Understanding the potentiality of accelerator based-boron neutron capture therapy for osteosarcoma: dosimetry assessment based on the reported clinical experience

Osteosarcoma is the most frequent primary malignant bone tumour, and its incidence is higher in children and adolescents, for whom it represents more than 10% of solid cancers [1]. Currently, the standard treatment of this malignancy consists in preoperative (neo-adjuvant) chemotherapy followed by surgical removal of all detectable disease (including metastases, that are yet present in 80% of patients at diagnosis), and postoperative (adjuvant) chemotherapy [2]. The standard route of neo adjuvant chemotherapy, surgery and adjuvant chemotherapy is effective in approximately 70% of the patients with localized disease, but for patients with metastases, the long term survival rate is lower than 20% [3]. Neo-adjuvant chemotherapy allows surgery to achieve wide or radical margins while adjuvant chemotherapy is essential to control micro-metastases (responsible for distant metastases and local recurrences) [4, 5]. In the last decades, the objective of avoiding limb amputation and preserving the functional and cosmetic status of patients without decreasing the survival rate has gained importance, leading to surgical techniques named limb-salvage, nowadays performed in 80% of the patients [6].

Osteosarcoma has been generally considered as a radio-resistant tumour, however it has been suggested that a large single fraction dose of radiotherapy may be effective. Radiotherapy has been used in small series of patients, and has been reported as an option for local treatment of unresectable tumours, or as palliation. Given the difficulty to deliver high photon doses in single fraction due to the tolerance of surrounding tissues, patients have been recently treated with charged particles (protons and carbon ions), that allow a very conformal dose distribution. Some other patients have received intraoperative radiotherapy and even extracorporeal irradiation [7‐9].

Boron Neutron Capture Therapy (BNCT) is an experimental form of radiotherapy based on the irradiation of a tumour with low energy neutrons after the target enrichment with atoms of 10-boron (¹⁰B) [10]. Low energy neutrons are captured by ¹⁰B with high probability, generating two high-Linear Energy Transfer (LET), short range particles that release all their energy in a distance comparable to a cell diameter (Fig. 1b). In this path, the charged particles cause non reparable damages to the DNA, thus killing the cell. If ¹⁰B is taken up preferentially in the tumour, the neutron irradiation can deliver a therapeutic dose, without affecting the surrounding healthy tissues. The most interesting characteristic of this treatment is its selectivity, that depends on the bio-distribution of ¹⁰B, rather than on the irradiation beam. Potentially, it can be effective also for disseminated metastases or for isolated tumour cells surrounded by normal tissues. The drug currently used in clinical practice to carry ¹⁰B into tumour is Boronophenylalanine (BPA) [11].

Up to now BNCT has been applied in many centres in the world provided with research nuclear reactors, the only neutron source intense enough to produce neutron beams for the therapy, and clinical trials have been dedicated to brain tumours, head and neck recurrent and primary tumours, skin melanoma, lung cancer [10, 12, 13]. Today, technology is ready to produce neutron beams from high-current proton accelerators coupled with Li or Be targets, and the possibility to install these machines in hospitals makes BNCT a more widespread option. Two BNCT clinical facilities based on accelerators technology are already operating at Kyoto University Research Reactor Institute (KURRI) and at Southern Tohoku BNCT Research Centre in Japan [14].

Italian National Institute of Nuclear Physics (INFN) designed and constructed a Radio Frequency Quadrupole (RFQ) proton accelerator, meant to be used as a neutron source for BNCT clinical applications [15]. The RFQ delivers 30 mA of 5 MeV protons. It is coupled to a beryllium target, and has a neutron yield of about 10¹⁴ s⁻¹ with maximum energy around 3.2 MeV. A Beam Shaping Assembly (BSA) is being designed in order to obtain an epithermal neutron beam peaked between 1 and 10 keV, energy range that was proven to be the most indicated for an advantageous dose distribution for deep-seated tumours like lung cancer [16]. The BSA design is based on the physical properties of the beam and it is optimized according to the dose distributions obtained in realistic clinical cases by treatment planning simulations (Fig. 1a).

Dosimetry in particle therapy requires the use of a formalism to express the dose in terms of photon conventional radiotherapy.

BNCT is characterized by a mixed radiation field composed by: high LET alpha and lithium ions from thermal neutron capture by ¹⁰B, intermediate LET protons from thermal neutron capture by ¹⁴N and neutron elastic collision with H nuclei, and low LET gamma-ray from the thermal neutron capture reaction by H and the gamma radiation in the irradiation beam. The energy released by the high and intermediate LET particles causes a higher severity of biological damage than photons.

Classically, BNCT dose has been reported as equivalent dose (in Gy-Eq), obtained by multiplying each dose component by Biological Effectiveness/Compound Biological Effectiveness (RBE/CBE) factors derived from cell survival curves, for a fixed end point [17]. González and Santa Cruz [18] showed that BNCT outcome as a function of the dose delivered to melanoma nodules, is hardly comparable with photon therapy clinical outcome, the RBE-weighted doses being artificially high. Their new approach of photon iso-effective dose (in Gy (IsoE)) has demonstrated to be more coherent in terms of dose-response outcome. In this work, both the fixed weighting factors and the iso-effective dose have been applied to calculate dose distribution in a representative case of parosteal osteosarcoma of the distal femur.

BNCT has been clinically applied for the first time in Japan in two cases: a recurrent maxilliary sarcoma (fibroblastic osteosarcoma) [19] and a skull radio-induced osteosarcoma [20]. The latter treatment ensured a stable local control of the tumour that did not show any recurrence for 17 months, and the overall status of the patient markedly improved already few days after the treatment. This clinical case was used as a reference for the dosimetry calculations in the representative case of parosteal osteosarcoma of the distal femur, and to draw conclusions about the feasibility of this therapeutic approach.

As other types of radiotherapy, BNCT needs to take into account the radio-sensitivity of the tissues and organs involved in the irradiation. Fast neutron radiotherapy has been used to treat different types of sarcomas, such as soft tissue sarcomas and osteosarcomas [37‐40]. Although the local control rate with this therapy was encouraging, the incidence of late complications was sometimes considered unacceptably high as reported in the cited papers. A trial for soft-tissue sarcoma was stopped because of severe complications in the treated patients, including early damage (erythema, mucositis, dry or moist desquamation developing within 3 months of commencement of irradiation) and late damage (fibrosis, telangiectasia or tissue necrosis) [41]. Reported complications for sarcomas involved predominantly the skin and were related to the high neutron doses delivered and/or the large radiotherapy fields used. The presence of hot spots in dose distribution to the skin has been identified as a source of severe side effects. Compared to fast neutron therapy, BNCT has the potential to reduce the dose to the skin and to the other healthy tissues because, as mentioned before, its selectivity strongly depends on boron bio-distribution rather than on the neutron beam.

Indeed, in clinical BNCT dose is prescribed to the most sensitive organ at risk, considering that tumour receives a higher dose due to the differential boron uptake. From literature, certain levels of toxicity have been reported, along with the protocol to handle the adverse effects and the final results of the treatment. BNCT has been applied especially to brain and head and neck cancer, however, to make predictions about the possible toxicity that may derive from osteosarcoma BNCT, the relevant examples are BNCT of skin melanoma in Argentina, where limbs were irradiated at the RA-6 reactor with hyper-thermal neutrons [42] and, of course, BNCT of skull osteosarcoma. In the first case, the organ at risk is the skin, thus it is a good reference also for limb osteosarcoma BNCT. The cited paper reports an acceptable toxicity, with 7 out of 10 evaluable areas showing a very mild erythema (G1) and 3 out of 10 moist desquamation and ulceration that in all the cases was solved with medical care and medication (30% toxicity grade 3). In particular, the most severe adverse effects were observed in the skin of the foot, an anatomic area that in case of osteosarcoma would be spared from irradiation. Also in the case of BNCT for skull osteosarcoma no radiation injury of the scalp was caused, the only adverse effect being hair loss in neutron field [20]. This experience also gives information about possible effects in normal bone: in this case dose was limited to the normal brain, but thanks to the tumour-to-normal bone boron concentration ratio, the skull was not damaged. The other published experience of BNCT for tumours infiltrated in normal bone [19] show that a severe side effect can be indeed produced to the normal bone: Case 5 reported in the cited paper had radio-osteonecrosis that required surgery. In the same paper, however, Case 2 who was treated for a maxillary osteosarcoma, showed no adverse effect in normal bone. These findings push further studies on boron distribution in normal bone in order to evaluate the dose in a more refined way for each patient. In this sense, more bio-distribution studies in animals and PET with F18-BPA will be the response in a single-patient based study.

A natural question arising is how the BNCT results obtained in this work can be compared with reported clinical radiation therapy in osteosarcoma. In literature, a number of protocols of osteosarcoma irradiation with photons is reported, ranging from conventional fractionated schemes to extremely high single photon doses delivered by intraoperative radiotherapy or extracorporeal irradiation. Shwartz et al., analysed the outcomes of irradiated patients in the Cooperative Osteosarcoma Study Group (COSS)-Registry, including 100 osteosarcoma patients that received radiotherapy for curative or for palliation intent [43]. The dose distribution of the different fractionated protocols included in the analysis, were described as: dose for external RT was 55.8 Gy (30–120); for preoperative RT, 50 Gy (30–68); postoperative RT, 54 Gy (36–72); for RT without surgery, 56 Gy (30–75,6); for primary RT, 59.7 Gy (20–120); for local recurrence, 50.4 Gy (30–70); and for metastases, 45 Gy (30–66). The conclusions of this review were that a clear dose-response relationship could not be demonstrated. However, in the same work it is mentioned that Machak et al. [44] and in a number of patients treated by Shwartz, long-term local control after fractionated doses of 60 Gy and chemotherapy could be demonstrated. Moreover, Gaitan-Yanguas et al. [45] showed that there was no control below 30 Gy, and 100% control above 90 Gy. Based on these experiences, a possible starting point to compare BNCT single fraction dose with the mentioned protocols, is that a fractionated dose of at least 90 Gy would be necessary to achieve complete local control. A dose of 90 Gy in a 2 Gy/fraction scheme corresponds to a single dose of about 15 Gy using Linear Quadratic Model and the α_Ref, β_Ref obtained for UMR-106 cell survival curve (Table 1). Then, regardless of the formalism employed to calculate doses both in limb (Fig. 6) and in skull osteosarcoma (Fig. 7), the dose distributions obtained by BNCT would be potentially therapeutic to treat this disease.

Regarding the single fraction schemes, the reported delivered photon doses are quite variable. Aporchayanon et al. irradiated 21 osteosarcoma patients with bone auto-transplantation delivering 100 Gy [9]; Mottard et al. delivered 70 or 90 Gy with the same technique [46]. Both authors obtained excellent local control. In addition, intraoperative irradiation was carried out in some centres, using for example 10-20 Gy of photons in children and adolescents [47], or 45-80 Gy of photons including also adult patients [7]. Again, BNCT dose values calculated with both formalisms fall in this wide range of single photon doses, RBE-weighted doses being even higher.

In papers describing clinical radiotherapy of osteosarcoma, the main effects limiting the dose to be administrated to the limb are the pathologic bone fracture risk and the tolerance of major nerves and vessels. Unlike for photon therapy, BNCT dose distribution is selective for tumour, thus normal bone absorbs lower dose. In particular, for the analysed femur osteosarcoma, the mean absorbed dose in normal bone (100 cm³) is 15.4 Gy-Eq (minimum 11 Gy-Eq, maximum 18.7 Gy-Eq), a value considerably lower of the single dose administered in intraoperative or extracorporeal bone irradiation with photons.

Oya et al. report that in most intraoperative photon therapy for osteosarcomas in the extremities, the delivered dose was limited to 20 Gy in a single fraction to the tumour bed in order to spare major vessels, nerves, and surrounding soft tissue [7]. Their dose escalation was possible because they excluded nerves and vessels from the irradiation field. Dose distribution obtained with BNCT in the neurovascular bundle falls between a minimum dose of 14 Gy-Eq and a maximum of 18 Gy-Eq, with a mean value of 16 Gy-Eq, calculated with an RBE of 2.5 and a CBE equal to 1.3, as assumed for the central nervous system in the treatment of the brain tumours. These values are safe according to the published tolerance dose of femoral arteries, that is as high as 30 Gy in single fraction [48], and of peripheral nerve, that is 20 Gy for intraoperative radiotherapy [49].

With the aim to understand if calculated doses in the femur case are potentially therapeutic, the dosimetry of the femur case was analysed in the light of the Japanese clinical case re-calculated in this work with the new osteosarcoma parameters and the iso-effective formalism. It can be seen that the maximum dose obtained in limb osteosarcoma using the accelerator (Table 4) is as high as the maximum RBE-weighted doses calculated with the two sets of RBE/CBE factors in skull osteosarcoma. The same holds for the minimum dose in limb osteosarcoma (87 Gy-Eq) when compared to the minimum of KURRI (67.7 Gy-Eq) and to the one calculated with RBE/CBE from UMR-106 cell line (92 Gy-Eq). Note that while the minimum dose in limb is the absolute minimum value in the whole tumour volume, the minimum value in skull corresponds to the lowest dose in the central beam profile (at 5.8 cm from the patient surface). Therefore, given that the patient showed a very good local tumour control, these results support that the same outcome could be obtained in limb with the accelerator beam. The iso-effective dose profile shows values sensibly lower than RBE-weighted dose: maximum iso-effective dose in patient is about 37.8 Gy (IsoE), minimum in tumour along the profile is 31.5 Gy (IsoE). It is worth mentioning the tumour was reduced dramatically without radiation injury of the scalp, therefore the same outcome can be expected in the femur osteosarcoma case.

This work explores the feasibility of BNCT for osteosarcoma using a neutron beam from a proton accelerator, from the point of view of the dose distribution. A human parosteal osteosarcoma of the distal femur was used as an example to calculate treatment planning. Dose was calculated both using traditional formalism of RBE-weighted dose and the new formalism of photon iso-effective dose that proved to give robust results in BNCT of melanoma and cerebral tumours. For the first time, survival curves for photons, neutrons and BNCT were obtained for an osteosarcoma cell line, allowing the derivation of the RBE/CBE factors and of the parameters for the iso-effective dose in osteosarcoma. Moreover, boron concentration values were measured in vivo in a small animal model bearing this tumour.

The results show that the dose distribution obtained in femur osteosarcoma is potentially therapeutic if compared to the photon therapy protocols reported in literature and at least as good as the BNCT dose delivered to the radio-induced skull osteosarcoma in Japan. This supports the suitability of an accelerator-based neutron beam peaked between 1 and 10 keV for the treatment of deep-seated bone tumours.

Several authors explain that osteosarcoma is a radio-resistant tumour not because of the sensitivity of tumour cells to radiation, but rather due to factors such as hypoxia, reoxygenation, tumor size, repair and proliferation rate, that limit the effectiveness of photon therapy [36, 47]. In particular, tumour cell repair of sublethal radiation damages may play a role in the poor outcome of standard radiotherapy [44]. These elements are much less crucial in the case of BNCT that is characterized by high-LET radiation with localized energy deposition inside tumour cells, and that may thus represent a more effective option to treat this disease.

The possibility to apply BNCT to osteosarcoma would allow a multimodal treatment consisting in neo-adjuvant chemotherapy, high-LET selective radiation treatment and a more conservative surgery followed by adjuvant chemotherapy.