Optimum dose range for the amelioration of long term radiation-induced hyposalivation using prophylactic pilocarpine treatment.

BACKGROUND: To determine dose and time dependency of pilocarpine pre-treatment protection from late damage after unilateral irradiation of the rat parotid gland. METHODS AND MATERIALS: The right parotid gland of saline (1mg/ml) or pilocarpine (4 mg/kg) pre-treated rats was irradiated with 10, 15 and 20 Gy. Saliva was collected from the irradiated and shielded parotid before, 30, 60, 120 and 240 days after irradiation. The number of acinar cells/gland was determined 30, 120 and 240 days after irradiation by histological examination. RESULTS: Pilocarpine pre-treated rats, protection of parotid gland function was seen in the early-intermediate phase (0-120 days) after 15 Gy and in the late phase (>120 days) after 10 and 15 Gy. Although no protection was observed after 20 Gy, a stimulatory effect of pilocarpine on the non-irradiated gland resulted in a significant increase in total saliva secretion. The increase in function after pilocarpine treatment was paralleled by a significant increase in the number of acinar cells in both the irradiated and shielded glands. CONCLUSIONS: Pre-irradiation treatment with pilocarpine induces compensatory response, at lower doses, in the irradiated and at higher doses in the non-irradiated gland reducing late damage, due to stimulation of unirradiated or surviving cells to divide.     

Comparison of radiosensitivity of rat parotid and submandibular glands after different radiation schedules.

BACKGROUND AND PURPOSE: To investigate the radiosensitivity of rat parotid and submandibular gland functioning after local single dose, conventional fractionated and accelerated fractionated irradiation. METHODS: The salivary glands of male albino Wistar rats were locally irradiated with a single dose (15 Gy) or a calculated (alpha/beta; 9.6) biological effective dose of fractionated irradiation equal to this, viz. conventional fractionation (32 Gy in 16 fractions of 2 Gy/day, five times/week) or accelerated fractionation (32 Gy in 16 fractions of 2 Gy, two fractions/day). Parotid and submandibular/sublingual saliva samples were collected by means of miniaturized Lashley cups before and up to 240 days after irradiation. Salivary flow rate, lag phase and amylase secretion were used as parameters for the assessment of salivary gland function. At the end of the experiments the animals were sacrificed and the glands processed for histopathological examination. RESULTS: Up to 120 days after irradiation no differences were observed between the glands or between the different irradiation schedules. Beyond 120 days, however, the parotid gland performed better in flow rate and lag phase after fractionated irradiation, when compared to the submandibular gland. The observed differences in function corresponded with the observed late histopathological changes. The parotid gland contained more acinar cells and had a higher gland weight. No differences were observed between both fractionation schedules on each gland. CONCLUSIONS: The main observation from this study is the higher radiosensitivity of the submandibular gland compared to the parotid gland for late effects after fractionated irradiation. This may have implications for the treatment planning in case of radiotherapy for head and neck cancer.     

Volume effects and region-dependent radiosensitivity of the parotid gland

PURPOSE: To detect volume effects and possible regional differences in radiosensitivity of the rat parotid gland. METHODS AND MATERIALS: Parotid glands of male albino Wistar rats were locally X-irradiated, with collimators with conformal radiation portals used to supply 100% volume and 50% cranial/caudal partial volumes. High-resolution magnetic resonance imaging was used to provide the outlines of the parotid glands. Single doses of up to 40 Gy were applied, and the effects on saliva secretion, measured with the aid of miniaturized Lashley cups, were followed up to 365 days after the irradiation. RESULTS: Under conditions of equal mean absorbed doses and small variations in dose distribution, a pertinent volume effect was observed for late but not for early radiation damage. The late effects were different for the cranial part as compared with the caudal part of the parotid gland. The reduction in flow rate was much more severe after irradiation in the cranial part. After a single dose of 30 Gy, the reductions in flow rates were approximately 65% and 25% for the cranial and caudal parts, respectively. At that dose, no saliva flow was observed after irradiation of 100% of the gland. CONCLUSION: From the rat model studies presented, it is concluded that late radiation damage after partial irradiation of parotid glands shows region-dependent volume effects. This finding is expected to be relevant to the radiosensitivity of human salivary glands, and it implies that the predictive power of the mean dose concept in radiotherapeutic practice is limited. The finding of region-dependent late radiation damage also challenges the basic assumptions of most current normal tissue complication probability models for parotid gland function.     

The function of the parotid gland following radiation therapy for head and neck cancer

The parotid gland was selected for study of its salivary output before and after radiation therapy for head and neck cancer. Before radiation therapy, a sialogram of the parotid gland was performed with the patient's head positioned for radiation therapy; a lateral radiographic view of the parotid gland was used to compare with the radiation treatment portal to determine the portion of the parotid gland to he irradiated. Samples of stimulated saliva were collected from the parotid gland before and at 1 and 6 months post-radiation. Eighteen patients with head and neck cancer who received radiation therapy were studied. The data showed that in the irradiation of nasopharyageal, advanced oropbaryngeal and Waldeyer's ring lesions, 100% of the parotid gland was irradiated; for the early oropharyageal and hypopbaryugeal lesions, from 30 to 90% of the parotid gland was irradiated and for the supragiottic and oral cavity lesions, 25–30% of the parotid gland was irradiated. When 100% of the parotid gland was irradiated, no saliva was produced at 1 month post-radiation; this remained the same when re-tested at 4–8 months, however, when any portion of the parotid gland was not irradiated, there was residual salivary function.     

Effects of single doses of X-rays on renal function in unilaterally irradiated pigs

The right kidney of 13 Large White female pigs was irradiated with single doses of 250 kV X-rays in the range 7-12.6 Gy. Sequential measurements of individual kidney glomerular filtration rate (GFR) and effective renal plasma flow (ERPF) were carried out by means of 99mTc-DTPA and 131I-hippuran renography for periods up to 24 weeks after irradiation. GFR levels increased in irradiated and unirradiated contralateral kidneys 2 weeks after treatment compared with age-matched controls. ERPF values exhibited a small increase in a proportion of animals. Renal function then declined in irradiated kidneys in a dose-dependent manner. A dose of 7 Gy resulted in a decline followed by subsequent recovery. After doses of greater than or equal to 8.8 Gy GFR and ERPF declined rapidly, reaching minimal levels by 6-12 weeks, the time depending on the dose. The reduction in ERPF was quantitatively greater than that for GFR. In animals receiving greater than 8.8 Gy the irradiated kidney contributed in the order of only 10% of the total ERPF. The reduction in GFR resulted in a prompt functional compensatory response in GFR in the unirradiated contralateral kidney. In terms of ERPF, a compensatory response was not evident until weeks 20-24. The results indicated that the radiation tolerance dose of the pig kidney following unilateral irradiation with single doses of X-rays was approximately 8 Gy.     

Regeneration of Parotid Acinar Cells After High Radiation Doses: A morphological study in rat

The acute and late effects of fractionated irradiation on rat parotid gland acinar cells were studied by light and electron microscopy. At 10 days after the last irradiation session (6 Gy or 9 Gy daily during five consecutive days) no effects were seen. At 180 days, minor loss of acini was detectable after a total dose of 30 Gy. After 45 Gy a massive acinar loss was seen at that time; the number of acini had diminished and minor duct-like structures and scattered amounts of fibrous stroma dominated the slides. The remaining acini were disorganized and usually larger compared with the control side and to non-irradiated animals. The acinar cells appeared larger than in the controls. The ducts were better preserved but the intercalated ducts often seemed to be larger than normal. We suggest that this phenomenon indicates a remaining capacity of the parotid gland to regenerate acinar cells even after high radiation doses.     

The effects of radiation on parotid salivary function

Postoperative electron beam irradiation of patients with parotid cancer has been used regularly at the Mallinckrodt Institute of Radiology to spare the opposite parotid and to preserve salivary function. Only anecdotal reports of amount of radiation required to ablate salivary function exist. To establish a dose-response curve for the human parotid, selective measurements of right and left parotid salivary flow were done for 15 age-matched control patients whose parotids were not irradiated, 17 patients who had both parotids irradiated, and 12 whose parotids were irradiated by unilateral electron beam technique. Point calculations of absorbed dose 1 cm below the surface were done for all 88 parotids and correlated with stimulated parotid salivary flow, pH, and secretory IgA (SIgA). Increasing doses of radiation resulted in progressive reduction of parotid salivary flow, pH and SIgA. The technique, dosimetry, and clinical application of unilateral electron beam irradiation to spare the opposite parotid will be discussed.     

Secondary radiation damage as the main cause for unexpected volume effects: a histopathologic study of the parotid gland

PURPOSE: To elucidate with a histopathological study the mechanism of region-dependent volume effects in the partly irradiated parotid gland of the rat. METHODS AND MATERIALS: Wistar rats were locally X-irradiated with collimators with conformal radiation portals for 100% volume and 50% cranial/caudal partial volumes. Single doses up to 40 Gy were applied. Parotid saliva samples were collected, and the three lobes of the parotid gland were examined individually on the macro- and micromorphologic level up to 1 year after irradiation. RESULTS: Dose-dependent loss of gland weight was observed 1 year after total or partial X-irradiation. Weight loss of the glands correlated very well with loss of secretory function. Irradiating the cranial 50% volume (implicating a shielded lateral lobe) resulted in substantially more damage in terms of weight loss and loss of secretory function than 50% caudal irradiation (shielding the ventral and dorsal lobe). Histologic examinations of the glands 1 year after irradiation revealed that the shielded lateral lobe was severely affected, in contrast to the shielded ventral and dorsal lobes. Time studies showed that irradiation of the cranial 50% volume caused late development of secondary damage in the shielded lateral lobe, becoming manifest between 240 and 360 days after irradiation. The possible clinical significance of this finding is discussed. CONCLUSION: It is concluded that the observed region-dependent volume effect for late function loss in the rat parotid gland after partial irradiation is mainly caused by secondary events in the shielded lateral lobe. The most probable first step (primary radiation event) in the development of this secondary damage is radiation exposure to the hilus region (located between the ventral and dorsal lobe). By injuring major excretory ducts and supply routes for blood and nerves in this area, the facility system necessary for proper functioning of the nonexposed lateral lobe is seriously affected. The unexpected volume effect in the rat might have consequences for treatment strategies in radiotherapy, implicating not only salivary glands but also other organs with a seemingly homogeneous distribution of radiosensitive elements, a situation wherein volume effects have not been anticipated up to now.     

Radioprotection by WR-2721 of gamma-irradiated rat parotid gland: effect on gland weight and secretion at 8-10 days post irradiation

Changes in rat parotid salivary gland weight and functional parameters were evaluated at 8 to 10 days post irradiation in WR-2721 protected and non-protected animals following exposure to a single 15.3 Gy dose of Cs-137 radiation to the head. Glandular fluid secretory capacity was assessed by maximum flow rate, total volume of saliva and duration of secretion following pilocarpine stimulation. Protection against radiomucositis was also evaluated indirectly by daily monitoring of food and water intake, body weight and paraoral symptomatology. WR-2721 provided a significant degree of protection for all glandular functional parameters as well as gland weight. Relative protective factors (RPF) were computed for irradiated protected and non-protected animals compared to their sham-irradiated, pair-fed controls. The calculated RPFs were: Gland weight 1.9, maximum flow rate 2.9, volume of saliva 2.1 and duration of secretion 2.1 for a mean "relative protection" of 2.25. Substantial protection against radiomucositis in protected animals was evident by a progressive gain in body weight and lack of oral signs and symptoms as compared to non-protected animals. Protection against radiomucositis and preservation of residual parotid gland secretory capacity as determined by functional parameters suggests that WR-2721 may be of significant benefit in alleviating oral symptoms and maintaining salivary gland function for patients receiving radiotherapy for head and neck tumors.     

Increase in mast cells and hyaluronic acid correlates to radiation-induced damage and loss of serous acinar cells in salivary glands: the parotid and submandibular glands differ in radiation sensitivity.

The detailed mechanisms which can explain the inherent radiosensitivity of salivary glands remain to be elucidated. Although DNA is the most plausible critical target for the lethal effects of irradiation, interactions with other constituents, such as cell membrane and neuropeptides, have been suggested to cause important physiological changes. Moreover, mast cells seem to be closely linked to radiation-induced pneumonitis. Therefore, in the present study the effects of fractionated irradiation on salivary glands have been assessed with special regard to the appearance of mast cells and its correlation with damage to gland parenchyma. Sprague-Dawley strain rats were unilaterally irradiated to the head and neck with the salivary glands within the radiation field. The irradiation was delivered once daily for 5 days to a total dose of 20, 35 and 45 Gy. The contralateral parotid and submandibular glands served as intra-animal controls and parallel analysis of glands was performed 2, 4, 10 or 180 days following the last radiation treatment. Morphological analysis revealed no obvious changes up to 10 days after the irradiation. At 180 days a radiation dose-dependent loss of gland parenchyma was seen, especially with regard to serious acinar cells in parotid gland and acinar cells and serous CGT (convoluted granular tubule) cells in the submandibular gland. These changes displayed a close correlation with a concomitant dose-dependent enhanced density of mast cells and staining for hyaluronic acid. This cell population seems to conform with the features of the connective tissue mast cell type. The parotid seems to be more sensitive to irradiation than the submandibular gland. Thus, the present results further strengthen the role of and the potential interaction of mast cells with radiation-induced tissue injury and alterations in normal tissue integrity.     

Individual radiation response of parotid glands investigated by dynamic 11C-methionine PET.

BACKGROUND AND PURPOSE: Previously, we showed that the net metabolic clearance of 11C-methionine of the parotid gland, K, calculated from dynamic 11C-methionine PET, can be used as a measure of parotid gland function. The aim of this study was to investigate by dynamic 11C-methionine PET the individual radiation dose response relationship of parotid glands in head and neck cancer patients. PATIENTS AND METHODS: Twelve head and neck cancer patients were examined by dynamic 11C-methionine PET after radiotherapy. Parametric images of K were generated, co-registered and compared voxel-by-voxel with the 3D radiation dose plan within the parotid gland to assess the individual radiation dose-function relationship. RESULTS: In each patient, voxel-values of K decreased with increasing radiation dose. Population based analysis showed a sigmoid dose response relationship of parotid gland, from which we estimated a threshold radiation dose of 16 Gy and a mean TD50 of 30 Gy. TD50 ranged from 7 to 50 Gy in the group of patients. CONCLUSIONS: Individual radiation dose response of parotid glands can be measured by dynamic 11C-methionine PET. The dose response analysis revealed a sigmoid relationship, a threshold radiation dose of 16 Gy, and a mean TD50 of 30 Gy.     

The impact of dose on parotid salivary recovery in head and neck cancer patients treated with radiation therapy

PURPOSE: A common side effect experienced by head and neck cancer patients after radiation therapy (RT) is impairment of the parotid glands' ability to produce saliva. Our purpose is to investigate the relationship between radiation dose and saliva changes in the 2 years after treatment. METHODS AND MATERIALS: The study population includes 142 patients treated with conformal or intensity-modulated radiotherapy. Saliva flow rates from 266 parotid glands are measured before and 1, 3, 6, 12, 18, and 24 months after treatment. Measurements are collected separately from each gland under both stimulated and unstimulated conditions. Bayesian nonlinear hierarchical models were developed and fit to the data. RESULTS: Parotids receiving higher radiation produce less saliva. The largest reduction is at 1-3 months after RT followed by gradual recovery. When mean doses are lower (e.g., <25 Gy), the model-predicted average stimulated saliva recovers to pretreatment levels at 12 months and exceeds it at 18 and 24 months. For higher doses (e.g., >30 Gy), the stimulated saliva does not return to original levels after 2 years. Without stimulation, at 24 months, the predicted saliva is 86% of pretreatment levels for 25 Gy and <31% for >40 Gy. We do not find evidence to support that the overproduction of stimulated saliva at 18 and 24 months after low dose in 1 parotid gland is the result of low saliva production from the other parotid gland. CONCLUSIONS: Saliva production is affected significantly by radiation, but with doses <25-30 Gy, recovery is substantial and returns to pretreatment levels 2 years after RT.     

Solitary extramedullary plasmacytoma in the oropharynx: advantages of intensity-modulated radiation therapy

We report a case of a 59-year-old man with solitary extramedullary plasmacytoma in his oropharynx. Because the diagnosis is rare and there is only limited experience in the literature based on retrospective data, the optimal planning target volume and optimal dose of radiation therapy (RT) are still controversial. The frequently discussed problem is the necessity of first echelon lymph node irradiation because it is associated with a higher rate of complications such as xerostomia caused by damage to salivary glands. In order to prevent late toxicity, intensity-modulated RT with the use of simultaneous integrated boost and parotid salivary gland sparing was used in this patient's treatment. The RT was performed in 23 identical fractions, the primary tumor region was irradiated with a dose of 46 Gy and the first echelon lymph node region with the risk of subclinical disease with a dose of 41.4 Gy; the dose per fraction was 2 Gy and 1.8 Gy, respectively. The patient is alive and well > 20 months after the irradiation, without any evidence of disease. Parotid gland function remained intact, and no xerostomia occurred. This is the first report of the use of intensity-modulated RT with parotid gland sparing in the treatment of solitary extramedullary plasmacytoma in the head and neck region.     

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.     

Skin sensitization by misonidazole: a demonstration of uniform mild hypoxia.

Skin reactions on irradiated mouse feet were used to measure the radiosensitization of normal tissues by misonidazole (MISO). Fractionation schedules of 1, 2, 5 and 10 daily doses of X-rays were combined with either 100 mg/kg or 670 mg/kg MISO. When unanaesthetized mice were irradiated in air, significant sensitization was observed with both the high and low drug doses, in all fractionation schedules. There was no decrease in sensitization with fractionation, even using fractions as small as 5 Gy. This indicates that many of the cells in mouse skin may be marginally hypoxic, and that sensitization at low doses is possible. Irradiation in O2 without MISO rendered the skin more sensitive to X-rays than in air. MISO given 30 min before single doses of radiation further sensitized the skin, but for 10 fractions in O2 no MISO sensitization was detected. There was little evidence for cytotoxic killing in skin by MISO. Repair of radiation damage was slightly reduced when MISO was present, during or after irradiation.     

The changes in irradiated salivary gland function of patients with head and neck tumors treated with radiotherapy

BACKGROUND: To investigate and analyze changes in irradiated salivary gland function of patients with head and neck tumors treated with radiotherapy. METHODS: Thirty-seven patients with head and neck tumors, who received 40-70 Gy of irradiation to all major salivary glands, were analyzed. The weights of saliva secreted for 10 minutes at rest, and for 5 minutes with vitamin C stimulation, were measured. The salivary gland function was defined by the weight of saliva. RESULTS: With vitamin C stimulation, the weight of saliva in patients whose doses were < or =50 Gy, was significantly higher than that of patients whose doses were > or = 58 Gy (2.48 +/- 0.33 g vs. 0.73 +/- 0.18 g, P = 0.0003). When doses administered to salivary glands were < or =50 Gy, the stimulated saliva secretion recovered over time, after irradiation. However, when the doses of irradiation were > or = 58 Gy, there was no recovery in saliva secretion even after a few years. Multiple regression analysis showed that age and chemotherapy may not affect salivary gland function even years after radiotherapy. CONCLUSION: When salivary glands were irradiated with doses < or =50 Gy, gradual recovery of salivary gland function was observed over time, whereas there was no significant recovery when the irradiation dose was >58 Gy.     

The radiation dose-response relationship in a human glioma xenograft and an evaluation of the influence of glutathione depletion by buthionine sulfoximine.

We have used an extensively characterized human glioma cell line in an athymic mouse model to evaluate new therapeutic approaches for human supratentorial high grade gliomas. The tumor, D-54MG, is a subline of a human anaplastic glioma. Eight days after homozygous nu/nu BALB/c athymic mice received intracranial (IC) injections of a tumor homogenate, the whole brain was irradiated with either single fractions of 4, 8, 9, and 12 Gy or twice daily fractions, separated by least 6 hr, of 2.28 Gy x 2 or 7.53 Gy x 2. To evaluate whether or not glutathione depletion influenced animal survival, animals at each dose level received either intraperitoneal (IP) buthionine sulfoximine (BSO) alone or I.P. BSO plus BSO in the drinking water. There was a stepwise prolongation of animal survival with increasing doses of external beam radiation. Mean survival in 9 of the 10 control groups (8-12 animals per group) ranged from 14.1 to 18.8 days. Mean survival ranged from 15.3 to 22.5 days at 4 Gy, 25 to 30 days at 8 Gy, 22.3 to 29.7 days at 9 Gy, and 32.9 to 33.6 days at 12 Gy single dose irradiation. At 2.28 Gy x 2 split dose irradiation mean survival was 29.3 days, for 7.53 Gy x 2 mean survival was over 47 days. The data for single fraction irradiation fit a linear regression line (r = 0.908) of mean animal survival = (1.22 [dose in Gy] + 16.7) days. Tumor GSH levels were decreased with all BSO dosing regimens tested. The most aggressive regimen (I.P. BSO+oral BSO for 5 days), reduced tumor GSH to 6.2% of control. Increased survival in irradiated glutathione depleted mice versus mice receiving radiation alone was not seen.     

鼻咽癌面颈联合野放疗对唾液腺功能影响的临床分析

目的 :利用放射性核素99mTcO4-动态显像唾液腺定量测定鼻咽癌面颈联合野放疗前、中、后唾液腺功能的变化并探讨与放疗剂量之间的关系。方法 :2 0 0 3年 2月 1日～ 2 0 0 3年 10月 3 1日 ,分别对 2 0例鼻咽癌面颈联合野放疗患者于放疗前、放疗至 10、3 6～ 40、70Gy时进行99mTcO4-动态显像定量测定其唾液腺(腮腺、颌下腺 )摄锝率 (UR)、泌锝率 (ER)变化 ,同时观察其口干程度进行临床分级。结果 :2 0例鼻咽癌面颈联合野放疗剂量为 10和3 6～ 40Gy时 ,其泌锝率明显低于放疗前 ,P <0 0 5 ,在放疗 70Gy时降到最低 ,P <0 0 1。与临床观察到的口干严重程度一致 ,而其摄锝率在 3 6～ 40Gy照射以前变化不明显 ,无统计学意义。结论 :鼻咽癌面颈联合野照射患者放疗前无明显唾液腺功能障碍 ,随着放疗剂量的增加 ,唾液腺功能明显下降 ,其ER较UR下降明显 ,在确保患者放疗疗效的同时 ,应尽可能提高放疗技术 ,减少唾液腺照射剂量和放疗体积 ,保护唾液腺功能 ,以提高患者的生活质量。     

Rat Salivary Gland Function after Fractionated Irradiation

The aim of this study was to investigate longitudinal effects of fractionated irradiation, with various total doses, on salivary gland function in the rat. Female Sprague-Dawley rats were irradiated with 4, 5, 6, 7 or 8 Gy per day on five consecutive days. Irradiation was given to the head and neck region. Whole saliva was collected before and 2, 15 and 26 weeks after irradiation. In general the effects of irradiation on salivary gland function were found to be related to dose and time after exposure. Secretion rates were significantly decreased two weeks after irradiation with doses of 30 Gy or higher, after 15 weeks with 25 Gy or higher, and after 26 weeks with 20 Gy or higher. Response patterns to irradiation differed between the salivary constituents. Thus, the conclusions from this study are that early and late effects display different patterns and that the model used to study variations in salivary, gland function after fractionated irradiation must be adjusted to the question addressed.     

Dose-response relationships within the parotid gland after radiotherapy for head and neck cancer.

BACKGROUND AND PURPOSE: To determine the salivary function, after parotid-sparing radiotherapy (RT), of different regions within the parotid gland and to evaluate dose-function relationships within the parotid glands and between patients. PATIENTS AND METHODS: Sixteen head and neck cancer patients, irradiated between September 1999 and November 2000 using a conformal parotid-sparing technique, were included in this study. Before RT and 7 months after RT (range 6-10 months), a salivary gland scintigraphy was performed in all patients combined with a single photon emission computed tomography (SPECT). The salivary excretion fraction (SEF) was measured, after stimulation, in 8-12 transverse 5mm SPECT slices of each parotid. Loss of salivary excretion fraction (dSEF %) of these slices was calculated as the proportion of SEF after RT as compared to SEF before RT. Since the planning CT-scan and the SPECT-scintigraphy were performed in the same treatment position, the dose to a transverse slice within the parotid gland could be matched to the loss of salivary excretion fraction of that respective slice. A non-linear model was fitted to the dose-loss of function data and the dose resulting in 50% loss of salivary excretion fraction (D50) was calculated. RESULTS: Before RT, all but one patient presented with normal salivary excretion fractions (SEF) of both parotid glands. Within the same parotid gland, the SEF's of the different slices were almost equal. Seven months after RT, the reduction in SEF was statistically significant (P-value<0.0001). A significant difference in loss of salivary excretion fraction (dSEF) was also observed between both parotid glands (P<0.0001) as a result of the parotid-sparing technique. When plotting the dSEF of a slice versus the dose given to that slice, doses as low as 10-15 Gy could result in a serious loss of function (dSEF>50%). After fitting a non-linear model to these plots, the mean dose resulting in 50% loss of salivary excretion fraction (D50) 7 months after RT was 22.5 Gy. A large inter-patient variability was found in D50. CONCLUSIONS: Salivary SPECT is a useful tool for the evaluation of the salivary function of different slices within the parotid gland. Before irradiation, the different slices within one parotid gland act as functional sub-units contributing equally to the function of the entire gland. Seven months after an average dose of 22.5 Gy (D50) the functional sub-unit has lost 50% of its excretion fraction. The high inter-patient variability in D50 and the observation that low doses (10-15 Gy) can induce serious loss of function should prompt us in the clinic to reduce the dose to the parotids even lower than the threshold of 22.5 Gy.