252Cf Brachytherapy Research Articles

Microdosimetry-based investigation of biological effectiveness of 252Cf brachytherapy source: TOPAS Monte Carlo study

Objective. To investigate biological effectiveness of 252Cf brachytherapy source using Monte Carlo-calculated microdosimetric distributions. Approach. 252Cf source capsule was placed at the center of the spherical water phantom and phase-space data were scored as a function of radial distance in water (R = 1–5 cm) using TOPAS Monte Carlo code. The phase-space data were used to calculate microdosimetric distributions at 1 μm site size. Using these distributions, Relative Biological Effectiveness (RBE), mean quality factor () and Oxygen Enhancement Ratio (OER) were calculated as a function of R. Main results. The overall shapes of the microdosimetric distributions are comparable at all the radial distances in water. However, slight variation in the bin-wise yield is observed with R. RBE, and OER are insensitive to R over the range 1–5 cm. Microdosimetric kinetic model based RBE values are about 2.3 and 2.8 for HSG tumour cells and V79 cells, respectively, whereas biological weighting function-based RBE is about 2.8. ICRP 60 and ICRU 40 recommendation-based values are about 14.5 and 16, respectively. Theory of dual radiation action based RBE is 11.4. The calculated value of OER is 1.6. Significance. This study demonstrates the relative insensitivity of RBE, and OER radially away from the 252Cf source along the distances of 1–5 cm in water.

Detailed analysis of dose difference in using water as tissue-equivalent material in 252Cf brachytherapy.

The purpose of this study is to analyse how small variations in the elemental composition of soft tissue lead to differences in dose distributions from a 252Cf brachytherapy source and to determine the error percentage in using water as a tissue-equivalent material. Water is normally used as a tissue-equivalent phantom material in radiotherapy dosimetry. Neutron energy spectra, neutron and gamma-ray dose rate distributions were calculated for a 252Cf AT source located at the center of a spherical phantom filled with various types of tissue compositions: adipose, brain, muscle, International Commission on Radiation Units and Measurements (ICRU) report No. 44 9-component soft tissue and water, using Monte Carlo simulation. The obtained results showed differences between total dose rates in various tissues relative to water varying between zero and 4.94%. The contributions of neutron and total gamma ray doses to these differences are, on average, 81% and 19%, respectively. It was found that the dose differences between various soft tissues and water depend not only on the soft tissue composition, but also on the beam type emitted from the 252Cf source and the distance from the source. Assuming water as a tissue-equivalent material, although leads to overestimation of dose rate (except in the case of adipose tissue), is acceptable and suitable for use in 252Cf brachytherapy treatment planning systems based on the recommendation by the ICRU that the uncertainties in dose delivery in radiotherapy should be lower than 5%.

Gimdos kūno ir kaklelio vėžio pacienčių, gydytų suderinta fotonų ir neutronų radioterapija nuo vietiškai išplitusio vėžio, ilgalaikio stebėjimo rezultatai

Jonizuojančioji spinduliuotė, naudojama piktybinių navikų gydymui, laikui bėgant gali sukelti antruosius pirminius piktybinius navikus (APPN). Tyrimo metu buvo vertinamas APPN paplitimas tarp 1143 spinduline terapija (ST) 1989–1999 metų laikotarpiu gydytų IIB ir IIIB stadijos gimdos kaklelio ir I-III stadijos gimdos kūno vėžiu sergančių pacienčių. Viena grupė pacienčių (N=538) buvo gydyta distancine 60Co gama terapija ir didelės dozės galios (DDG) fotonų 60Co brachiterapija, o kita grupė (N=605) – distancine 60Co gama terapija ir DDG neutronų 252Cf brachiterapija. Yra pakankamai literatūros duomenų, rodančių APPN dažnį, nustatytą po distancinės 60Co gama terapijos taikymo, APPN lokalizacijas ir organo apšvitos dozę. Apie APPN dažnį, nustatytą po neutronų 252Cf brachiterapijos, gydant gimdos kūno ir kaklelio vietiškai išplitusį vėžį, literatūros nebuvo rasta. Mūsų tyrimas 25 metus stebint onkologines pacientes, gavusias DDG fotonų 60Co arba neutronų 252Cf brachiterapiją, parodė, kad tarp visų stebėtų 1143 pacienčių buvo diagnozuoti 83 (7,26 proc.) APPN atvejai. 46 (4,02 proc.) APPN atvejai nustatyti pacienčių, gavusių neutronų 252Cf brachiterapiją ir 37 atvejai (3,24 proc.) – gavusių fotonų 60Co brachiterapiją (nėra statistiškai reikšmingo skirtumo: p=0,554). APPN dažniai tarp Lietuvos pacienčių, gavusių neutronų 252Cf brachiterapiją ir gavusių fotonų 60Co brachiterapiją, nesiskyrė nuo kitų šalių tyrėjų publikacijose pateiktų APPN dažnių. Vietiškai išplitusio vėžio pacienčių išgyvenamumas II B stadijos pacientėms buvo toks pat abiejose (fotonų 60Co, ir neutronų 252Cf) brachiterapijos grupėse (p=0,737). III B stadijos pacientėms nustatytas statistiškai reikšmingai geresnis išgyvenamumas neutronų 252Cf brachiterapijos grupėje (skirtumas 10–12 proc.: p=0,004). Didžiausi APPN dažniai buvo stebėti tarp didelę ir mažą apšvitos dozę gavusių kūno anatominių sričių. Vidutinę apšvitos dozę gavusiose srityse APPN buvo stebėti rečiausiai. Neutronų 252Cf brachiterapija gydytoms pacientėms buvo rasta statistiškai reikšmingai mažiau navikų recidyvų. Remiantis gautais rezultatais išspausdinti trys straipsniai aukšto reitingo tarptautiniuose (ISI) žurnaluose (du – Q1 ir vienas Q2 lygio).

Long-term results for Stage IIIB cervical cancer patients receiving external beam radiotherapy combined with either HDR 252Cf or HDR 60Co intracavitary brachytherapy

PurposeThe aim of this work was to compare the long-term curative effects and complications of patients diagnosed with cervical cancer International Federation of Gynecology and Obstetrics IIIB (n = 430) as treated with Californium-252 (252Cf) or cobalt-60 (60Co) intracavitary brachytherapy (ICBT) combined with external beam radiotherapy (EBRT). Methods and MaterialsCervical cancer cases with a history of treatment with 252Cf or 60Co ICBT combined with EBRT were selected from the Lithuanian National Cancer Institute database. Complications and second primary malignancies were compared in both patients groups. Estimates of the 5-, 10-, and 15-year overall survival and disease-free survival rates were computed with the Kaplan–Meier method and a Cox proportional hazards model applied using STATA software. ResultsAt 5, 10, and 15 years, the overall survival rates were 46.9%, 39.3%, and 34.6% for the 252Cf group and 35.4%, 26.9%, and 22.5% for the 60Co group (p = 0.004), respectively. The disease-free survival rates were 42.1%, 35.0%, and 31.0% for the 252Cf group and 32.0%, 25.1%, and 21.4% for the 60Co group (p = 0.009), respectively. Histopathologic type of adenocarcinoma increased the risk of death for the 252Cf group (hazard ratio 3.62). Histopathologic tumor type (hazard ratio 7.48) and recurrence (hazard ratio 2.83) were factors that statistically and significantly influenced the patient prognosis for the 60Co group. ConclusionsApplying 252Cf ICBT with EBRT was effective for International Federation of Gynecology and Obstetrics IIIB cervical cancer patients. Moreover, long-term followup data demonstrated higher survival rates in patients treated with 252Cf ICBT than 60Co ICBT. Complications in patients treated with neutron ICBT were not more frequent or severe than those treated with 60Co ICBT.

Second primary malignancies after radiotherapy including HDR (252)Cf brachytherapy for cervical cancer.

Second primary malignancies (SPMs) are among the most serious late adverse effects after radiotherapy experienced over time by the increasing population of cancer survivors worldwide. The study aim was to determine the rate and distribution of SPMs for neutron- and photon-emitting brachytherapy (BT) sources for patients treated for cervical cancer. The cohort comprised 662 patients with invasive cervical cancer (Stages IIB and IIIB) and contributed 5,224 patient-years (PY) of observation. These patients were treated by radiotherapy during the 1989-1999 year period with cobalt-60 source ((60)Co) teletherapy. The first group of patients (N = 375; 3,154 PY) received high-dose-rate (HDR) californium-252 source ((252)Cf) BT, whereas the second group (N = 287; 2,070 PY) received HDR (60)Co BT. Over a 25-year period, 35 SPMs were observed, amounting to 5.3% of all observed patients: in 16 (2.4%) heavily, 2 (0.3%) moderately, 14 (2.1%) lightly irradiated body sites, and 3 (0.5%) other sites. Of these, 21 cases (5.6%) were observed in the HDR (252)Cf BT group, whereas 14 cases (4.9%) were observed in the HDR (60)Co BT group. Exposures received during (60)Co teletherapy and HDR BT with either (252)Cf or (60)Co had statistically equivalent (p = 0.68) effects on SPM development. Cure rates are improving, and therefore, there are more long-term survivors from cervical cancer. This study shows no significant difference in rates or distribution of SPMs in women treated with neutron BT compared with photon BT (p = 0.68). After reviewing related literature and our research results, it is evident that a detailed investigation of SPM frequency, localization, and dose to adjacent organs is a suitable topic for further research.

Intracavitary moderator balloon combined with (252)Cf brachytherapy and boron neutron capture therapy, improving dosimetry in brain tumour and infiltrations.

This article proposes a combination of californium-252 ((252)Cf) brachytherapy, boron neutron capture therapy (BNCT) and an intracavitary moderator balloon catheter applied to brain tumour and infiltrations. Dosimetric evaluations were performed on three protocol set-ups: (252)Cf brachytherapy combined with BNCT (Cf-BNCT); Cf-BNCT with a balloon catheter filled with light water (LWB) and the same set-up with heavy water (HWB). Cf-BNCT-HWB has presented dosimetric advantages to Cf-BNCT-LWB and Cf-BNCT in infiltrations at 2.0-5.0 cm from the balloon surface. However, Cf-BNCT-LWB has shown superior dosimetry up to 2.0 cm from the balloon surface. Cf-BNCT-HWB and Cf-BNCT-LWB protocols provide a selective dose distribution for brain tumour and infiltrations, mainly further from the (252)Cf source, sparing the normal brain tissue. Malignant brain tumours grow rapidly and often spread to adjacent brain tissues, leading to death. Improvements in brain radiation protocols have been continuously achieved; however, brain tumour recurrence is observed in most cases. Cf-BNCT-LWB and Cf-BNCT-HWB represent new modalities for selectively combating brain tumour infiltrations and metastasis.

EP-1904: Long-term results of the 252-Cf brachytherapy combined with EBRT for carcinoma of cervix uteri

EP-1904: Long-term results of the 252-Cf brachytherapy combined with EBRT for carcinoma of cervix uteri

The more important heavy charged particle radiotherapy of the future is more likely to be with heavy ions rather than protons

The more important heavy charged particle radiotherapy of the future is more likely to be with heavy ions rather than protons

Comparative dosimetry in intracavitary balloon catheter brachytherapy with I-125 and in Cf-252 brachytherapy combined with BNCT for brain tumors

Objective Comparative analysis of dosimetry in intracavitary balloon catheter brachytherapy with I-125 and in Cf-252 brachytherapy combined with BNCT for treatment of brain tumors. Materials and Methods Simulations of intracavitary balloon catheter brachytherapy with I-125 and in Cf-252 brachytherapy combined with BNCT were performed with the MCNP5 code, modeling the treatment of a brain tumor on a voxel computational phantom representing a human head. Absorbed dose rates were converted into biologically weighted dose rates. Results Intracavitary balloon catheter brachytherapy with I-125 produced biologically weighted mean dose rates of 3.2E-11, 1.3E-10, 1.9E-11 and 6.9E-13 RBE.Gy.h-1.p-1.s, respectively, on the healthy tissue, on the balloon periphery and on the I 1 and I 2 tumor infiltration zones. On the other hand, Cf-252 brachytherapy combined with BNCT produced a biologically weighted mean dose rate of 5.2E-09, 2.3E-07, 8.7E-09 and 2.4E-09 RBE.Gy.h-1.p-1.s, respectively on the healthy tissue, on the target tumor and on the I 1 and I 2 infiltration zones. Conclusion Cf-252 brachytherapy combined with BNCT delivered a selective irradiation to the target tumor and to infiltration zones, while intracavitary balloon catheter brachytherapy with I-125 delivered negligible doses on the tumor infiltration zones.

Effect of tissue Inhomogeneities on dose distributions from Cf-252 brachytherapy source

The Monte Carlo method was used to determine the effect of tissue inhomogeneities on dose distribution from a Cf-252 brachytherapy source. Neutron and gamma-ray fluences, energy spectra and dose rate distributions were determined in both homogenous and inhomogeneous phantoms. Simulations were performed using the MCNP5 code. Obtained results were compared with experimentally measured values published in literature. Results showed a significant change in neutron dose rate distributions in presence of heterogeneities. However, their effect on gamma rays dose distribution is minimal.

Neutron and gamma ray doses from a 252Cf brachytherapy source in a water phantom

Axial and radial doses of Neutrons and gamma rays from an Isotron 252Cf Brachytherapy source were calculated in a Water phantom using Maienshein’s prompt fission gamma rays data and Maxwellian neutron energy spectrum. It was observed that neutron dose due to the source casing thickness does not contribute significantly to the total dose. Further the calculated secondary gamma ray dose rate is very small compared to the calculated primary gamma dose rate. Neutron and secondary gamma ray dose calculated in this study agree with the published data. Results of this study will be presented here.

Gamma Dose Rate Near a New 252Cf Brachytherapy Source

A new generation of medical grade 252Cf sources was developed in 2002 at the Oak Ridge National Laboratory. The combination of small size and large activity of 252Cf makes the new source suitable to be used with the conventional high-dose-rate remote afterloading system for interstitial brachytherapy. A recent in-water calibration experiment showed that the measured gamma dose rates near the new source are slightly greater than the neutron dose rates, contradicting the well established neutron-to-gamma dose ratio of approximately 2:1 at locations near a 252Cf brachytherapy source. Specifically, the MCNP-predicted gamma dose rate is a factor of two lower than the measured gamma dose rate at the distance of 1 cm, and the differences between the two results gradually diminish at distances farther away from the source. To resolve this discrepancy, we updated the source gamma spectrum by including in the ORIGEN-S data library the experimentally measured 252Cf prompt gamma spectrum as well as the true 252Cf spontaneous fission yield data to explicitly model delayed gamma emissions from fission products. We also investigated the bremsstrahlung X-rays produced by the beta particles emitted from fission product decays. The results show that the discrepancy of gamma dose rates is mainly caused by the omission of the bremsstrahlung X-rays in the MCNP runs. By including the bremsstrahlung X-rays, the MCNP results show that the gamma dose rates near a new 252Cf source agree well with the measured results and that the gamma dose rates are indeed greater than the neutron dose rates.

Brain tumour and infiltrations dosimetry of boron neutron capture therapy combined with 252Cf brachytherapy

This article presents a dosimetric investigation of boron neutron capture therapy (BNCT) combined with (252)Cf brachytherapy for brain tumour control. The study was conducted through computational simulation in MCNP5 code, using a precise and discrete voxel model of a human head, in which a hypothetical brain tumour was incorporated. A boron concentration ratio of 1:5 for healthy-tissue: tumour was considered. Absorbed and biologically weighted dose rates and neutron fluency in the voxel model were evaluated. The absorbed dose rate results were exported to SISCODES software, which generates the isodose surfaces on the brain. Analyses were performed to clarify the relevance of boron concentrations in occult infiltrations far from the target tumour, with boron concentration ratios of 1:1 up to 1:50 for healthy-tissue:infiltrations and healthy-tissue:tumour. The average biologically weighted dose rates at tumour area exceed up to 40 times the surrounding healthy tissue dose rates. In addition, the biologically weighted dose rates from boron have the main contribution at the infiltrations, especially far from primary tumour. In conclusion, BNCT combined with (252)Cf brachytherapy is an alternative technique for brain tumour treatment because it intensifies dose deposition at the tumour and at infiltrations, sparing healthy brain tissue.

612 poster CLINICAL RESULTS OF THE COMBINED TREATMENT INCLUDING INTRALUMINAL CF-252 BRACHYTHERAPY FOR BREAST CANCER

Purpose: The aim of this study was to evaluate the impact the preoperative radiotherapy including intraluminal Cf-252 brachytherapy on risk of locoregional recurrence and long-term survival in breast cancer patients treated with mastectomy in 1987-1997. Materials: Weretrospectively reviewed records for 155 patientswith invasive medial and central breast cancer (T1-T3, N0-2) who received preoperative irradiation of the chest wall, axillary, and supraclavicular, and parasternal regions and underwentmastectomy.Median age was 55 years, 56.5 % of the patients were lymph node negative. Patients received of conventional 36-45 Gy radiation dose in 2-Gy fractions and intraluminal Cf-252 neutron brachytherapy to parasternal lymph nodes. During surgery catheters were placed in the arteria thoracica interna and on the first or second post-operative day, flexible Cf-252 sources were inserted into catheters. Their active length was 10-12 cm, irradiation dose at a distance of 1 cm from the centre of source was 4045 Gyeq. There was no Tamoxifen given. The five-year actuarial and 10-year of loco-regional recurrence and survival probabilities were estimated by the Kaplan-Meier method and differences between patient subgroups were compared by the log-rank test. Results: Overall survival was 73.8, 72.2 and 44.0% at 3, 5 and 10 years. The 5-yer acturial overall survival was 38% for patients with chest wall, axillary, supraclavicular or internal mammary lymph node (one case) recurrence. Recurrence was observed in 31 of 154 (20 %) patients at the end of this study. Loco-regional recurrence rate was 7.4, 13.9 and 20.4% at 3, 5 and 10 years. The recurrence rate was 1.96% in patients without lymphovascular permeation, and it increased to 21.36 % in patients with in patients with lymphovascular permeation (p=0.03). Patients younger than 50 years at primary diagnosis had statistically significantly higher rates of 10 year recurrence [16 of 53 (30.2%)] than older patients [15 of 101 (15 %)], p=0.03. The side effects included of pneumonitis (3 cases, 1.9%), lung fibrosis (26 cases, 16.8 %) and oedema of the arm (37 cases, 23.9%). Conclusions: The long-term data show that the preoperative radiotherapy including intraluminal Cf-252 brachytherapy has positive impact on time of loco-regional recurrence and survival in patients after mastectomy.

Clinical analysis of combination of (252)Cf neutron intracavitary brachytherapy and external beam radiotherapy for 110 cervical cancer patients

To evaluate the curative effect and complication of (252)Cf brachytherapy on cervical cancer. From Nov 2002 to Nov 2007, 110 cervical cancer patients were treated by combination of (252)Cf neutron intracavitary brachytherapy and external beam radiotherapy. There were 2 cases of stage Ib, 5 stage IIa, 57 stage IIb, 2 stage IIIa, 41 stage IIIb, 2 stage IVa, and 1 stage IVb. The whole pelvic cavity was irradiated with 8 MV X-ray, 1.8 Gy/fraction, 4 - 5 times per week. The total dose of external beam radiotherapy was 40 - 50 Gy (the center of whole pelvic field was blocked by 4 cm in width after 20 - 30 Gy). (252)Cf neutron intracavitary brachytherapy was delivered at 6 - 8 Gy(i)/fraction, and the total dose of reference point A was 30-51 Gy(i). The median dose was 42 Gy(i). The overall 3-year survival rate of all patients was 79.2%, and the local control rate was 90.0%. In particular, the 3-year survival rate was 1/2 for stage I, 84.3% for stage II, 53.7% for stage III. The difference between stage II and stage III was statistically significant (P < 0.05). The 3-year survival in cervical lesions larger than or equal to 4 cm and those less than 4 cm was 68.1% and 71.2%, respectively (P > 0.05). The 3-year survival rate of patients with and without anemia was 42.4% and 78.2%, respectively (P < 0.05). The 3-year survival rate of patients with squamous cell carcinoma and adenocarcinoma was 78.5% and 76.9%, respectively (P > 0.05). The late radiation complications of rectum and bladder was 11.8% (13/110) and 2.7% (3/110), respectively. It is concluded that (252)Cf is a better source for intracavitary brachytherapy. According to our initial experience, (252)Cf has advantages of a high local control rate, especially to bulky tumor and adenocarcinoma.

Detailed dose distribution prediction of Cf-252 brachytherapy source with boron loading dose enhancement

The purpose of this work is to evaluate the dose rate distribution and to determine the boron effect on dose rate distribution for 252Cf brachytherapy source. This study was carried out using a Monte Carlo simulation. To validate the Monte Carlo computer code, the dosimetric parameters were determined following the updated TG-43 formalism and compared with current literature data. The validated computer code was then applied to evaluate the neutron and photon dose distribution and to illustrate the boron loading effect.

WE-D-BRB-04: An Evaluation Study of Treatment Planning of Brain Tumor Using Implanted Neutron Brachytherapy and Compared with Photon IMRT

Purpose: The objective of this study is to perform a plan evaluation using Californium-252 neutron brachytherapy for the treatment of malignant gliomas and compare the neutron brachytherapy planning with photon IMRT planning. Materials/Methods: After a phase I trial of neutron brachytherapy (Cf-252 implant) for the treatment of malignant gliomas, there is renewed interest to evaluate the effectiveness of radiation therapy using neutron brachytherapy compared with photon IMRT. Here we performed a dosimetric comparison of two treatment plans based on the same patient using equivalent prescribed dose. Isodose lines and dose volume histogram of brain tumor and adjacent critical structures were used for plan evaluation. Dose of neutron brachytherapy was calculated using CT-converted Monte Carlo model and simulated by Monte Carlo code MCNPX. The tissue component of Monte Carlo model was adapted from the sectioned images of human cadavers of the Visible Human Project of NLM. Dose of IMRT plan was calculated using Pinnacle3 TPS. A relative biological effectiveness of 6 was used to determine the neutron equivalent dose (ncGy) for central nervous system (CNS) tissues. An equivalent dose of 6000 cGy was prescribed for both the IMRT plan and the neutron brachytherapy plan. Results: The targets were well covered by the 95% isodose line in both IMRT and neutron brachytherapy plans. Comparing the IMRT and Cf-252 neutron brachytherapy, the mean dose was 6102 cGy and 6708 ncGy for target, 403 cGy and 177 ncGy for brainstem, 1022 cGy and 311 cGy for chiasm, 311 cGy and 275 ncGy for pituitary, and 705 cGy and 420 ncGy for brain, respectively. Conclusions: Cf-252 Brachytherapy provided conformal dose distribution to the brain tumor and reduced the dose to the surrounding critical organs compared to IMRT. The implanted Cf-252 source provides high dose to brain tumor and reduces the radiation exposure of normal brain.

SU-GG-T-12: Neutron Energy and Dose Considerations of Cf-252 Brachytherapy Sources for Various Geometries

Purpose: To examine the effect of the 252 Cf source size to the neutronenergy spectrum and dose contribution in a tissue equivalent material, with and without boronneutron capture enhancement. Method and Materials: Version 2.5.0 of the MCNPXcomputer code (Pelowitz, 2005) was used in this study to calculate the neutron energy spectra and dose distribution with and without 10B loading for 252Cf sources of various geometries. A spherical phantom geometry with a centrally positioned 252 Cf point‐source was first implemented in order to verify our simulation code by reproducing existing data in the literature. The neutronenergy flux was calculated for various 10 B loadings and various distances from the source. Cylindrical sources of different sizes were simulated in a 30×30×30 cm3 water phantom and the neutronenergy flux and energy deposition in the medium was calculated on cylindrical surfaces enclosing each source at various distances from each source surface. The boron enhancement of 30 ppm was also studied in this case. The neutronenergy spectrum was modeled as an isotropic Watt distribution and all calculated spectra were normalized assuming the same amount of 252 Cf distributed uniformly inside the source volume. Results: Calculated neutron energy spectra for the 252 Cf point‐source geometry showed very good agreement with existing literature, verifying our simulation model. For the cylindrical 252 Cf sources, preliminary results showed increased fast neutron contribution from the compact source, as compared to the conventional one, especially at small distances from the source surface. Conclusion: Smaller size 252 Cf sources can be more beneficial for brachytherapy treatments, not only due to localized dose distribution, but also due to higher dose contribution from fast neutrons of high RBE at small distances from the source.

Towards HDR 252Cf brachytherapy: Shielding assessment of a linear accelerator vault

Towards HDR 252Cf brachytherapy: Shielding assessment of a linear accelerator vault

A Radiobiological Model for the Relative Biological Effectiveness of High-Dose-Rate252Cf Brachytherapy

While there is significant clinical experience using both low- and high-dose-rate 252Cf brachytherapy, there are minimal data regarding values for the neutron relative biological effectiveness (RBE) with both modalities. The aim of this research was to derive a radiobiological model for 252Cf neutron RBE and to compare these results with neutron RBE values used clinically in Russia. The linear-quadratic (LQ) model was used as the basis to characterize cell survival after irradiation, with identical cell killing rates (S(N) = S(gamma)) between 252Cf neutrons and photons used for derivation of RBE. Using this equality, a relationship among neutron dose and LQ radiobiological parameter (i.e., alpha(N), beta(N), alpha(gamma), beta(gamma)) was obtained without the need to specify the photon dose. These results were used to derive the 252Cf neutron RBE, which was then compared with Russian neutron RBE values. The 252Cf neutron RBE was determined after incorporating the LQ radiobiological parameters obtained from cell survival studies with fast neutrons and teletherapy photons. For single-fraction high-dose-rate neutron doses of 0.5, 1.0, 1.5 and 2.0 Gy, the total biologically equivalent doses were 1.8, 3.4, 4.7 and 6.0 RBE Gy with 252Cf neutron RBE values of 3.2, 2.9, 2.7 and 2.5, respectively. Using clinical data for late-responding reactions from 252Cf, Russian investigators created an empirical model that predicted high-dose-rate 252Cf neutron RBE values ranging from 3.6 to 2.9 for similar doses and fractionation schemes and observed that 252Cf neutron RBE increases with the number of treatment fractions. Using these relationships, our results were in general concordance with high-dose-rate 252Cf RBE values obtained from Russian clinical experience.