Clinical significance of multi-leaf collimator positional errors for volumetric modulated arc therapy

Background and purpose

Multi-leaf collimator (MLC) positional errors occur during intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) deliveries. The impact of such errors has been evaluated for IMRT but not VMAT. The purpose of this work is to understand how random and systematic VMAT MLC positional errors affect the patient dose distribution.

Materials and methods

Eight head and neck single arc (360°) VMAT treatment plans were created. Random and two types of systematic MLC errors were simulated for error magnitudes of 0.25, 0.5, 1, 2 and 5 mm. The two types of systematic MLC errors were: (1) MLC banks are shifted in the same direction (left or right) and (2) MLC banks are shifted in opposing directions resulting in smaller or larger field shapes. The MLC errors were simulated, for all control points, on both banks of active MLC leaves only.

Results

There is a linear correlation of MLC errors with gEUD for all error types. The gEUD dose sensitivities with MLC error for the PTV70 were −0.2, −0.9, −2.8 and 1.9 Gy/mm for random, systematic shift, systematic close and systematic open MLC errors, respectively. The sensitivity of VMAT plans to MLC positional errors was similar to those of IMRT plans with less than 50 segments but much less than those created for a step and shoot with more than 50 segments or sliding-window delivery technique. To maintain the PTV70 to within 2% would require that MLC open/close errors be within 0.6 mm.

Conclusions

Radiation therapy centers should have adequate quality assurance programs in place to assess open/close MLC errors (i.e. leaf gap errors) as they tend to be more impactful than random or systematic MLC shift errors.     

Evaluation of the ‘dose of the day’ for IMRT prostate cancer patients derived from portal dose measurements and cone-beam CT

Purpose

High geometrical and dosimetrical accuracies are required for radiotherapy treatments where IMRT is applied in combination with narrow treatment margins in order to minimize dose delivery to normal tissues. As an overall check, we implemented a method for reconstruction of the actually delivered 3D dose distribution to the patient during a treatment fraction, i.e., the ‘dose of the day’. In this article results on the clinical evaluation of this concept for a group of IMRT prostate cancer patients are presented.

Materials and methods

The actual IMRT fluence maps delivered to a patient were derived from measured EPID-images acquired during treatment using a previously described iterative method. In addition, the patient geometry was obtained from in-room acquired cone-beam CT images. For dose calculation, a mapping of the Hounsfield Units from the planning CT was applied. With the fluence maps and the modified cone-beam CT the ‘dose of the day’ was calculated. The method was validated using phantom measurements and evaluated clinically for 10 prostate cancer patients in 4 or 5 fractions.

Results

The phantom measurements showed that the delivered dose could be reconstructed within 3%/3 mm accuracy. For prostate cancer patients, the isocenter dose agreed within −0.4 ± 1.0% (1 SD) with the planned value, while for on average 98.1% of the pixels within the 50% isodose surface the actually delivered dose agreed within 3% or 3 mm with the planned dose. For most fractions, the dose coverage of the prostate volume was slightly deteriorated which was caused by small prostate rotations and small inaccuracies in fluence delivery. The dose that was delivered to the rectum remained within the constraints used during planning. However, for two patients a large degrading of the dose delivery was observed in two fractions. For one patient this was related to changes in rectum filling with respect to the planning CT and for the other to large intra-fraction motion during treatment delivery, resulting in mean underdosages of 16% in the prostate volume.

Conclusions

A method to accurately assess the ‘dose of the day’ was evaluated for prostate cancer patients treated with IMRT. To correct for observed dose deviations off-line dose-adaptive strategies will be developed.     

Radiation therapy with unflattened photon beams: Dosimetric accuracy of advanced dose calculation algorithms

Purpose

To compare the dosimetric accuracy of advanced dose calculation algorithms for flattened (FF) and unflattened (FFF) photon beams.

Material and methods

We compared the enhanced collapsed cone (eCC) algorithm implemented in OncentraMasterplan and the XVMC (MC) code in Monaco. Test plans were created for 10 MV FF and FFF beams. Single beam tests were delivered to radiochromic films positioned within a solid water phantom and evaluated with 1D γ-index analysis. Conformal plans were verified with ion chambers in an anthropomorphic thorax phantom. IMRT plans were applied to the Delta4 system and evaluated with γ-criteria of 3% and 3 mm.

Results

1D γ-index evaluation revealed significantly lower (p < 0.05) average γ_mean-values of 0.46 ± 0.22 for MC calculated FFF profiles compared to average values of 0.53 ± 0.27 detected for FF beams. Respective values for eCC were 0.42 ± 0.27/0.38 ± 0.26 (FF/FFF). When considering off-axis profiles separately, we found significantly reduced average γ_mean-values for FFF and both algorithms (MC: 0.55 ± 24 vs. 0.45 ± 0.21, eCC: 0.41 ± 0.24 vs. 0.35 ± 0.22). No significant differences were detected on-axis. Absolute dosimetry in the anthropomorphic phantom revealed superior results for MC based dose calculation, with mean deviations of 0.8 ± 0.8/0.0 ± 1.0% compared to −0.1 ± 1.7/−0.5 ± 0.1.7% (FF/FFF) for the eCC algorithm. IMRT plans showed similar results for both linac modes.

Conclusions

The dose calculation accuracy for unflattened beams was found to be at least as high as for flattened beams. The slightly improved dose calculation accuracy observed for off-axis profiles for single FFF beams did not directly translate into better verification results for composite IMRT plans.     

Advanced kernel methods vs. Monte Carlo-based dose calculation for high energy photon beams

Purpose

The aim of this study was to compare the dose calculation accuracy of advanced kernel-based methods and Monte Carlo algorithms in commercially available treatment planning systems.

Materials and methods

Following dose calculation algorithms and treatment planning (TPS) systems were compared: the collapsed cone (CC) convolution algorithm available in Oncentra Masterplan, the XVMC Monte Carlo algorithm implemented in iPlan and Monaco, and the analytical anisotropic algorithm (AAA) implemented in Eclipse. Measurements were performed with a calibrated ionization chamber and radiochromic EBT type films in a homogenous polystyrene phantom and in heterogeneous lung phantoms. Single beam tests, conformal treatment plans and IMRT plans were validated. Dosimetric evaluations included absolute dose measurements, 1D γ-evaluation of depth-dose curves and profiles using 2 mm and 2% dose difference criteria for single beam tests, and γ-evaluation of axial planes for composite treatment plans applying 3 mm and 3% dose difference criteria.

Results

Absolute dosimetry revealed no large differences between MC and advanced kernel dose calculations. 1D γ-evaluation showed significant discrepancies between depth-dose curves in different phantom geometries. For the CC algorithm γ_mean values were 0.90 ± 0.74 vs. 0.43 ± 0.41 in heterogeneous vs. homogeneous conditions and for the AAA γ_mean values were 1.13 ± 0.91 vs. 0.41 ± 0.28, respectively. In general, 1D γ results obtained with both MC TPS were similar in both phantoms and on average equal to 0.5 both for profiles and depth-dose curves. The results obtained with the CC algorithm in heterogeneous phantoms were slightly better in comparison to the AAA algorithm. The 2D γ-evaluation results of IMRT plans and four-field plans showed smaller mean γ-values for MC dose calculations compared to the advanced kernel algorithms (γ_mean for four-field plan and IMRT obtained with Monaco MC were 0.28 and 0.5, respectively, vs. 0.40 and 0.54 for the AAA).

Conclusion

All TPS investigated in this study demonstrated accurate dose calculation in homogenous and heterogeneous phantoms. Commercially available TPS with Monte Carlo option performed best in heterogeneous phantoms. However, the difference between the CC and the MC algorithms was found to be small.     

A national dosimetric audit of IMRT

Background and purpose

A dosimetric audit of IMRT has been carried out within the UK between June 2009 and March 2010 in order to provide an independent check of safe implementation and to identify problems in the modelling and delivery of IMRT.

Methods and materials

A mail based audit involving film and alanine dosimeters was utilized. Measurements were made for each individual field in an IMRT plan isocentrically in a flat water-equivalent phantom at a depth of 5 cm. The films and alanine dosimeters were processed and analysed centrally; additional ion chamber measurements were made by each participating centre.

Results

57 of 62 centres participated, with a total of 78 plans submitted. For the film measurements, all 176 fields from the less complex IMRT plans (including prostate and breast plans) achieved over 95% pixels passing a gamma criterion of 3%/3 mm within the 20% isodose. For the more complex IMRT plans (mainly head and neck) 8/245 fields (3.3%) achieved less than 95% pixels passing a 4%/4 mm gamma criterion. Of the alanine measurements, 4/78 (5.1%) of the measurements differed by >5% from the dose predicted by the treatment planning system. Three of these were large deviations of −77.1%, −29.1% and 14.1% respectively. Excluding the three measurements outside 10%, the mean difference was 0.05% with a standard deviation of 1.5%. The out of tolerance results have been subjected to further investigations.

Conclusions

A dosimetric audit has been successfully carried out of IMRT implementation by over 90% of UK radiotherapy departments. The audit shows that modelling and delivery of IMRT is accurate, suggesting that the implementation of IMRT has been carried out safely.     

Secondary radiation doses of intensity-modulated radiotherapy and proton beam therapy in patients with lung and liver cancer

Purpose

To compare the secondary radiation doses following intensity-modulated radiotherapy (IMRT) and proton beam therapy (PBT) in patients with lung and liver cancer.

Methods and materials

IMRT and PBT were planned for three lung cancer and three liver cancer patients. The treatment beams were delivered to phantoms and the corresponding secondary doses during irradiation were measured at various points 20-50 cm from the beam isocenter using ion chamber and CR-39 detectors for IMRT and PBT, respectively.

Results

The secondary dose per Gy (i.e., a treatment dose of 1 Gy) from PBT for lung and liver cancer, measured 20-50 cm from the isocenter, ranged from 0.17 to 0.086 mGy. The secondary dose per Gy from IMRT, however, ranged between 5.8 and 1.0 mGy, indicating that PBT is associated with a smaller dose of secondary radiation than IMRT. The internal neutron dose per Gy from PBT for lung and liver cancer, 20-50 cm from the isocenter, ranged from 0.03 to 0.008 mGy.

Conclusions

The secondary dose from PBT is less than or compatible to the secondary dose from conventional IMRT. The internal neutron dose generated by the interaction between protons and body material is generally much less than the external neutron dose from the treatment head.     

Modulated electron radiotherapy treatment planning using a photon multileaf collimator for post-mastectomized chest walls

Background and purpose

To evaluate the feasibility of using a photon MLC (xMLC) for modulated electron radiotherapy treatment (MERT) as an alternative to conventional post-mastectomy chest wall (CW) irradiation. A Monte Carlo (MC) based planning system was developed to overcome the inaccuracy of the ‘pencil beam’ algorithm. MC techniques are known to accurately calculate the dose distributions of electron beams, allowing the explicit simulation of electron interactions within the MLC.

Materials and methods

Four real clinical CW cases were planned using MERT which were compared with the conventional electron treatments based on blocks and by a straightforward approach using the MLC, and not the blocks (as an intermediate step to MERT) to shape the same segments with SSD between 60 and 70 cm depending on PTV size. MC calculations were verified with an array of ionization chambers and radiochromic films in a solid water phantom.

Results

Tests based on gamma analysis between MC dose distributions and radiochromic film measurements showed an excellent agreement. Differences in the absolute dose measured with a plane-parallel chamber at a reference point were below 3% for all cases. MERT solution showed a better PTV coverage and a significant reduction of the doses to the organs at risk (OARs).

Conclusion

MERT can effectively improve the current electron treatments by obtaining a better PTV coverage and sparing healthy tissues. More directly, block-shaped treatments could be replaced by MLC-shaped non-modulated segments providing similar results.     

Superficial dose distribution in breast for tangential radiation treatment, Monte Carlo evaluation of Eclipse algorithms in case of phantom and patient geometries

Purpose

The aim of this study is to examine experimentally and by the Monte Carlo method the accuracy of the Eclipse Pencil Beam Convolution (PBC) and Analytical Anisotropic Algorithm (AAA) algorithms in the superficial region (0-2 cm) of the breast for tangential photon beams in a phantom case as well as in a number of patient geometries. The aim is also to identify differences in how the patient computer tomography data are handled by the treatment planning system and in the Monte Carlo simulations in order to reduce influences of these effects on the evaluation.

Materials and methods

Measurements by thermoluminescent dosimeters and gafchromic film are performed for six MV tangential irradiation of the cylindrical solid water phantom. Tangential treatment of seven patients is investigated considering open beams. Dose distributions are obtained by the Eclipse PBC and AAA algorithms. Monte Carlo calculations are carried out by BEAMnrc/DOSXYZnrc code package. Calculations are performed with a calculation grid of 1.25 × 1.25 × 5 mm³ for PBC and 2 × 2 × 5 mm³ for AAA and Monte Carlo, respectively. Dose comparison is performed in both dose and spatial domains by the normalized dose difference method.

Results

Experimental profiles from the surface toward the geometrical center of the cylindrical phantom are obtained at the beam entrance and exit as well as laterally. Full dose is received beyond 2 mm in the lateral superficial region and beyond 7 mm at the beam entrance. Good agreement between experimental, Monte Carlo and AAA data is obtained, whereas PBC is seen to underestimate the entrance dose the first 3-4 mm and the lateral dose by more than 5% up to 8 mm depth. In the patient cases considered, AAA and Monte Carlo show agreement within 3% dose and 4 mm spatial tolerance. PBC systematically underestimates the dose at the breast apex. The dimensions of region out of tolerance vary with the local breast shape. Different interpretations of patient boundaries in Monte Carlo and the Eclipse are found to influence the evaluation. Computer tomography marker wire may introduce local disturbance effects on the comparison as well. These factors are not related to the accuracy of the calculation algorithms and their effect is taken into account in the evaluation.

Conclusions

The accuracy of AAA in the case of the solid water phantom is comparable with that of the Monte Carlo method. The AAA-Monte Carlo differences in the patient cases considered are within 3%, 4 mm tolerance. The PBC algorithm does not give equivalent results. In the phantom case, PBC underestimates the lateral dose by more than 5% up to 8 mm depth. The PBC-Monte Carlo differences in the patient cases are outside the tolerance at the breast apex. The dimension of region varies with the breast shape being typically 8-10 mm long and 6-8 mm deep.     

Dosimetric effect of setup motion and target volume margin reduction in pediatric ependymoma

Purpose

Quantify the dosimetric effect of inter- and intrafractional motion on intensity-modulated radiation therapy (IMRT) and three-dimensional (3D) planning via changes in the generalized equivalent uniform dose (gEUD), predicted tumor control probability (TCP) and normal tissue complication probability (NTCP) for pediatric ependymoma.

Methods and materials

Twenty patients treated between 1998 and 2002 with a 3D plan (CTV = 1 cm, PTV = 5 mm) were selected. Two IMRT plans were created for the 1 cm CTV (PTV = 5 mm and PTV = 0 mm), and a third IMRT plan for a 5 mm CTV (PTV = 0 mm).Direct simulation with inter- and intrafractional motion was performed for 3D and IMRT plans based on daily pre and post-treatment cone beam CT information obtained from 20 well-matched patients (age, supine/prone, use of GA) on a localization protocol. Calculated TCP, NTCP, Conformity Index (CI), and predictive IQ were compared.

Results

IMRT improved the calculated TCP by 2.8 ± 2.8 vs. 3D (p < 0.001). Inter- and intrafractional motion results in a TCP loss of 0.4 ± 0.7 (p = 0.02) and 0.0 ± 0.1 (p = 0.14) for the IMRT plan with PTV = 0 mm. Mean NTCP for 3D and IMRT with PTV = 5 mm, PTV = 0 mm, and CTV = 5 mm for the cochlea was: 66.6, 29.4, 8.7. Mean NTCP change due to motion was <5%. CI was 0.70 ± 0.06 for IMRT and 0.5 ± 0.10 for 3D. Predictive IQ was 10.0 ± 10.3 points higher for IMRT vs. 3D.

Conclusions

IMRT improves calculated TCP vs. 3D. Daily localization can allow for a safe reduction in the PTV margin, while maintaining target coverage; reducing the CTV margin can further reduce NTCP and may reduce future side-effects.     

A comparison of several modulated radiotherapy techniques for head and neck cancer and dosimetric validation of VMAT

Purpose

Volumetric modulated arc therapy (VMAT) has the potential to shorten treatment times for fluence modulated radiotherapy. We compared dose distributions of VMAT, step-and-shoot IMRT and serial tomotherapy for typical head and neck (H&N) planning target volumes (PTV) with sparing of one parotid, a complex paradigm and a situation often encountered in H&N radiotherapy. Finally, we validated the dosimetric accuracy of VMAT delivery.

Material and methods

Based on CT datasets of 10 patients treated for H&N cancer (PTV1:60 Gy/PTV2:56 Gy) with IMRT (7/9 fields), serial tomotherapy (MIMiC) and VMAT were compared with regard to plan quality and treatment efficiency. Plan quality was assessed by calculating homogeneity/conformity index (HI/CI), mean dose to parotid and brain stem and the maximum dose to the spinal cord. For plan efficiency evaluation, total treatment time (TTT) and number of monitor units (MU) were considered. A dosimetric evaluation of VMAT was performed using radiosensitive film, ion chamber and 2D-array.

Results

For MIMiC/IMRT_7F/IMRT_9F/VMAT, mean CI was 1.98/2.23/2.23/1.82, HI_PTV1 was 1.12/1.20/1.20/1.11 and HI_PTV2 was 1.11/1.15/1.13/1.12. Mean doses to the shielded parotid were 19.5 Gy/14.1 Gy/13.9 Gy/14.9 Gy and the spinal cord received maximum doses of 43.6 Gy/40.8 Gy/41.6 Gy/42.6 Gy. The mean MU’s were 2551/945/925/521 and the mean TTT was 12.8 min/7.6 min/8.5 min/4.32 min. The ion chamber measurements showed an absolute deviation of 0.08 ± 1.10% and 98.45 ± 3.25% pixels passed γ-analyses for 3%/3 mm and 99.95 ± 0.09% for 5%/5 mm for films. 2D-array measurements reported an agreement for 3%/3 mm of 95.65 ± 2.47%-98.33 ± 0.65% and for 5%/5 mm 99.79 ± 0.24%-99.92 ± 0.09% depending on the measurement protocol.

Conclusion

All treatment paradigms produced plans of excellent quality and dosimetric accuracy with IMRT providing best OAR sparing and VMAT being the most efficient treatment option in our comparison of treatment plans with high complexity.     

A method to improve target dose homogeneity of craniospinal irradiation using dynamic split field IMRT

Purpose

Craniospinal irradiation (CSI) is technically very challenging and field edge matching is needed because of the mechanical limitations of standard linear accelerators. We assessed the feasibility of intensity-modulated radiotherapy (IMRT) in CSI to overcome the standard feathering and dose inhomogeneities associated with the standard feathering technique in the junction areas.

Materials and methods

The use of IMRT in CSI was studied with five patients CT scanned in the supine position. Isocentric treatment plans of three dimensional conventional radiotherapy (3D-CRT) and split field IMRT (sfIMRT) with dynamic intrafractional feathering were created with the same field setup and the resulted dose distributions were compared. The effect of treatment inaccuracy was simulated with an intentional shift of ±3 mm with both treatment plans. Dosimetric verification of the sfIMRT treatment plan was performed with radiographic films placed in a phantom.

Results

The sfIMRT treatment plans resulted in a better dose coverage and uniformity in the target volume. The ±3 mm shift had only a minor effect on the dose distribution of the sfIMRT treatment plan whereas with the 3D-CRT the shift resulted in an error of ±38% of the calculated dose in the spinal cord. The measured dose distribution of the sfIMRT treatment plan correlated well with the calculations.

Conclusions

Improved dose homogeneity in the target volume was achieved with the sfIMRT compared to the conventional 3D-CRT treatment plan. With the sfIMRT technique only a single treatment plan is required to deliver the total treatment dose and the resulting dose distribution is also less volatile for technical uncertainties of the treatment.     

Validation of functional imaging with pathology for tumor delineation in the prostate

Introduction

A study was performed to validate magnetic resonance (MR) based prostate tumor delineations with pathology.

Material and methods

Five patients with biopsy proven prostate cancer underwent a T2 weighted (T2w), diffusion weighted MRI (DW-MRI) and dynamic contrast-enhanced MRI (DCE-MRI) scan before prostatectomy. Suspicious regions were delineated based on all available MR information. After prostatectomy whole-mount hematoxylin-eosin stained (H&E) sections were made. Tumor tissue was delineated on the H&E stained sections and compared with the MR based delineations. The registration accuracy between the MR images and H&E stained sections was estimated.

Results

A tumor coverage of 44-89% was reached by the MR based tumor delineations. The application of a margin of ∼5 mm to the MR based tumor delineations yielded a tumor coverage of 85-100% in all patients. Errors created during the registration procedure were 2-3 mm, which cannot completely explain the limited tumor coverage.

Conclusions

An accurate tissue processing and registration method was presented (registration error 2-3 mm), which enables the validation of MR based tumor delineations with pathology. Reasonable tumor coverage of about 85% and larger was found when applying a margin of ∼5 mm to the MR based tumor delineations.     

A randomised controlled trial of forward-planned radiotherapy (IMRT) for early breast cancer: Baseline characteristics and dosimetry results

Background and purpose

This large trial was designed to investigate whether correction of dose inhomogeneities using intensity-modulated radiotherapy (IMRT) reduces late toxicity and improves quality of life in patients with early breast cancer. This paper reports baseline characteristics of trial participants and dosimetry results.

Materials and methods

Standard tangential plans of 1145 trials were analysed. Patients with inhomogeneous plans, defined by ICRU recommendations, were randomised to forward-planned IMRT or standard radiotherapy.

Results

Twenty-nine percentage of patients had adequate dosimetry with standard 2D radiotherapy. In the randomised patients, the decreases in mean volumes receiving greater than 107% (Vol > 107) and less than 95% (Vol < 95) of the prescribed dose in the IMRT compared with the control group were 34.0 cm³ (95% CI 26.4-41.6; P < 0.0001) and 48.1 cm³ (95% CI 34.4-61.9; P < 0.0001), respectively. In this study, 90% of patients who had a breast separation greater ?21 cm had Vol > 107 > 2 cm³ on standard radiotherapy plans.

Conclusion

This large trial, in which patients with all breast sizes were eligible, confirmed that breast dosimetry can be significantly improved with a simple method of forward-planned IMRT and has little impact on radiotherapy resources. It is shown that patients with larger breasts are more likely to have dose inhomogeneities and breast separation gives some indication of this likelihood. Photographic assessment of patients at 2 years after radiotherapy, as the next part of this randomised controlled trial, will show whether these results for IMRT translate into improved cosmetic outcome in patients with early breast cancer. This would provide impetus for the widespread adoption of 3D planning and IMRT.     

Geometric accuracy of a novel gimbals based radiation therapy tumor tracking system

Purpose

VERO is a novel platform for image guided stereotactic body radiotherapy. Orthogonal gimbals hold the linac-MLC assembly allowing real-time moving tumor tracking. This study determines the geometric accuracy of the tracking.

Materials and methods

To determine the tracking error, an 1D moving phantom produced sinusoidal motion with frequencies up to 30 breaths per minute (bpm). Tumor trajectories of patients were reproduced using a 2D robot and pursued with the gimbals tracking system prototype. Using the moving beam light field and a digital-camera-based detection unit tracking errors, system lag and equivalence of pan/tilt performance were measured.

Results

The system lag was 47.7 ms for panning and 47.6 ms for tilting. Applying system lag compensation, sinusoidal motion tracking was accurate, with a tracking error 90% percentile E_90% < 0.82 mm and similar performance for pan/tilt. Systematic tracking errors were below 0.14 mm. The 2D tumor trajectories were tracked with an average E_90% of 0.54 mm, and tracking error standard deviations of 0.20 mm for pan and 0.22 mm for tilt.

Conclusions

In terms of dynamic behavior, the gimbaled linac of the VERO system showed to be an excellent approach for providing accurate real-time tumor tracking in radiation therapy.     

A study of middle ear function in the treatment of nasopharyngeal carcinoma with IMRT technique

Purpose

This study evaluates the difference in damage to middle ear function with CRT and IMRT techniques in the treatment of nasopharyngeal carcinoma (NPC). We explore the isthmus of the Eustachian tube (ET) as the key anatomic site for the prevention of radiation-induced otitis media with effusion.

Methods and materials

Eighty-two patients with NPC were divided into two groups: 40 patients treated with CRT and 42 patients treated with IMRT. The difference between dosage over the middle ear cavity and the isthmus of the ET was evaluated in both CRT group and IMRT group. All patients underwent hearing tests including pure tone audiometry and impedance audiometry before and after RT.

Results

The dosage difference to the middle ear cavity and isthmus between these two groups was statistically significant (p < 0.05). The difference in hearing test results between these two groups was also statistically significant (p < 0.05). If we limited the dose to the middle ear cavity under 34 Gy and the dose to the isthmus under 53 Gy with IMRT, we may decrease radiation-induced OME even with the larger 2.25 Gy fraction size.

Conclusions

IMRT may have better protected the middle ear function compared with the CRT technique, even with larger fraction sizes than for the conventional CRT technique.