Dose-volume relationships for moderate or severe neck muscle atrophy after intensity-modulated radiotherapy in patients with nasopharyngeal carcinoma

This study aimed to identify the dosimetric parameters and radiation dose tolerances associated with moderate or severe sternocleidomastoid muscle (SCM) atrophy after intensity-modulated radiotherapy (IMRT) in nasopharyngeal carcinoma (NPC). We retrospectively analysed 138 patients treated with IMRT between 2011 and 2012 for whom IMRT treatment plans and pretreatment and 3-year post-IMRT MRI scans were available. The association between mean dose (Dmean), maximum dose (Dmax), VX (% SCM volume that received more than X Gy), DX (dose to X% of the SCM volume) at X values of 20, 25, 30, 35, 40, 45, 50, 55, 60, 65, 70, 75, 80 and SCM atrophy at 3 years after IMRT were analyzed. All dosimetric parameters, except V40, V50 and V80, were significantly associated with moderate or severe SCM atrophy. Multivariate analysis showed that V65 was an independent predictor of moderate or severe SCM atrophy (P < 0.001). Receiver operating characteristic (ROC) curve indicated a V65 of 21.47% (area under ROC curves, 0.732; P < 0.001) was the tolerated dose for moderate or severe SCM atrophy. We suggest a limit of 21.47% for V65 to optimize NPC treatment planning, whilst minimizing the risk of moderate or severe SCM atrophy.

Thus, the impact of SCM injury on the normal motion of the neck can be enormous. However, IMRT-induced SCM atrophy, which may contribute to the development of weakness and further impair quality of life, has received little investigation. The precise mechanism regarding radiotherapy-induced neck muscle atrophy remains unknown, but it is believed that the incidence and severity of neck muscle atrophy is associated with volume and dose of neck muscle irradiated 12 .
Hence, we conducted this retrospective study to investigate the incidence of SCM atrophy in patients with NPC at three years after IMRT and also investigate the dose-response relationships for moderate or severe SCM atrophy using a dose-volume-outcome analysis. The purpose of this study was to offer guidelines for optimizing IMRT treatment plans in order to reduce the incidence of neck muscle atrophy and weakness in patients with NPC.

Materials and Methods
Patient selection. Between January 2011 and January 2012, 549 newly diagnosed patients with NPC with no distant metastases were treated using IMRT at our centre. A total of 355 patients were excluded due to the IMRT treatment plan documents being unavailable. Forty-three patients were excluded because they had not had MRI scans pre-treatment and/or 3 years post-IMRT. Of the remaining 151 patients, 6 were excluded due to them having diabetes mellitus, 2 were excluded due to them having connective tissue disorders; and 5 were excluded due to them having had previous neck surgery. From the original 549 patients, 138 were included in this study. This retrospective study was approved by the Institutional Review Board of Sun Yat-sen University Cancer Center, and was conducted in accordance with Good Clinical Practice guideline. Informed consent was obtained from all patients.
Pretreatment baseline evaluation included a complete medical history, physical examination, haematology and biochemistry profiles, MRI of the neck and nasopharynx, chest radiography, abdominal sonography, and bone scan emission computed tomography (SPECT). All patients were staged as part of their routine treatment according to the 7th edition of the AJCC staging system 13 . The clinical features of the patients are summarised in Table 1. Treatment methods. All patients underwent radical IMRT treatment. The target volumes were delineated according to our institutional protocol, which is in agreement with the International Commission on Radiation Units and Measurements Reports 50 and 62 14 . In summary, the prescribed dose was limited to 68-70 Gy in 30-33 fractions to the planning target volume (PTV) of the gross tumour volume of the primary (GTV-P), 60-68 Gy to the PTV of the nodal gross tumour volume (GTV-N), 60 Gy to the PTV of (CTV)-1 (high-risk region) and Data collection. Muscle volume measurement. All patients underwent MRI using a 1.5-Tesla system (Signa CV/i; General Electric Healthcare, Chalfont St. Giles, United Kingdom). The area from the suprasellar cistern to the inferior margin at the sternal end of the clavicle was examined with a head and neck combined coil, as previously described 15 . Measurement of the muscle volumes of the left and right sides of the SCM was performed separately by volume rendering. MRI scanning images obtained at pretreatment and 3-years post-IMRT for each study subject were conveyed to Advantage Workstation 4.4 (General Electric Healthcare). The outline of the SCM was traced on each T2-weighted axial image, beginning at the manubrium and collarbone and ending at the mastoid, and then the bilateral SCM muscle volumes were calculated by volume rendering. A radiologist and clinician trained to use the software to delineate the SCM contoured the bilateral muscles independently, any disagreements about the SCM delineation were resolved by consensus. Then the average of the measurements was taken as the final muscle volume.
Grading of muscle atrophy. The following formula was used to calculate the extent of SCM atrophy: SCM atrophy ratio (%) = [(SCM volume at pre-treatment -SCM volume at 3-years post-IMRT)/SCM volume at pre-treatment] × 100. SCM atrophy was graded according to a 3-point visual grading system as follows: Grade 0, no atrophy; Grade 1, minimal to mild atrophy, 0 < SCM atrophy ratio ≤ 40%; Grade 2, moderate atrophy, 40% < SCM atrophy ratio ≤ 70%; Grade 3, severe atrophy, SCM atrophy ratio > 70%. This system is similar to that proposed by Hyun Kyong Lim et al. 16 . Grade 2 and 3 were defined as the endpoint of the study.
Grading of muscle weakness. Neck muscle weakness was assessed at follow-up appointments, approximately 3 years post IMRT. Muscle weakness was graded using the Common Terminology Criteria for Adverse Events (CTCAE) V3.0: grade 0, no weakness; grade 1, asymptomatic, but weakness on physical exam; grade 2, symptomatic weakness interfering with function, but not interfering with activities of daily living; grade 3, symptomatic weakness interfering with activities of daily living; grade 4, life-threatening or disabling weakness; grade 5, death due to muscle weakness.
DVH factors for muscles. The following parameters were collected from the dose-volume histograms (DVH) for the bilateral SCM muscles for each patient: mean dose (Dmean), maximum dose (Dmax), VX (the percentage of the SCM volume that received more than X Gy) and DX (

Follow-up and statistical analysis.
Patients were examined at least every 3 months in the first 2 years, and thereafter, patients returned for follow-up every six months (or until death). A complete physical examination, chest radiography and abdominal sonography were performed as routine follow-up, and neck and nasopharyngeal MRI was performed every 6 to 12 months. All data analysis was performed using SPSS version 13.0 (SPSS, Chicago, IL, USA). The endpoint of this research was the occurrence of moderate or severe (grade 2-3) SCM atrophy. To identify significant dosimetric parameters for moderate or severe SCM atrophy, the dosimetric parameters Dmean, Dmax, D20, D25, D30, D35, D40, D45, D50, D55, D60, D65, D70, D75, D80, V20, V25, V30, V35, V40, V45, V50, V55, V60, V65, V70, V75, V80 were compared in individuals with grade 0-1 SCM atrophy to those in individuals with grade 2-3 SCM atrophy using univariate analysis and the Wilcoxon rank-sum test. Binary logistic regression analysis was used for multivariate analyses to test whether the significant dosimetric parameters identified in univariate analysis were independent risk factors for moderate or severe SCM atrophy. Then, ROC (Receiver operating characteristic) curves were used to assess the dose tolerances for the independent significant factors with respect to moderate or severe SCM atrophy. The relationship between neck muscle atrophy and neck muscle weakness was analyzed using spearman rank correlation analysis. Two-sided P values ≤ 0.05 were considered statistically significant.

Results
Patient characteristics and incidence of SCM atrophy. The  Dosimetric analysis with respect to moderate or severe SCM atrophy. Significant dosimetric parameters and independent predictors of moderate or severe SCM atrophy. The median values and interquartile ranges of the dosimetric parameters for the SCMs with and without moderate or severe muscle atrophy are listed Table 2. The Wilcoxon rank-sum test showed that the dosimetric parameters Dmean, Dmax, D20, D25, D30, D35, D40, D45, D50, D55, D60, D65, D70, D75, D80, V20, V25, V30, V35, V45, V55, V60, V65, V70 and V75 were significantly associated with the development of moderate or severe SCM atrophy. On the contrary, the V40 (P = 0.055), V50 (P = 0.132) and V80 (P = 0.762) were not significantly associated with moderate or severe SCM atrophy ( Table 2). All of the significant dosimetric parameters were included in multivariate analysis using binary logistic regression. Moderate or severe SCM atrophy was significantly related to the V65 (β = 0.055, SE = 0.013, OR = 1.056, 95% CI = [1.029-1.084], P < 0.001), indicating that V65 is an independent predictor of moderate or severe SCM atrophy at 3-years post IMRT in patients with NPC.
Dose tolerance of the SCM for moderate or severe atrophy. Inclusion of the V65 in ROC curve analysis was used to identify the dose tolerance cut-off point for the SCM with respect to moderate or severe atrophy. The cut-off for significant parameters was selected using the Youden index with a level of P < 0.05. The dose tolerance of the SCM for moderate or severe atrophy was found to be a V65 of 21.47% (sensitivity 0.680 and specificity of 0.709). The area under the ROC curves for a V65 of 21.47% was 0.732 (Fig. 1) .The distribution of V65 is detailed in Table 3

Discussion
Despite recent advances in radiation technology that minimize exposure of the surrounding normal tissues, many patients undergoing radiotherapy using modern techniques still experience muscle atrophy [17][18][19][20] . Neck muscle atrophy after radical IMRT in patients with NPC, an irreversible phenomenon that -in our experience -can develop several years after IMRT, affects muscle cell function and reduces muscle contraction force, leading to the clinical manifestation of muscle weakness. Hence, measures to reduce the occurrence of neck muscle atrophy in patients treated with IMRT are attracting more and more attention. However, the pathophysiological mechanisms leading to late-onset radiation-induced muscle atrophy are complex and not completely understood. We explored the risk factors (age, gender, T stage, N stage, chemotherapy, the length of time after IMRT and weight change ratio) for IMRT-induced SCM atrophy in previous research. Only the N stage and the length of time after IMRT were found to be associated with IMRT-induced SCM atrophy 21 . Therefore, in the present study patients were examined 3 years after the completion of IMRT in order to exclude the effect of time on the occurrence of this late complication. Given that the N stage is related to the radiation dose delivered to the neck region, we speculated that, when the length of time after IMRT is constant, the frequency of IMRT-induced SCM atrophy will depend on the radiation dose to the neck region.
The total radiation dose received may be a significant factor in the disruption of muscle homeostasis; high dose radiation may be directly related to muscle precursor cell injury 22,23 . Additionally, Berthrong et al. 24 postulated   that radiation may indirectly affect muscles by promoting capillary and vessel injury. Therefore, we believe that the degree of muscle atrophy may be dependent on the radiation dose. In this study, SCMs that developed grade 2-3 atrophy received a significantly higher dose of radiation than SCMs with grade 0-1 atrophy ( Table 2), demonstrating that the severity of muscle atrophy at 3-years after IMRT is dependent on the radiation dose for the SCM. To our knowledge, only one case series has previously studied the relationship between the absorbed dose and atrophy of the SCM. Popovtzer et al. 20 investigated the dose-effect relationship for SCM atrophy at 3 months after chemo-irradiation in head and neck cancer; however, they showed that the muscle did not become significantly thinner if the mean dose to the SCM was < 50 Gy. In contrast, a significant reduction in thickness was observed in SCMs that received a mean dose of > 50 Gy. These differences indicate that SCM atrophy develops in a dose-dependent manner and that reducing the radiation dose to the SCM may effectively reduce the occurrence of severe muscle atrophy. However, Popovtzer et al. did not investigate the dose-effect relationship for muscle atrophy, and the dose tolerance of the SCM in terms of moderate or severe muscle atrophy had not been defined until now.
This study ascertained the dose tolerance of the SCM for moderate or severe atrophy. Grade 2-3 SCM atrophy (SCM atrophy ratio > 40%) was chosen as the study endpoint, as the risk of developing neck muscle weakness appears to increase when the degree of neck muscle atrophy is higher. Our previous and current research showed significant associations between the grade of SCM atrophy and the grade of neck weakness 21 . The current study found that 9.1% of SCMs suffered moderate or severe SCM atrophy, and 22.46% of patients had grade 2-3 neck weakness, 3-years post-IMRT. The high incidence of moderate or severe SCM atrophy and grade 2-3 neck weakness and the correlation of these two syndromes confirm the need to be alert for this complication. In univariate analysis, all dosimetric parameters except for the V40, V50 and V80 were significantly associated with the occurrence of moderate and severe SCM atrophy. Multivariate analysis indicated that the V65 was independently associated with moderate or severe SCM atrophy. ROC analysis suggested that keeping the V65 of the SCM below 21.47% is likely to decrease the incidence of moderate or severe SCM atrophy.
The use of V65 cut-off value to reduce the incidence of moderate or severe SCM atrophy may be clinically important. As the prescribed dose is limited to 60-68 Gy for the PTV of the nodal gross tumour volume (GTV-N) and 54 Gy for the PTV of the CTV-2 (neck nodal regions) and the SCM is adjacent to the irradiated target volume of the cervical lymph node regions, the dose to the SCM could be too high if an established dose limit is not applied for this OAR. However, IMRT has been proven to significantly spare the adjacent normal structures without compromising tumour target coverage 9,25-28 . Therefore IMRT may be capable of reducing the dose to the SCM while simultaneously delivering high doses to the tumour targets. By delivering high-dose irradiation to defined tumour targets while minimizing the dose to surrounding normal organs and tissues, including the oral cavity, the brain/ brainstem, and the spinal cord, it should be possible for radiation oncologists using IMRT technology to deliver a lower dose to the SCM as an OAR and obtain a V65 below 21.47% for the SCM.
If this dose limitation to the SCM is not possible, our results still provide a theoretical basis with which to predict the degree of radiation-induced muscle atrophy likely and thereafter explore possible salvage measures to reduce the severity of the resulting muscle atrophy. To date, exercise therapy is the only treatment method considered to be effective for muscle atrophy 29 . However, there are literature reports suggesting that there are around 19 drugs which may influence muscle atrophy, and may therefore have therapeutic potential 30 .
To the best of our knowledge, this is first study to investigate the dose tolerance of the SCM for IMRT-induced moderate or severe atrophy. The limitations of this study are the same major limitations of any retrospective study. We believe that this study provides some useful data; additional practical evidence is required to assess the feasibility of applying dose limitation to the SCM during treatment planning and confirm that the dose limitations reduce the incidence of moderate or severe SCM atrophy in patients with NPC treated with IMRT.