Patient-reported sexual toxicity after radiation therapy in long-term prostate cancer survivors.

BACKGROUND: To present an overview of patient-reported sexual toxicity in sexually active long-term prostate cancer survivors treated with radiation therapy. METHODS: We used patient-reported outcomes from a study-specific questionnaire surveying symptoms after prostate cancer radiation therapy. Data from 518 men treated at the Sahlgrenska University Hospital in Sweden from 1993 to 2006 were analysed. The men had undergone primary or salvage external beam radiation therapy (EBRT) or EBRT combined with high-dose rate brachytherapy (BT). We also used information from 155 non-treated reference men from the general population with no history of prostate cancer, matched for age and residency. RESULTS: Median time from treatment to follow-up was 5 years (range: 1-14 years). Among the 16 investigated symptoms on erectile function, libido, orgasm, and seminal fluid, 9 symptoms in the primary EBRT group and 10 in both the salvage EBRT and the EBRT+BT groups were statistically significantly more prevalent in survivors than in reference men. Erectile dysfunction was influenced by both age and time to follow-up, whereas symptoms relating to orgasm and seminal fluid were influenced by time to follow-up only. Not being sexually active was almost one and a half times as common in survivors as in reference men. CONCLUSIONS: The presented symptom profiles can help to develop personalized therapy for prostate cancer through a better understanding of which radiation-induced toxicities to be addressed in the clinic and can also assist in identifying suitable interventions for existing symptoms.

Patient-reported genitourinary toxicity for long-term prostate cancer survivors treated with radiation therapy.

BACKGROUND: The objective of this study is to provide comprehensive overviews of patient-reported urinary symptoms for long-term prostate cancer survivors treated with radiation therapy and for untreated, healthy men. METHODS: We performed a population-based cross-sectional study using a study-specific postal questionnaire assessing symptoms among 1007 men consecutively treated at the Sahlgrenska University Hospital, Göteborg, Sweden from 1993-2006 (primary or salvage external beam radiation therapy (EBRT) or EBRT and high-dose rate brachytherapy). We also randomly recruited 350 non-pelvic-irradiated matched control men from the Swedish Total Population Register. Symptom prevalence and prevalence ratios were computed. RESULTS: Survey participation rate was 89% (874/985) for eligible survivors and 73% (243/332) for eligible controls. Median time from treatment to follow-up was 5 years (range, 1-14 years). Among the 21 investigated symptoms reflecting obstruction, frequency, urgency, pain and incontinence, we found significantly higher prevalence compared with controls for 9 symptoms in the EBRT group, 10 in the EBRT+brachytherapy group and 5 in the salvage EBRT group. The prevalence for a majority of the symptoms was stable over time. CONCLUSION: The presented toxicity profiles provide a thorough understanding of patient-reported urinary symptoms that can assist in developing personalised therapy for prostate cancer.

Surface attachment and pre-penetration stage development by plant pathogenic fungi.

Fungal pathogens cause many of the most serious crop diseases. One of the principal reasons for the success of this group is their ability to locate and perceive appropriate host surfaces and then to elaborate specialized infection structures. Here we review the processes implicated in surface attachment, germ tube elongation, and development of appressoria. The involvement of surface-acting proteins such as fungal hydrophobins and integrins in these processes is evaluated, along with a description of studies that have revealed the existence of conserved signaling pathways that regulate appressorium formation. Finally, we anticipate the prospect of genome-level analysis of fungal pathogens and the key research questions that will need to be addressed.

Predominance of the metastatic phenotype in somatic cell hybrids of the K-1735 murine melanoma.

The purpose of these studies was to determine whether the metastatic phenotype will dominate when metastatic and nonmetastatic clones of the K-1735 mouse melanoma are hybridized by somatic cell fusion. Three nonmetastatic and three metastatic clones were transfected with DNA from plasmids pSV2neo or pSV2hygro, which confer resistance to the drugs neomycin or hygromycin, respectively. The metastatic properties of the six clones were not altered by these transfections. The tumorigenicity and metastatic capacity of cell hybrids formed by somatic cell fusion of nonmetastatic and metastatic clones were examined. To do so, near-tetraploid hybrids containing a nearly complete chromosomal complement from both parental cells were injected i.v. into syngeneic mice, and the number of metastatic nodules in the lung was determined at 45 days or when the mice became moribund. Seven of nine hybrids produced from the fusion of metastatic and nonmetastatic clones exhibited a highly metastatic phenotype, although in most cases the metastatic potential of the hybrids was lower than that of the metastatic parent cells. Very similar results were obtained in athymic nude mice. The metastatic potential of the hybrids was directly correlated with their growth in the subcutis of nude mice. These results indicate that the metastatic capacity of K-1735 cells predominates in somatic cell hybrids between nonmetastatic and metastatic cells. When fusion of nonmetastatic and metastatic cells yields a hybrid with nonmetastatic properties, it may be due to suppression of growth.

Biological effect of epidermal growth factor on the in vitro growth of human tumors.

The effect of epidermal growth factor (EGF) on the in vitro growth of 186 malignant human tumor specimens (45 melanomas, 32 sarcomas, and 56 lung, 16 gynecological, 14 breast, 12 genitourinary, and 11 gastrointestinal carcinomas) was evaluated in the cellular adhesive matrix human tumor culture system supplemented with transferrin, insulin, hydrocortisone, and estradiol. EGF increased tumor growth by at least 50% in 81% of the 186 tumors and by over 100% in 54%. The enhanced growth induced by EGF was related to an accelerated cellular division independent of tumor type and not to an increase in the actual number of clonogenic units. The drug concentrations of cell cycle-independent Adriamycin and cisplatin needed to achieve a 90% tumor cell kill were not altered by the responsiveness of the tumor to EGF.

Stage IV diffuse large-cell lymphoma: a long-term analysis.

A long-term analysis of the clinical outcome of previously untreated adult patients who presented with stage IV diffuse large-cell lymphoma at diagnosis was performed to identify possible prognostic factors. Sixty-one patients were seen between 1974 and 1981; all were treated with cyclophosphamide, doxorubicin, vincristine, prednisone, and bleomycin followed by cyclophosphamide, vincristine, prednisone, and bleomycin for a total of one year. Overall five-year survival was 48.5%, with a median follow-up of 53 months. Of the 56 patients evaluable for remission status, 41 achieved a complete remission, and 27 are alive and disease free. Clinical factors of prognostic importance for survival included age, constitutional symptoms, lactate dehydrogenase (LDH) level, presence of mediastinal disease, large-cell infiltration of bone marrow, and number of extranodal sites of disease. The proportional hazards model then identified age, number of extranodal sites, and, to a lesser extent, serum LDH level as independent risk factors for survival. Four distinct patient risk groups were identified using these three factors. Younger patients with only one extranodal site of disease and normal LDH levels responded well on this therapy, with 100% alive at five years. In contrast, survival was less than 30% at five years for patients in the lowest risk group. There were 11 relapses; LDH level, constitutional symptoms, and mediastinal disease predicted for relapse. Knowledge of these risk factors permits individualization of treatment planning and allows more meaningful comparisons with the results of treatment studies using other intensive regimens.

Tumor burden assessment and its implication for a prognostic model in advanced diffuse large-cell lymphoma.

Previously untreated adult patients who presented with advanced diffuse large-cell lymphoma (DLCL) at diagnosis were studied to identify possible prognostic factors. One hundred five patients were seen between 1974 and 1981; 45 patients were stage III and 60 patients were stage IV. All patients received cyclophosphamide, doxorubicin, vincristine, prednisone, and bleomycin (CHOP-Bleo). Stage III patients also received radiation therapy alternated with chemotherapy. Overall survival was 50% at 5 years and 43% at 8 years. Seventy-four patients achieved a complete remission (CR) and 37 are alive and disease-free with a median follow-up of 72 months. There was no difference in clinical outcome between stage III and stage IV. However, a proportional hazards model identified lactic dehydrogenase (LDH) level and tumor burden, among all clinical factors studied, as independent risk factors for survival. These two factors were also important for achievement of remission and relapse-free survival. Three distinct patient risk groups were identified with 5-year survival rates of 87%, 48%, and 20%, respectively. The measure of tumor burden proposed herein, along with LDH level, can be used for developing treatment programs, and for meaningful comparison of different treatment regimens, as well as assessment of prognosis.

Small noncleaved cell lymphoma in adults: superior results for stages I-III disease.

Small noncleaved cell lymphoma (SNCCL), a rare lymphoma in adults, is associated with not only a rapid complete response (CR) to chemotherapy but also with the potential to rapidly relapse both systemically and in the CNS. We treated 44 assessable adults with two similar protocols, consisting of three sequential chemotherapy combinations and intrathecal prophylaxis with methotrexate and cytarabine. The overall CR rate was 80%; it was 100% in patients with Ann Arbor (AA) stages I-III disease and 57% in those with stage IV disease. The overall survival (OS) rate at 5 years was 52%. The overall 5-year freedom from tumor mortality (FTM) rate was 63%; it was 95% for patients with AA stages I-III disease, and 29% for those with stage IV disease. Stepwise multivariate analysis of factors associated with remission duration and survival indicated that advanced-disease stage and age of 40 years or over were predictors of poor prognosis. Twelve patients with positive human immunodeficiency virus (HIV) serology were also included in this series. They had an 83% CR rate and an 83% 5-year FTM, but only a 36% 5-year OS; most deaths were secondary to opportunistic infection. Histologic subtype (Burkitt's lymphoma [BL] or non-Burkitt's lymphoma [NBL]) did not correlate with patient age, site of tumor presentation, response to therapy, or survival. Both protocols achieved comparable results. The approach used in these protocols is highly effective for patients with early staged disease, regardless of their HIV status; however, better therapy is necessary for those with SNCCL presenting in an advanced stage.

Prognostic factors for response and survival after high-dose cyclophosphamide, carmustine, and etoposide with autologous bone marrow transplantation for relapsed Hodgkin's disease.

Sixty-one patients with relapsed Hodgkin's disease who had failed a mechlorethamine, vincristine, procarbazine, and prednisone (MOPP)- and a doxorubicin, bleomycin, vinblastine, and dacarbazine (ABVD)-like regimen were treated with a high-dose combination chemotherapy containing cyclophosphamide, carmustine, and etoposide (CBV) and autologous bone marrow transplantation (ABMT). Fifty-nine patients were treated in relapse and two were intensified early in third remission. Following therapy, 29 patients (47%) were in complete remission (CR), 18 patients (30%) achieved a partial response (PR), and 14 patients (23%) had progressive disease (PD). Among the partial responders, six patients achieved a CR following addition of local radiation therapy to sites of residual nodal disease. For a minimum follow-up of 2 years, 23 patients (38%) are alive and free of disease. High-dose CBV therapy produced severe myelosuppression, and there were four (7%) treatment-related deaths. A multivariate analysis identified failure of more than two prior chemotherapy treatments and poor performance status as important adverse risk factors for survival. Patients who had no adverse risk factor and/or were intensified with CBV while Hodgkin's disease was still responding to conventional chemotherapy, had a CR rate of 63%, with 77% projected 3-year survival; whereas, all other patients had a CR rate of 31%, and a projected 3-year survival of only 18%. Our results demonstrated that CBV and ABMT can induce remission duration of 2 years or greater in a significant proportion of patients with relapsed Hodgkin's disease.

Recombinant human granulocyte colony-stimulating factor hastens granulocyte recovery after high-dose chemotherapy and autologous bone marrow transplantation in Hodgkin's disease.

To determine whether recombinant human granulocyte colony-stimulating factor (rhG-CSF) can accelerate granulocyte recovery after high-dose combination chemotherapy with autologous bone marrow transplantation (ABMT) in patients with Hodgkin's disease, we performed a nonrandomized phase II study using historical controls as a comparison. Eighteen relapsed/refractory Hodgkin's disease patients who received cyclophosphamide at 1.5 g/m2/day (days -6 to -3), carmustine (BCNU) at 300 mg/m2 (day -6), and etoposide (VP-16) at 125 mg/m2 every 12 hours (days -6 to -4), followed by ABMT (day 0) were treated with rhG-CSF at 60 micrograms/kg/day for a maximum of 28 days beginning on day 1. rhG-CSF dosage was gradually diminished and stopped once an adequate granulocyte count was maintained. rhG-CSF significantly accelerated absolute granulocyte count (AGC) compared with historical controls recovery to the 100/microL level (median, 9 days v 13 days; P = .103 x 10(-4), 500/microL level (median, 13 days v 22 days; P = 0.189 x 10(-2), and 1000/microL level (median, 16 days v 30 days levels; P = .125 x 10(-5). Platelet recovery to 50,000/microL was not significantly altered (P = .370). rhG-CSF was well tolerated, bone pain and myalgia being the only side effects noted. rhG-CSF hastens granulocyte recovery after high-dose chemotherapy with ABMT in patients with relapsed/refractory Hodgkin's disease without significant toxicity.

BCR rearrangement-negative chronic myelogenous leukemia revisited.

PURPOSE: To document the characteristics of patients with major breakpoint cluster region (M-bcr) rearrangement-negative chronic myelogenous leukemia (CML). PATIENTS AND METHODS: The hematopathologist, who was blinded to patients' molecular status, reviewed the referral bone marrows and peripheral-blood smears from 26 patients with Philadelphia (Ph) translocation-negative CML who lacked Bcr rearrangement (and other evidence of a Bcr-Abl anomaly) and 14 patients (controls) with chronic-phase Ph-positive CML. Clinical data was ascertained by chart review. RESULTS: Among the 26 M-bcr rearrangement-negative CML patients, three pathologic subtypes emerged: (1) patients indistinguishable from classic CML (n = 9), (2) patients with atypical CML (n = 8), and (3) patients with chronic neutrophilic leukemia (n = 9). Among the 14 patients with Ph-positive CML who were included in the blinded review, 13 were classified as classic CML, and one was classified as atypical CML. The only statistically significant difference between M-bcr rearrangement-negative subgroups was in the proportion of patients having karyotypic abnormalities, an observation common only in patients with atypical CML (P = 0.008). However, the small number of patients in each subgroup limited our ability to differentiate between them. Interferon alfa induced complete hematologic remission in five of 14 patients; four of these remissions lasted more than 5 years. Only one of 26 patients developed blast crisis. The median survival of the 26 patients was 37 months. CONCLUSION: Patients with M-bcr rearrangement-negative CML fall into three morphologic subgroups. Disease evolution does not generally involve blastic transformation. Instead, patients show progressive organomegaly, leukocytosis, anemia, and thrombocytosis. Some patients in each subgroup can respond to interferon alfa.

Impact of systemic treatment on local control for patients with lymph node-negative breast cancer treated with breast-conservation therapy.

PURPOSE: To determine the impact of tamoxifen and chemotherapy on local control for breast cancer patients treated with breast-conservation therapy. PATIENTS AND METHODS: The data from 484 breast cancer patients who were treated with breast-conserving surgery and radiation were analyzed. Only patients with lymph node-negative disease were studied to provide comparative groups with a similar stage of disease and a similar competing risk for distant metastases. Actuarial local control rates of the 277 patients treated with systemic therapy (128, chemotherapy with or without tamoxifen; 149, tamoxifen alone) were compared with the rates for the 207 patients who received no systemic treatment. Only 10% of the patients had positive (2%), close (3%), or unknown margin status (5%). RESULTS: Patients treated with systemic therapy had improved 5-year (97.5% v 89.8%) and 8-year (95.6% v 85.2%) local control rates compared with those that did not receive systemic treatment (P =.004, log-rank test). There was no statistical difference in local control between patients treated with chemotherapy and patients treated with tamoxifen alone (P =.219). Systemic treatment, margin status, young patient age, estrogen and progesterone receptor status, and primary tumor size were analyzed in a Cox regression analysis. The use of systemic treatment was the most powerful predictor of local control: patients who did not receive systemic treatment had a relative risk of local recurrence of 3.3 (95% confidence interval, 1.5 to 7.5; P =.004). CONCLUSION: In this retrospective analysis, systemic therapy appears to contribute to long-term local control in patients with lymph node-negative breast cancer treated with breast-conservation therapy.

A model for regulation of the cell cycle incorporating cyclin A, cyclin B and their complexes.

A mathematical model for the cell cycle is proposed that incorporates the known biochemical reactions involving both cyclin A and cyclin B, the interactions of these cyclins with cdc2 and cdk2, and the controlling effects of cdc25 and weel. The model also postulates the existence of an as yet unknown phosphatase involved in the formation of maturation promoting factor. The model produces solutions that agree qualitatively with a wide variety of experimentally observed cell-cycle behaviour. Conditions under which the model could explain the initial rapid divisions of embryonic cells and the transition to the slower somatic cell cycle are also discussed.

Evidence for individual differences in the radiosensitivity of human skin.

Previously published clinical data have been re-analysed to investigate individual differences in the radiosensitivity of human skin. In the clinical studies, acute and late skin reactions were recorded for 254 breast cancer patients receiving radiotherapy to the internal mammary nodes following simple or modified radical mastectomy. Each patient was treated bilaterally with different fractionation schedules to the right and left fields. Patients were assigned prospectively to 10 different treatment groups of 11-35 patients each, with all patients in a group receiving the same pair of fractionation schedules to the right and left fields. In the present study, correlations between the skin reactions in the two treatment fields per patient were investigated. For each of three different endpoints--peak reflectance measure of erythema, peak acute skin reaction score, and a ranking measure of the progression rate of telangiectasia--significant correlations were found between the levels of skin injury to the right and left treatment fields of the patients in most treatment groups. Although there were correlations between the absorbed doses in the right and left fields, statistical analyses indicated that dose effects were not sufficient to explain fully the patient-to-patient differences in skin response. Thus, these data provide evidence for the existence of individual differences in the radiation response of human skin, both for early and late effects. Whether these differences are dominated by heterogeneity in intrinsic cell radiosensitivity or by other factors has yet to be determined. However, there was no clear evidence of a correlation between the acute and late endpoints, suggesting that the individual differences in radiosensitivity are not dominated by a common genetic component expressed equally in all cells.

Tests for the fit of the linear-quadratic model to radiation isoeffect data.

The linear-quadratic (LQ) model for cell survival is frequently extended to describe multifraction isoeffect data via the formula: ln(response) = -n(alpha d + beta d2), where d is the dose per fraction and n is the number of fractions. However, estimates of the quantity "alpha/beta" derived from such data are meaningless unless the use of the model is justified. Two methods are proposed for testing the fit of the multifraction LQ model to isoeffect data. If the use of the model cannot be rejected, each method also provides a new technique for estimating alpha/beta. The two methods are applied to published data from spleen, kidney, and colon. In each case, consistent results are obtained from the two methods concerning the quality of the fit.

The effect of patient-to-patient variability on the accuracy of predictive assays of tumor response to radiotherapy: a theoretical evaluation.

Mathematical modeling was used to investigate the relative accuracy that might be expected from predictive assays of tumor response to radiotherapy based on one of the following four tumor characteristics: intrinsic tumor-cell radiosensitivity, doubling time of the clonogenic tumor cells, number of clonogens in the tumor at the start of treatment, and extent of hypoxia in the tumor. In particular, the influence of inter-tumor heterogeneity on predictive accuracy was investigated. Wide patient-to-patient variability in a tumor characteristic contributing to treatment response adds to the accuracy of a predictive assay based on that characteristic, but variability from patient to patient in factors influencing response, but not measured by the assay, has a confounding effect and reduces predictive accuracy. The results of this theoretical study suggest that predictive assays based on intrinsic tumor-cell radiosensitivity are much more likely to be correlated with clinical outcome than are assays based on clonogen doubling time, hypoxic fraction, or clonogen number, since individual differences in tumor radiosensitivity can seriously confound assays based on the other factors. It is concluded that it may be necessary to correct for individual differences in intrinsic radiosensitivity before predictive assays based on other tumor characteristics, such as potential doubling time, might have any detectable clinical significance.

Flexure dose: the low-dose limit of effective fractionation.

Total radiation dose often can be increased without subsequent increases in the severity of tissue injury by using reduced doses per fraction. The flexure dose, df, is defined as the largest fractional dose for which further fractionation produces no significant change in the total dose required to reach a specified effect level. Thus, df is clinically relevant in that it represents the limit of effective dose fractionation. For those tissues in which injury reflects depletion of a critical proportion of target cells, the flexure dose is a measure of the extent of the initial, nearly linear portion of the dose-survival curve. More generally, the flexure dose is a measure of the extent of the initial, nearly linear portion of a dose-response curve in organized tissue, whatever its relationship to clonogenic target cells might be. Several quantitative expressions for df are derived. The characteristic common to these is that each defines the flexure dose as a multiple of the ratio alpha/beta of the parameters of the linear-quadratic model of cell survival or dose response, where the multiple is a measure of experimental or statistical resolution. These multiples tend to fall within a limited range, thereby defining the "region of flexure" via the inequality 0.05 (alpha/beta) less than or equal to df less than or equal to 0.15 (alpha/beta). Estimates of the region of flexure are presented for a variety of normal and neoplastic tissues.

Radiation fractionation sensitivity of epiphyseal cartilage in a weanling rat model.

Tibial growth at 100 days of age was measured in Sprague-Dawley rats treated at 21 days to the proximal tibia with various courses of fractionated radiation. In split-dose and multiple-fraction experiments, a minimum interval of 5-6 hr was required to achieve maximal sparing of growth arrest. Total doses required to reduce growth to 80% of untreated controls were computed from dose-response curves for fractionated radiation (dose/fraction 1.0-10 Gy). When fitted to a linear-quadratic model of radiation response the data described an estimated alpha/beta of 4.47 (95% C.I. (3.71, 5.23) Gy). This value suggests that the fractionation sensitivity of the epiphyseal plate is substantially greater than that of most neoplasms, predicting a favorable therapeutic gain with the use of hyperfractionated radiation therapy.

Residual radiation damage in murine lung assessed by pneumonitis.

The amount of radiation damage remaining in mouse lung has been assessed by retreatment from 1 to 6 months after a range of first doses. Pneumonitis at 196 days after retreatment was used as the endpoint. Lungs were first irradiated with a range of single doses (6-10 Gy). Ten Gy was the highest dose that, on its own, produced no changes in breathing rate or deaths due to pneumonitis. One to 6 months later lungs were retreated with a full range of single doses. Isoeffect doses were calculated for lethality for all retreatment times after each priming dose. The amount of residual damage remaining in the lung has been calculated as both a proportion of first doses and as the effect equivalent of remembered dose. Following a 10 Gy first dose, there was evidence of remembered irradiation injury at all retreatment intervals. After a 6 Gy priming dose, the lungs could be retreated to tolerance. The amount of residual damage was proportional to the size of first dose and was highest at 1 month (27% after 6 Gy and 70% after 10 Gy) and lowest after 3 months (0% after 6 Gy and 46% after 10 Gy). This partial recovery of lung function between 1 and 3 months was followed by an increase in amount of damage "remembered"; that is, a reduction in the retreatment dose that could be delivered. The proportion of residual damage after 10 Gy was never less than 25%. The data suggest an early target cell depletion and regeneration in the lung (within 3 months), the extent of which is dependent on the size of initial injury.

Spatial heterogeneity of the volume effect for radiation pneumonitis in mouse lung.

PURPOSE: In a previous study to determine the effect of partial volume irradiation on damage and morbidity from pneumonitis in mouse lung, a critical determinant of the volume effect was the spatial location of the irradiated subvolume within the lung. The goals of the present study were to (a) define the dose-volume effect curves for radiation pneumonitis in mouse lung, (b) define the threshold volume, and (c) further investigate the spatial heterogeneity of the radiosensitivity of mouse lung. METHODS AND MATERIALS: Eight fractional volumes ranging from 94% to 17% of the lungs of C3Hf/Kam mice were irradiated with single doses ranging from 12 to 22 Gy, depending on the volume irradiated. The fractional volumes irradiated were determined from computed tomographic scans of mouse lung. To determine the effect of location of irradiated subvolume, equivalent volumes in the base and the apex were irradiated by shielding the prescribed adjacent volume in the apex or base respectively. Dose-response curves of breathing rate at 22 weeks and lethality at 28 weeks were constructed for each subvolume irradiated in the apex or base and fitted by logit analysis, and ED50s and LD50s with 95% confidence limits obtained, respectively. Lungs from dead mice or mice sacrificed when moribund were examined for histologic signs of pneumonitis. RESULTS: Irradiation of any of the eight subvolumes in the base yielded a consistently lower isoeffect dose for both assays of radiation pneumonitis than if the same irradiated subvolume was located in the apex. Plots of isoeffect dose for breathing rate as a function of subvolume irradiated in the base or apex showed that these curves were not linear but exhibited a plateau between irradiated volumes of 70% and 80% in both the apex and base. A similar curve was obtained for lethality and volume irradiated in the base. A threshold volume, i.e., irradiation of that volume that should produce no changes in breathing rate or mortality, was dependent on the location of the irradiated subvolume. CONCLUSION: The response of mouse lung to partial volume irradiation is heterogeneous and is critically dependent on the specific location of the irradiated subvolume in the lung, i.e., a given subvolume in the base is consistently more sensitive than the same subvolume in the apex using either breathing rate or lethality as assays of radiation pneumonitis. We suggest that this heterogeneity is due to the anatomy of the tracheobronchial tree, i.e., to the distribution of non-gas exchange-conducting airways in the irradiated volume. These data have implications for the modeling of dose-volume effects in the lung and the prediction of normal tissue complication probabilities for radiation pneumonitis in humans.