4.3 Dose-Response Modeling of Rat Data and Extrapolation
4.3.2 Lung Tumors
4.3.2.1 Rat data
Dose-response data from chronic inhalation studies in rats exposed to TiO2 were used to estimate working lifetime exposures and lung cancer risks in humans. These studies are de-scribed in more detail in Table 4–4 and include fine (pigment-grade) rutile TiO2 [Lee et al.
1985; Muhle et al. 1991] and ultrafine anatase TiO2 [Heinrich et al. 1995]. The doses for fine TiO2 were 5 mg/m3 [Muhle et al. 1991] and 10, 50, and 250 mg/m3 [Lee et al. 1985]. For ultra-fine TiO2 there was a single dose of approxi-mately 10 mg/m3 [Heinrich et al. 1995]. Each of these studies reported the retained particle mass lung burdens in the rats. The internal dose measure of particle burden at 24 months of exposure was used in the dose-response models, either as particle mass or particle sur-face area (calculated from the reported or esti-mated particle surface area per gram).
The relationship between particle surface area dose of either fine or ultrafine TiO2 and lung tumor response (including all tumors or tu-mors excluding the squamous cell keratinizing cysts) in male and female rats was shown in Chapter 3. Statistically significant increases in
lung tumors were observed at the highest dose of fine TiO2 (250 mg/m3) or ultrafine TiO2 (ap-proximately 10 mg/m3), whether or not the squamous cell keratinizing cysts were included in the tumor counts.
Different strains and sexes of rats were used in each of these three TiO2 studies. The Lee et al.
[1985] study used male and female Sprague-Dawley rats (crl:CD strain). The Heinrich study used female Wistar rats [crl:(WI)BR strain]. The Muhle et al. [1991] study used male and female Fischer-344 rats but reported only the average of the male and female lung tumor proportions.
The body weights and lung weights differed by rat strain and sex (Table 4–4). These lung mass differences were taken into account when cal-culating the internal doses, either as mass (mg TiO2/g lung tissue) or surface area (m2 TiO2/m2 lung tissue).
4.3.2.2 Critical dose estimates in rats
Statistical models for quantal response were fitted to the rat tumor data, including the suite of models in the BMDS [EPA 2003]. The re-sponse variable used was either all lung tumors or tumors excluding squamous cell keratiniz-ing cystic tumors. Figure 4–5 shows the fit of the various BMD models [EPA 2003] to the lung tumor response data (without squamous cell keratinizing cysts) in male and female rats chronically exposed to fine or ultrafine TiO2 [Lee et al. 1985; Heinrich et al. 1995; and Muhle et al. 1991].
The lung tumor response in male and female rats was significantly different for “all tumors”
but not when squamous cell keratinizing cys-tic tumors were removed from the analysis (Appendix A, Table A–2). In other words, the male and female rat lung tumor responses were equivalent except for the squamous cell
Table 4–4. Summary of chronic inhalation studies in rats exposed to TiO2 Treated rats Particle size and type; studyRat strain
Mean body weigh
t of controls at 24 months (g)
Mean lung weigh
t of controls at 24 months (g)Particle size MMAD (µm) and specific SA (m2/g TiO2)
Exposure concentration (mg/m3)
Retained mean dose (mg TiO2 /lung)*Tumor proportion (rats with tumors/total rats) FemaleMaleFemaleMaleFemaleMaleFemaleMaleAverage Fine TiO2 (≥ 99% rutile): Lee et al. [1985, 1986a]Sprague-Dawley (crl: CD)5577802.353.25MMAD: 1.5 to 1.7
SA: 4.99 [Dr
iscoll 1996]
0 10 50 250
0 32.3 130 545.8
0 20.7 118.3 784.8
0/77 0/75 0/74 14/74 2/79 2/71 1/75 12/77
†
— — — —
Muhle et al.
[1989, 1991, 1994]; B ellman et al. [1991]
Fischer-3443374031.051.38
MMAD: 1.1 (GS
D: 1.6) SA: 4.99 (estimate)
0 5 0 2.72
— —
— —
— — 3/100 2/100
§ (Continued) See footnotes at end of table.
Treated rats Particle size and type; studyRat strain
Mean body weigh
t of controls at 24 months (g)
Mean lung weigh
t of controls at 24 months (g)Particle size MMAD (µm) and specific SA (m2/g TiO2)
Exposure concentration (mg/m3)
Retained mean dose (mg TiO2 /lung)*Tumor proportion (rats with tumors/total rats) FemaleMaleFemaleMaleFemaleMaleFemaleMaleAverage Ultrafine TiO2 (~80% anatase; ~20% rutile): Heinrich et al. [1995]; Muhle et al. [1994]
Wistar [crl:(WI) BR)417—1.44 MMAD: 0.80 (GS
D: 1.8) (agglomerates)
0.015–0.040 (individual particles) SA: 48 (SD: 2)
0 ~10
0 39.29 (SD: 7.36)
At 24 months: 0/10 (controls) 4/9 (all tumors) At 30 months: 1/217 (controls) 19/100 (no keratinizing cysts) 32/100 (all tumors)** GSD = geometric standard deviation; MMAD = mass median aerodynamic diameter; SA = surface area (mean or assumed mean); SD = arithmetic standard deviation; TiO2 = titanium diox- ide; crl:CD and crl:(WI)BR are the rat strain names from Charles River Laboratories, Inc. *Lung particle burdens in controls not reported; assumed to be zero. †Tumor types: controls, male: 2 bronchioloalveolar adenomas. At 10 mg/m3, males: 1 large cell anaplastic carcinoma and 1 bronchioloalveolar adenoma. At 50 mg/m3, male: 1 bronchioloal- veolar adenoma. At 250 mg/m3, females: 13 bronchioloalveolar adenomas and 1 squamous cell carcinoma; males: 12 bronchioloalveolar adenomas. In addition to the tumors listed above, 13 keratin cysts were observed: 1 in a 10 mg/m3 male, 1 in a 250 mg/m3 male, and 11 in 250 mg/m3 females. §Dose was averaged for male and female rats because the tumor rates were reported only for male and female rats combined. Tumor types: controls, 2 adenocarcinomas and 1 adenoma. At 5 mg/m3: 1 adenocarcinoma and 1 adenoma. **Tumor types: controls, at 30 months: 1 adenocarcinoma. At ~10 mg/m3: 20 benign squamous-cell tumors, 3 squamous-cell carcinomas, 4 adenomas, and 13 adenocarcinomas (includes 8 rats with 2 tumors each).
Table 4–4 (Continued). Summary of chronic inhalation studies in rats exposed to TiO2
keratinizing cystic tumor response, which was elevated only in the female rats. To account for the heterogeneity in the “all tumor” response among male and female rats [Lee et al. 1985;
Heinrich et al. 1995], a modified logistic regres-sion model was developed (Appendix A); this model also adjusted for the combined mean tu-mor response for male and female rats reported by Muhle et al. [1991]. As discussed in Chap-ter 3, many pathologists consider the rat lung squamous cell keratinizing cystic tumor to be
Figure 4–5. BMD models and three-model average fit to the lung tumor data (without squamous cell keratinizing cysts) in male and female rats chronically exposed to fine or ultrafine TiO2 Note: The specific models used in constructing the model average were the multistage, Weibull, and log-probit models. The confidence intervals represent 95% binomial confidence limits for the individual data points.
Data source: BMD models and three-model average [Wheeler and Bailer 2007], male and female rats exposed to TiO2 [Lee et al. 1985; Heinrich et al. 1995; Muhle et al. 1991]
irrelevant to human lung pathology. Excess risk estimates of lung tumors were estimated both ways—either with or without the squamous cell keratinizing cystic tumor data. The full results of the analyses including squamous cell keratin-izing cystic tumors can be found in Appendix A.
Inclusion of the keratinizing cystic tumors in the analyses resulted in slightly higher excess risk es-timates in females, but not males. Since the male and female rat tumor responses may be combined when the squamous cell keratinizing cystic
sub-linear; the multistage model is cubic in form, the Weibull model is similar with a power of 2.94, and the log-probit model has a slope of 1.45. Therefore the dose-response relationship is quite steep, and the estimated risk drops sharply as the dose is reduced, as shown in Table 4–7.
4.3.2.3 Estimated human equivalent exposure
The critical dose estimates from Table 4–5 were converted to mass dose and extrapolated to humans by adjusting for species differences in lung surface area, as described in Section 4.2.3.
Pulmonary dosimetry modeling was used to estimate the occupational exposure concentra-tions corresponding to the benchmark dose estimates, as described in Section 4.2.4. Life-time occupational exposure concentrations estimated to produce human lung burdens equivalent to the rat critical dose levels for lung tumors are presented in Table 4–6 for fine and ultrafine TiO2. Estimated occupational ex-posure levels corresponding to various levels of lifetime excess risk are shown in Table 4–7.
This table shows the risk estimates for both fine and ultrafine TiO2. The 95% LCL estimates of the occupational exposure concentrations pected to produce a given level of lifetime ex-cess risk are shown in the right-hand column.
The concentrations shown in bold for fine and ultrafine TiO2 represent 1 per 1000 risk levels, which NIOSH has used as the basis for estab-lishing RELs. The REL for ultrafine TiO2 was rounded from 0.29 mg/m3 to 0.3.