Fast neutrons in prostatic adenocarcinomas: worldwide clinical experience

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.

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.

Paving the Road for Modern Particle Therapy - What Can We Learn from the Experience Gained with Fast Neutron Therapy in Munich?

While neutron therapy was a highly topical subject in the 70s and 80s, today there are only a few remaining facilities offering fast neutron therapy (FNT). Nevertheless, up to today more than 30,000 patients were treated with neutron therapy. For some indications like salivary gland tumors and malignant melanoma, there is clinical evidence that the addition of FNT leads to superior local control compared to photon treatment alone. FNT was available in Munich from 1985 until 2000 at the Reactor Neutron Therapy (RENT) facility. Patient treatment continued at the new research reactor FRM II in 2007 under improved treatment conditions, and today it can still be offered to selected patients as an individual treatment option. As there is a growing interest in high-linear energy transfer (LET) therapy with new hadron therapy centers emerging around the globe, the clinical data generated by neutron therapy might help to develop biologically driven treatment planning algorithms. Also FNT might experience its resurgence as a combinational partner of modern immunotherapies.

Current standards and future directions for prostate cancer radiation therapy

Definitive radiation therapy is a well-recognized curative treatment option for localized prostate cancer. A suitable technique, dose, target volume and the option of a combination with androgen deprivation therapy need to be considered. An optimal standard external beam radiotherapy currently includes intensity-modulated and image-guided radiotherapy techniques with total doses of ≥76-78 Gy in conventional fractionation. Protons or carbon ions are alternatives available only in specific centers. Data from several randomized studies increasingly support the rationale for hypofractionated radiotherapy. A simultaneous integrated boost with dose escalation focused on a computed tomography/PET- or MRI/magnetic resonance spectroscopy-detected malignant lesion is one option to increase tumor control, with potentially no additional toxicity. The application of a spacer is a promising concept for optimal protection of the rectal wall.

Particle therapy in prostate cancer: a review

While dose escalation is proving important to achieve satisfactory long-term outcomes in prostate cancer, the optimal radiation modality to deliver the treatment is still a topic of debate. Charged particle beams can offer improved dose distributions to the target volume as compared to conventional 3D-conformal radiotherapy, with better sparing of surrounding healthy tissues. Exquisite dose distributions, with the fulfillment of dose-volume constraints to normal tissues, however, can also be achieved with photon-based intensity-modulated techniques. This review summarizes the literature on the use of particle therapy in prostate cancer and attempts to put in perspective its relative merits compared to current photon-based radiotherapy.

Accelerator-based neutron brachytherapy

The development and evaluation of a new approach to neutron brachytherapy is described. This approach, accelerator-based fast neutron brachytherapy, involves the interstitial or intracavity insertion of a narrow, evacuated accelerator beam tube such that its tip, containing the neutron-producing target, is placed in or near the tumor. Tumor irradiation via brachytherapy should result in a reduction in the healthy tissue complication rate observed when poorly collimated and/or low energy external neutron beam are used for treatment. Use of a variable energy accelerator provides an advantage over isotope sources for neutron brachytherapy in that the neutron beam can be turned on and off and the neutron energy spectrum varied for different treatment applications. A prototype accelerator-based fast neutron brachytherapy device, 10 cm long and 6 mm outer diameter, has been constructed and evaluated in terms of its dosimetric output, treatment time, and practical feasibility. The prototype device is a tube-in-tube design with cooling water running between the inner and outer tubes to cool a beryllium target located at the tip of the inner tube. Cooling experiments were performed and coupled with Monte Carlo simulations to determine treatment times as a function of heat load for various neutron-producing reactions. Using the 9Be(d,n) 10B reaction at Ed= 1.5 MeV, 66 RBE-Gy (12 Gy physical dose) can be delivered to the boundary of a 4.5-cm-diam treatment volume in 8 min at a heat load of 130 W. Other reactions offer similar treatment times at somewhat higher bombarding energies and also show higher potential for dose enhancement with the boron-10 neutron capture reaction due to their softer neutron spectra. Dose distributions in a water phantom were measured with the prototype brachytherapy tube using the dual-ion chamber technique for the 9Be(d,n) 10B reaction at Ed = 1.5 MeV. The measurements and simulations agree within uncertainties and demonstrate that fast neutrons contribute more than 90% of the dose to the target volume.

Localized prostate cancer

Much controversy still surrounds the diagnosis and treatment of localized prostate cancer. Urologists generally believe that early detection and aggressive surgical therapy saves lives despite the absence of confirmatory randomized trials. Furthermore, a recent survey of radiation oncologists and urologists revealed marked polarization toward their own specialties when asked how they would counsel patients on therapy for newly diagnosed localized disease. Some issues are not controversial, however. There is general agreement that pretreatment tumor characteristics, including serum prostate-specific antigen level at diagnosis, tumor grade, and clinical stage as judged by digital rectal examination, are important prognosticators for treatment outcomes independent of the type of treatment. Also, there is sufficient experience with standard therapies (radical prostatectomy and external beam radiotherapy) to counsel patients on the chance for cure and the expected incidence of acute and chronic toxicities. A comparative evaluation of various therapies for prostate cancer should include consideration of cancer control, acute toxicity, treatment-related quality of life issues, salvage of treatment failures, and cost. Within this context, we believe that newly diagnosed patients should be counseled on all available treatment options before embarking on a course of therapy.