration geometry. The response of the IC-17 approximates well the kinetic energy released per unit mass (KERMA) in water for neutrons and photons for energies from a few tens of keV up to about 20 MeV. Above 20 MeV, the simulated model 17 ion chamber response is 20%-30% higher than the neutron KERMA in water. For CNTS neutrons, simulated on- and off-axis output factors in water match measured values within ~2%  ±  2% for rectangular and irregularly shaped field with equivalent square areas ranging in a side dimension from 2.8 cm to 30.7 cm. Wedge factors vary by less than 1.9% of the measured dose in water for clinically relevant field sizes. Simulated tissue maximum ratios in water match measured values within 3.3% at depths up to 20 cm. Although the absorbed dose for water and adipose tissue are within 2% at a depth of 1.7 cm, the absorbed dose in muscle and bone can be as much as 12 to 40% lower than the absorbed dose in water. The reported studies are significant from a historical perspective and as additional validation of a new tool for patient quality assurance and as an aid in ongoing efforts to clinically implement advanced treatment techniques, such as intensity modulated neutron therapy, at the UW.