Use of diffusion-weighted magnetic resonance imaging (DW-MRI) to investigate the effect of chemoradiotherapy on the salivary glands

Feasibility of DW-MRI analysis of salivary glands during head and neck radiotherapy

INTRODUCTION

With no effective treatment for xerostomia, there remains an unmet need to reduce radiation induced toxicity. Measuring physiological changes during RT in salivary glands using DW-MRI may predict which patients are most at risk of severe toxicity. This study evaluated the feasibility of measuring apparent diffusion coefficient (ADC) in the major salivary glands and describes the observed changes in volume and ADC during RT.

METHODS

Scans were acquired at baseline (MR_base) and after 10 fractions (MR_rpt). Sequences included T1 post contrast fat saturated (T1PCFS) and DW-MRI (5b values, 0-1000 s/mm2). Ipsilateral and contralateral parotid (iPG/cPG), submandibular (iSMG/cSMG) and sublingual glands (iSLG/cSLG) were delineated on T1PCFS, modified on b0 and copied to the ADC map.

RESULTS

31 patients with intermediate/high risk squamous cell carcinoma (SCC) of the oropharynx were evaluated. On 124 scans, SMG and SLG delineations were successful on all; parotids were fully contoured in 90.7%. Baseline mean ADC were significantly different between each gland type (p < 0.0001). IPG and cPG volume decreased during treatment by 6.7% and 11.2%. ISMG, cSMG, iSLG and cSLG volume increased by 6.9, 0.9, 60.8 and 60.3% respectively. All structures showed an increase in mean_ADC values. For each gland the increase in ADC was statistically significant p < 0.0001. A smaller mean percentage increase in ADC was observed in the group experiencing a higher grade (2 or > ) of toxicity.

CONCLUSION

It is feasible to measure volume and ADC of the salivary glands prior to and during RT for HNC. Early data suggests a lower rise in ADC during treatment is associated with more severe late xerostomia.

Apparent diffusion coefficient measurement of the parotid gland parenchyma

The measurements of apparent diffusion coefficient (ADC) with diffusion weighted magnetic resonance imaging (DW-MRI) is becoming a popular diagnostic and research tool for examination of parotid glands. However, there is little agreement between the reported ADC values of the parotid gland in published literature. In this review 43 studies on ADC measurement of the parotid glands were included. The analyses indicated several possible culprits of the observed ADC discrepancies. For example, DW-MRI examinations under gustatory stimulation gives higher ADC values compared to the unstimulated parotid gland (P=0.003). The diffusion weighting factors (b-values) can either increase (b-value <200 s/mm²) or decrease ADC values (b-values >1,000 s/mm²). The timing of follow-up DW-MRI after radiotherapy (RT) indicates correlation to the found ADC values (R² =0.39). Interestingly, the choice of regions of interest (ROI) appears not to affect the measurements of ADC (P=0.75). It can be concluded that there is a critical need for standardization of ADC measurement of the parotid glands to allow valid inter-study comparisons and eventually to reach consensus on the use of ADC as biomarker.

Diffusion-weighted imaging as a follow-up modality for evaluation of major salivary gland function in nasopharyngeal carcinoma patients: a preliminary study

PURPOSE

To investigate the value of diffusion-weighted imaging (DWI) in assessing dynamic changes of major salivary gland function during follow-up post radiotherapy (RT) in nasopharyngeal carcinoma (NPC) patients.

MATERIALS AND METHODS

31 consecutive patients with pathologically confirmed NPC scheduled for RT underwent six routine follow-up MRI examinations including DWI sequence prior to (pre-RT) and 1, 3, 6, 9, and 12 months post RT. Mean apparent diffusion coefficient (ADC) values of bilateral parotid glands (PGs) and submandibular glands (SMGs) were measured. Objective measurement of salivary flow rate (SFR) under unstimulated (uSFR) and stimulated conditions (sSFR) as well as subjective xerostomia assessment according to a patient-rated questionnaire were conducted before each MRI. Variance analysis was used to evaluate dynamic changes of ADC, SFR and xerostomia questionnaire summary scores (XQ-sum) at different timepoints and the correlation between ADC and XQ-sum. Pearson's correlation test was used to evaluate the correlations between pre- and post-RT changes of ADC (ΔADC) and SFR (ΔSFR) or mean RT dose.

RESULTS

At each timepoint, ADCs of PGs were significantly lower than of SMGs, uSFR was significantly lower than sSFR. For both PGs and SMGs, ADC_post-RT were all higher than ADC_pre-RT, with significant differences. ADC_1m-post-RT initially increased and changed little to ADC_3m-post-RT, ADC_6m-post-RT, ADC_9m-post-RT, and ADC_12m-post-RT, then gradually declined over time. The dynamic change trends of SFR were negatively paralleled to those of ADC, while that of XQ-sum was similar. Dose-response relationships were detected between salivary gland mean RT dose and ΔADC. In PGs, negative correlations between ΔsSFR_9m-post-RT and ΔADC_9m-post-RT, and ΔsSFR_12m-post-RT and ΔADC_12m-post-RT were detected. In SMGs, negative correlations between ΔsSFR_12m-post-RT and ΔADC_12m-post-RT, and ΔuSFR_12m-post-RT and ΔADC_12m-post-RT were also detected. The ADCs of patients with severe subjective xerostomia were significantly higher, while patients with moderate subjective xerostomia presented a tendency toward higher ADCs compared to those with mild xerostomia from 6 to 12 months post RT.

CONCLUSION

As part of routine follow-up MRI in NPC patients, DWI might be a promising modality for follow-up assessing the dynamic changes of major salivary gland function and might be more powerful in the late post-RT period.

Repetitive MRI of organs at risk in head and neck cancer patients undergoing radiotherapy

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First review on MRI changes in head and neck organs at risk during radiotherapy.

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Focus on dynamics in salivary gland, muscle and bone in the head and neck region.

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Pointing out the limitations in implementing MRI in guiding radiation therapy.

With emerging technical advances like real-time MR imaging during radiotherapy (RT) with an integrated MR linear accelerator, it will soon be possible to analyze changes in the organs at risk (OARs) during radiotherapy without additional effort for the patients. Until then, patients have to undergo additional MR imaging and often without the same immobilization devices as used for radiotherapy. Consequently, studies with repetitive MRI during the course of radiotherapy are rare, with low patient numbers and with the challenge of registration between the different MR sequences and the varying imaging time points.

This review focuses on studies with at least two MRIs, one before and another either during or post-RT, in order to report on RT-induced changes in normal tissues and their correlation with toxicity. We therefore included clinical studies published in English until March 2019, with repetitive MRI of OARs in head and neck cancer patients receiving external beam radiotherapy. OARs analyzed were salivary glands, musculoskeletal structures and bones. MR sequences used included T1, T2, dynamic contrast enhanced (DCE) imaging, diffusion-weighted imaging (DWI), DIXON and MR sialography.

Toward a more patient-specific model of post-radiotherapy saliva secretion for head and neck cancer patients

BACKGROUND

Reduction of saliva secretion is a common side effect following radiotherapy (RT) for cancer of the head and neck region. The aim of this study is to predict the post-RT salivary function for individual patients prior to treatment and to recognise possible differences in individual radiosensitivity.

MATERIAL AND METHODS

A predictive model for post-RT salivary function was validated for 64 head and neck cancer patients. The input parameters for the model were salivary excretion fraction (sEF) measured by 99mTc-pertechnetate scintigraphy, total stimulated salivary flow and mean absorbed dose for the major salivary glands. SEF values after RT relative to the baseline before RT (rEF) were compared among the patients using the distance ΔrEF between single gland rEF and the corresponding expected value at the dose response curve.

RESULTS

A significant correlation (R = 0.86, p = 0.018) was found between the modelled and the measured values of stimulated salivary flow six months after RT. The average prediction error for the saliva flow rate was 6 ml/15 min. A linear relationship between ΔrEF for the left and the right parotid glands was observed both six (R = 0.53) and 12 (R = 0.79) months after RT. The average of absolute values of ΔrEF was 0.20 for parotid glands and 0.22 for submandibular glands.

CONCLUSIONS

The salivary flow model was validated for 64 patients. The results imply, that one explanation for the discrepancies between the predicted and the measured salivary flow rate values and the common variations found in ΔrEF for the parotid glands may be differences in patients' individual response to radiation. However, quantitative extraction of individual radiosensitivity would require further studies in order to take it into account in predictive models.

The effect of region of interest strategies on apparent diffusion coefficient assessment in patients treated with palliative radiation therapy to brain metastases

BACKGROUND

Although diffusion-weighted magnetic resonance imaging (DW-MRI) is widely used in radiation therapy (RT) response studies, no standard of delineating the region of interest (ROI) exists. In this retrospective study, we evaluate the effect of four ROI strategies on the apparent diffusion coefficients (ADC) in patients receiving palliative RT to brain metastases.

MATERIAL AND METHODS

Twenty-two metastases from nine patients, treated with whole-brain irradiation (30 Gy in 10 fractions) were analyzed. Patients were scanned with a 1T MR system to acquire DW- (eight b-values), T2*W-, T2W- and T1W scans, before start of RT (pre-RT) and at the 9th/10th fraction (end-RT). The following ROI strategies were applied. ROIb800 and ROIb0: Entire tumor volume visible on DW(b = 800 s/mm(2)) and DW(b = 0 s/mm(2)) images, respectively. ROIb800vi: Viable tumor volume based on DW(b = 800 s/mm(2)). ROIb800rep: ROIb800 from pre-RT scan replicated to end-RT scan. Delineations were aided by co-registered T1W, T2W and T2*W images. ADC was estimated with two mono-exponential fits and one bi-exponential fit.

RESULTS

Differences in absolute ADC values were non-significant across ROI strategy independent of fitting method, while significantly different between fitting methods. Evaluation of individual metastases showed that ROI strategies disagreed on the relative ADC change (from pre-RT to end-RT) in 13 of the 22 metastases when all fitting methods were added up.

CONCLUSION

The ROI strategies have an effect on the relative ADC change, which may be important for the assessment of individual patient's response to RT and the interpretation of the current literature.