Antonetta C. Houweling

A COMPARISON OF DOSE-RESPONSE MODELS FOR THE PAROTID GLAND IN A LARGE GROUP OF HEAD-AND-NECK CANCER PATIENTS

PURPOSE The dose-response relationship of the parotid gland has been described most frequently using the Lyman-Kutcher-Burman model. However, various other normal tissue complication probability (NTCP) models exist. We evaluated in a large group of patients the value of six NTCP models that describe the parotid gland dose response 1 year after radiotherapy. METHODS AND MATERIALS A total of 347 patients with head-and-neck tumors were included in this prospective parotid gland dose-response study. The patients were treated with either conventional radiotherapy or intensity-modulated radiotherapy. Dose-volume histograms for the parotid glands were derived from three-dimensional dose calculations using computed tomography scans. Stimulated salivary flow rates were measured before and 1 year after radiotherapy. A threshold of 25% of the pretreatment flow rate was used to define a complication. The evaluated models included the Lyman-Kutcher-Burman model, the mean dose model, the relative seriality model, the critical volume model, the parallel functional subunit model, and the dose-threshold model. The goodness of fit (GOF) was determined by the deviance and a Monte Carlo hypothesis test. Ranking of the models was based on Akaikes information criterion (AIC). RESULTS None of the models was rejected based on the evaluation of the GOF. The mean dose model was ranked as the best model based on the AIC. The TD(50) in these models was approximately 39 Gy. CONCLUSIONS The mean dose model was preferred for describing the dose-response relationship of the parotid gland.

Sparing the contralateral submandibular gland in oropharyngeal cancer patients: a planning study.

BACKGROUND AND PURPOSE The submandibular glands are proposed to be important in preventing xerostomia in head-and-neck cancer patients. We investigated the feasibility of sparing the contralateral submandibular gland (cSMG) by reducing the dose to the contralateral planning target volume (PTV) and by reducing the clinical target volume (CTV)-to-PTV margin. MATERIALS AND METHODS Ten oropharyngeal cancer patients with a contralateral elective PTV were included in this planning study, using intensity modulated radiotherapy (IMRT). The effect on the mean dose to the cSMG of reducing the dose coverage to the contralateral elective PTV from 95 to 90% of the prescribed dose (54Gy in 1.8Gy daily fractions) was determined. The influence of reducing the margin for position uncertainty from 5 to 2mm was investigated. RESULTS The mean dose to the cSMG was reduced from 54Gy to approximately 40Gy if the dose coverage to the contralateral PTV was reduced to 90% of the prescribed dose. The estimated normal tissue complication probability (NTCP) was reduced below 50%. Reducing the margin from 5 to 2mm resulted in a decrease in the mean dose to the cSMG of approximately 6Gy. CONCLUSIONS Reducing the mean dose to the cSMG below 40Gy is possible with a reasonable dose coverage of the contralateral elective PTV.

MRI to quantify early radiation-induced changes in the salivary glands.

PURPOSE We investigated radiation-induced changes in the salivary glands, 6 weeks after RT, using MRI. MATERIALS AND METHODS Eighteen oropharyngeal cancer patients were treated with salivary gland sparing IMRT. All patients received a 3T MRI exam before and 6 weeks after the end of RT, including a T(1)-weighted (T(1)w), a T(2)-weighted (T(2)w), and a dynamic contrast-enhanced (DCE) MRI. For both time points separately, the parotid and submandibular glands were delineated on the MR images. Differences in median signal intensity and signal variation within the glands were tested for significance. Correlations were studied between the MR changes and the planned RT dose. RESULTS The volume of the glands reduced significantly by 25%. The T(1)w signal decreased by 10% and the T(2)w signal increased by 23%. The k(ep) value decreased, while the v(e) increased. A correlation of the changes in T(2)w signal with the mean dose was found in both glands. CONCLUSIONS Overall radiation-induced changes and volume loss were observed in the parotid and submandibular gland using MR. The observed differences indicated an increased water content such as found in oedema. The overall changes could be related to the mean dose, with a slightly greater impact in the high dose area.

Improved immobilization using an individual head support in head and neck cancer patients

The benefits of a patient-specific head support, developed to improve immobilization during radiotherapy, were determined in head and neck cancer patients. Cone-beam CTs were registered to the planning CT in five regions. Compared to the standard head support, the individual head support decreased the systematic and random errors of the inter- and intrafraction displacements and reduced deformations.

Magnetic resonance imaging at 3.0 T for submandibular gland sparing radiotherapy

PURPOSE Besides sparing the parotid gland, sparing the submandibular gland is considered to be important in preventing xerostomia in head-and-neck cancer patients. Delineation of the submandibular gland at CT, and even on T(1)- and T(2)-weighted MR images, is difficult, due to low contrast with the surrounding tissues. MR sialography might be used for delineation. METHODS AND MATERIALS Sixteen oropharyngeal cancer patients received a CT and MRI exam as part of the standard treatment imaging protocol. Patients were scanned in their five-point RT immobilization mask. The MRI exam included T(1)- and T(2)-weighted MRI scans and an MR sialography scan. Thirty submandibular glands were delineated on only CT, on the combined CT and T(1)- and T(2)-weighted MRI scans and on all MR images. A Wilcoxon signed-rank test was performed to test if the delineated volumes were significantly different. RESULTS The delineated volume of the submandibular gland was 7.3mL in the CT-delineation, 7.1mL in the CT/MRI-delineation and 8.1mL in the MRI-delineation. The MRI-delineation was significantly larger than the other delineations (p<0.001). The differences were mainly located in the cranial direction. CONCLUSION The delineation of the submandibular gland was improved in the cranial direction by using T(1)- and T(2)-weighted MRI and MR sialography, compared to the other delineations.

Auditing local methods for quality assurance in radiotherapy using the same set of predefined treatment plans

Background and purpose Local implementation of plan-specific quality assurance (QA) methods for intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) treatment plans may vary because of dissimilarities in procedures, equipment and software. The purpose of this work is detecting possible differences between local QA findings and those of an audit, using the same set of treatment plans. Methods A pre-defined set of clinical plans was devised and imported in the participating institute’s treatment planning system for dose computation. The dose distribution was measured using an ionisation chamber, radiochromic film and an ionisation chamber array. The centres performed their own QA, which was compared to the audit findings. The agreement/disagreement between the audit and the institute QA results were assessed along with the differences between the dose distributions measured by the audit team and computed by the institute. Results For the majority of the cases the results of the audit were in agreement with the institute QA findings: ionisation chamber: 92%, array: 88%, film: 76% of the total measurements. In only a few of these cases the evaluated measurements failed for both: ionisation chamber: 2%, array: 4%, film: 0% of the total measurements. Conclusion Using predefined treatment plans, we found that in approximately 80% of the evaluated measurements the results of local QA of IMRT and VMAT plans were in line with the findings of the audit. However, the percentage of agreement/disagreement depended on the characteristics of the measurement equipment used and on the analysis metric.

Single dose partial breast irradiation using an MRI linear accelerator in the supine and prone treatment position

Highlights • All MRI-linac plans met the coverage and predefined OAR constraints.• The prone approach appeared to be more favorable with respect to the chest wall, and ipsilateral lung dose compared to the supine position.• Overall, the MRI-linac and clinical plans were comparable, with minor absolute dosimetric differences.

SU-E-J-202: Performance of the Patient QA Systems in a Hybrid MRI-Linac

Purpose: The first clinical MR-linac will soon become operational, therefore patient plan QA procedures and equipment have to become MRcompatible. Reference dosimetry is affected by the magnetic field, however, relative dosimetry using patient QA systems haven’t been investigated extensively. The purpose of this study was to examine the performance characteristics of the MR-compatible ArcCHECK and Delta4 systems in a transverse 1.5 T magnetic field. Methods: Recently MR-compatible versions of both ArcCheck (Sun Nuclear) and Delta4 (Scandidos) have been developed. To examine the performance characteristics within the magnetic field, the reproducibility, dose linearity, dose rate dependence, field size and angular dependence were evaluated on the MR-linac (8 MV FFF beam, SAD of 142 cm) and a conventional linac (Elekta, 6MV). To allow comparison of the measurements with and without magnetic field, the measurement setup for the conventional linac is adapted to mimic the setup at the MR-linac if possible (e.g. SAD, dose rate). The results from the MR-linac were benchmarked to the results from the conventional linac as being the clinical reference. Results: At the moment of writing, measurements for the Delta4 are still running. Therefore, only the results of the ArcCheck are presented in this abstract. No significant difference was observed in the reproducibility of the ArcCheck (<0.06%) between both linacs. The maximum dose response difference when measuring the dose linearity was less than 0.4% and the varying dose rate resulted in maximal dose differences of 1.0% for both linacs. Response variation for varying field sizes was <2.6% at the conventional linac, and <1.0% at the MR-linac. Angular response was similar for both linacs. Conclusion: The reproducibility, dose linearity, dose rate dependence, field size and angular dependence of the MR compatible ArcCheck were not influenced by the presence of a transverse 1.5 T magnetic field. The results for the Delta4 are about to follow. Equipment is generously provided by Sun Nuclear Corporation (Melbourne, USA) and Scandidos (Uppsala, Sweden)