Effect of 14 MeV monoenergetic neutrons on HeLa and P388F cells in vitro

Modelling of the oxygen enhancement ratio for ion beam radiation therapy

The poor treatment prognosis for tumours with oxygen-deficient areas is usually attributed to the increased radioresistance of hypoxic cells. It can be expressed by the oxygen enhancement ratio (OER), which decreases with increasing linear energy transfer (LET) suggesting a potential clinical advantage of high-LET radiotherapy with heavy ion beams compared to low-LET photon or proton irradiation. The aim of this work is to review the experimental cell survival data from the literature and, based on them, to develop a simple OER model to estimate the clinical impact of OER variations. For this purpose, the standard linear-quadratic model and the Alper-Howard-Flanders model are used. According to our calculations for a carbon ion spread-out Bragg peak at clinically relevant intermediate oxygen levels (0.5-20 mmHg), the advantage of carbon ions might be relatively moderate, with OER values about 1%-15% smaller than for protons. Furthermore, the variations of OER with LET are much smaller in vivo than in vitro due to different oxygen partial pressures used in cell experiments or measured inside tumours. The proposed OER model is a simple tool to quantify the oxygen effect in a practical way and provides the possibility to do hypoxia-based biological optimization in treatment planning.

The influence of hypoxia on the relative sensitivity of human tumor cells to 62.5 MeV (p-->Be) fast neutrons and 4 MeV photons

Fast neutrons have been used in the clinical radiation therapy of tumors largely because of experimental evidence that their cytotoxic effects are much less dependent on oxygen levels than those of low-LET photons. The potential therapeutic advantage of fast neutrons based on hypoxia alone can be calculated as the "hypoxic gain factor", which is the ratio of the OERs for the fast-neutron compared to the photon beams. The hypoxic gain factor that is generally anticipated based on studies with established mammalian cell lines is about 1.6. However, surprisingly few studies have examined the influence of hypoxia on the fast-neutron radiosensitivity of human tumor cells of different histological types. For this reason, we have determined the OERs of five human tumor cell lines exposed to 62.5 MeV (p-->Be) cyclotron-generated fast neutrons or 4 MeV photons from a clinical linear accelerator. The OERs for four chemotherapy-naive cell lines, HT29/5, Hep2, HeLa and RT112, were invariably greater for photons than for neutrons, but all of these values were lower than expected on the basis of the previous literature. Despite their low OERs, these cell lines showed hypoxic gain factors that were within the range of 1.31-1.63, indicating that such effects cannot entirely explain the disappointing clinical results obtained with fast neutrons. In contrast, comparison of the surviving fractions at clinically relevant doses (1.6 Gy of neutrons and 2.0 Gy of photons) for these four tumor cell lines suggested that little benefit should result from neutron treatment. Only the cisplatin-resistant OAW42-CP line showed a significant hypoxic gain factor by this method of analysis. We conclude that, at the dose fractions used in clinical radiation therapy, there may not be a radiobiological precedent for higher local control rates after fast-neutron irradiation of hypoxic tumor cells.

A tumor/normal tissue advantage for low dose rate neutron brachytherapy

Reports on clinical study of Cf-252 pelvic brachytherapy are reviewed and show that complication frequency is low. Low dose rate (LDR) neutron brachytherapy has been shown to be effective against cervical and advanced pelvic cancers; and produces 5 year cures without a high frequency of normal tissue complications. This is attributed to a high relative biological effectiveness (RBE) for the bulky, hypoxic tumor which along with an oxygen enhancement ratio (OER) advantage and dose-rate independent effects, produces rapid tumor regression and good local tumor control. Adjacent normal tissues which are oxygenated have lower RBE values than that of hypoxic tumors. Cf-252 brachytherapy increases the dose differential, therapeutic gain and the probability of local tumor control, since the high RBE of Cf-252 for hypoxic tumor is much less in normal tissues. Cf-252 radiation concentrated in the tumor, and for this reason, has had much fewer attendant normal tissue complications compared to neutron beam therapy.

Systematic evaluation of cellular radiosensitivity parameters

Cellular radiosensitivity parameters of the track structure theory of Katz and co-workers are evaluated from a sum of squares minimizing computer program for nonlinear models. Based on these observations, suggestions are presented for efficient experiment design for the determination of these parameters from track-segment bombardments of high LET radiations.

RBE-dose relations for neutrons and pions

From cellular radiosensitivity parameters and theoretical particle-energy spectra in tissue, of the secondary particles from neutron and negative pion irradiations, RBE-Dose relations have been calculated. The theoretical results are compared with clinical and radiobiological data for normal tissue, tumours and cells in culture. Formulae for calculation, cellular parameters and the needed properties of equivalent 'track-segment bombardments' are given, for several mammalian cells irradiated with pions and with neutrons of several energies.