The effect on human neuroblastoma spheroids of fractionated radiation regimes calculated to be equivalent for damage to late responding normal tissues

A mathematical model for brain tumor response to radiation therapy

Gliomas are highly invasive primary brain tumors, accounting for nearly 50% of all brain tumors (Alvord and Shaw in The pathology of the aging human nervous system. Lea & Febiger, Philadelphia, pp 210-281, 1991). Their aggressive growth leads to short life expectancies, as well as a fairly algorithmic approach to treatment: diagnostic magnetic resonance image (MRI) followed by biopsy or surgical resection with accompanying second MRI, external beam radiation therapy concurrent with and followed by chemotherapy, with MRIs conducted at various times during treatment as prescribed by the physician. Swanson et al. (Harpold et al. in J Neuropathol Exp Neurol 66:1-9, 2007) have shown that the defining and essential characteristics of gliomas in terms of net rates of proliferation (rho) and invasion (D) can be determined from serial MRIs of individual patients. We present an extension to Swanson's reaction-diffusion model to include the effects of radiation therapy using the classic linear-quadratic radiobiological model (Hall in Radiobiology for the radiologist. Lippincott, Philadelphia, pp 478-480, 1994) for radiation efficacy, along with an investigation of response to various therapy schedules and dose distributions on a virtual tumor (Swanson et al. in AACR annual meeting, Los Angeles, 2007).

Intraoperative radiotherapy in the treatment of neuroblastoma: report of a pilot study

BACKGROUND

External beam radiotherapy in advanced neuroblastoma is limited by the volume of normal radiosensitive tissues included in the radiation field. Limitations to external radiation are the late effects to these tissues. Intraoperative radiotherapy (IORT) delivers a single high-radiation dose to a tumor while displacing normal tissues that would have been included in an external field. Standard external radiotherapy can still be done after "boost" IORT.

METHODS

Eight advanced-stage neuroblastoma patients who received IORT as part of their multimodality therapy were reviewed to identify the impact of IORT on operative time, complications, and tumor control in the treatment field. The IORT was accomplished by patient transport from the OR to the radiation therapy suite; these were separated by three floors.

RESULTS

IORT added 30-75 min to the operative procedure. Tumors in the resection/IORT fields showed no evidence of disease (one), stable tumor size (six), and tumor recurrence (one). Two complications were identified: a urinary fistula and CO2 retention, which was detected and corrected before the IORT. Neither of these complications was related to the IORT. Two patients who had subsequent tumor resection after IORT demonstrated tumor differentiation to ganglioneuromatous tissue.

CONCLUSIONS

IORT usually can be completed in less than an hour. No IORT-associated complications were identified. IORT along with maximal tumor resection, external radiation, and chemotherapy enhances local tumor control.

Heterogeneity in the fractionation sensitivities of human tumor cell lines: studies in a three-dimensional model system

PURPOSE

Current concepts to optimize the therapeutic gain of radiotherapy by hyperfractionation assume that human tumors are less sensitive to fractionation than late reacting normal tissues. The aim of this study was to investigate the extent of the intercell line heterogeneity of fractionation sensitivity of a wide variety of human tumor cell lines in a three-dimensional model system under fully oxic conditions using schedules with one to eight fractions. Biological characteristics of the tumors that correlate with fractionation sensitivity should be identified.

METHODS AND MATERIALS

A total of 21 cell lines from human tumors maintained as multicellular spheroids consisting of 1000-1500 cells were given fractionated irradiation within a total treatment time of maximally 50 h. Complete dose-spheroid control curves were determined for each fractionation scheme. The spheroid control data were adequately described by the linear quadratic model assuming Poisson statistics. In addition, the induction of a G2 block by a fractionated test dose of seven 3 Gy fractions given at 6-h intervals was determined in spheroid cells using flow cytometry of propidium bromide stained cell nuclei.

RESULTS

The fractionation sensitivities of human tumor cells in multicellular spheroids could be characterized by alpha/beta values, ranging from 2.8-37 Gy in dependence on the cell line. The log normally distributed alpha/beta values were positively correlated with the percentage increase in G2/M phase after the fractionated test dose compared to the controls (r = 0.72, p < 0.01), and were associated with the degree of tumor differentiation (p = 0.01, ANOVA F-test). No significant correlation between the log (alpha/beta) values and the surviving fractions at 2 Gy (SF2) or the total doses with 2 Gy per fraction necessary to control 50% of the spheroids (SCD50) was observed. Despite the intercell line variability of the alpha/beta values, the SCD50 values of the different cell lines, given with one and eight fractions or one fraction and 2 Gy per fraction, were closely associated (Spearman rank correlation coefficients: r = 0.89 or r = 0.90, p < 0.0001).

CONCLUSION

Human tumor cell lines showed a marked heterogeneity in the fractionation sensitivity when irradiated as multicellular spheroids and assayed in situ using the spheroid control end point. Therefore, the therapeutic gain of altered fractionation also depends on those biological characteristics of each individual tumor that affects its fractionation sensitivity. Parameters that correlate with fractionation sensitivity of the tumor lines in the spheroid system were identified as grade of tumor differentiation and percentage increase in G2/M cells at the end of an eight-fraction schedule.

Pharmacokinetics and efficacy of 131I-meta-iodobenzylguanidine in two neuroblastoma xenografts

The pharmacokinetics, biodistribution and efficacy of the radiopharmaceutical 131I-meta-iodobenzylguanidine (131I-mIBG) were determined in murine xenografts of two human neuroblastoma cell lines, SK-N-SH and SK-N-BE(2c). These lines have similar capacities in vitro for active uptake of 131I-mIBG, but different radiobiological characteristics. Groups of four mice were killed after injection of 131I-mIBG, and retained radioactivity in the tumour and normal tissues was measured at 8, 16, 24 and 48 h. Within each type there was heterogeneity of tumour uptake, although average values were similar for both. The per cent injected dose per gram of tumour retained at 24 h was (mean and 95% confidence interval) 0.95 (0.67-1.23) for SK-N-SH and 0.76 (0.47-1.05) for SK-N-BE(2c). The growth of tumours in groups of seven animals following injection of 35, 70 or 105 MBq 131I-mIBG was compared with that of controls. The specific regrowth delay (median and 95% confidence intervals) caused by 105 MBq 131I-mIBG was 4.2 (0.9-5.9) in SK-N-SH and 5.6 (0-11.3) in SK-N-BE(2c) bearing mice. SK-N-BE(2c) xenografts were significantly more sensitive to external beam irradiation than SK-N-SH xenografts.

Radiosensitivity, repair capacity, and stem cell fraction in human soft tissue tumors: an in vitro study using multicellular spheroids and the colony assay

Radiation doses necessary to control 50% of spheroids (SCD50) were determined for five human soft tissue tumor lines after single dose and fractionated irradiation. Spheroids with 1000-1500 cells were used throughout. A similar number of cells per spheroid resulted in different sized spheroids for the respective cell lines. The parameters alpha, beta, and the number of regenerating cellular units per spheroid (SRU) were estimated from the spheroid control data using a direct fit according to the linear quadratic model assuming Poisson statistics. The number of spheroid regenerating cellular units was also determined from the growth delay at doses required for 10% spheroid control. In addition, alpha, beta, and the fraction of clonogenic cells of the five cell lines were obtained from a soft agar colony forming assay. The most precise parameter for radiation sensitivity was the SCD50, with a coefficient of variation smaller than 5%. SCD50 values ranged from 5.9 to 11.0 Gy for the five soft tissue tumor lines. Two of the five cell lines showed significantly higher alpha values and lower calculated survival fractions after 2 Gy (SF2) in the soft agar clonogenic assay than in the spheroid control assay. This points to a resistance-enhancing effect in the spheroid system. Whereas the fractions of SRU from the number of cells per spheroid, estimated from the spheroid control and growth delay assays, agreed well, no significant correlation existed between the fraction of SRU and the fraction of clonogenic cells in the soft agar colony forming assay. The alpha/beta ratios as a descriptive measure of the fractionation sensitivity of the tumor cell spheroids in the spheroid control assay corresponded well with those derived from the dose-cell survival data using a soft agar colony forming assay. Two of the five cell lines showed high fractionation sensitivities with alpha/beta values smaller than 5 Gy while those of the remaining three ranged from 7.8 to 10.8 Gy. Spheroids are structurally more similar to in vivo tumors than monolayer cultures. From the observed lack of correlation in the radiosensitivity parameters alpha and SF2 as well as in the fraction of SRU or clonogenic cells obtained from the spheroid control assay or the colony forming assay, one would expect even greater differences between results from colony forming assays and the radiosensitivity of in vivo tumors, at least for human soft tissue sarcomas.