Uncertainties Of Organs At Risk Dose During Intensity-Modulated Radiotherapy For Nasopharynx Carcinoma

Purpose:To assess the uncertainties of organs at risk (OAR) dose during intensity-modulated radiotherapy for nasopharynx carcinoma, and to provide evidence of dose limiting on OAR protection.Method:First, the impact of OAR delineation difference on OAR dose was accessed.16 patients newly diagnosed with NPC were selected in the study. The OAR was delineated separately by three junior physicians and three senior physicians, and the geometric and dosimetric differences were assessed relative to the reference OAR. Delineation was performed again for the two OARs with the biggest difference after specific training in the two groups of physicians, and then differences were evaluated again.Then assess the Normal Tissue Complication Probability (NTCP) difference on lower cranial nerves (LCNs) between the Radiation Therapy Oncology Group (RTOG) based and China based target definition in IMRT planning for T1-2 nasopharyngeal carcinoma (NPC).20 patients with NPC were selected and the LCNs were contoured on CT simulation image respectively. The target was contoured and subsequent IMRT planning was designed based target definition recommend respectively by RTOG0225 and Working committee for the clinical stage NPC in China (2010 years). Calculate the dose and NTCP difference of LCNs between the two target definition methods subsequently.Finally,20 patients with T3-4 NPC were studied by the same may. The difference of dose and NTCP between the two groups was assessed.Results:On delineation of OAR, the maximum dose differences (Dmax) ofOAR in the junior and senior physicians were (2.33±12.06)%(-48.06%-137.82%) and (0.09±4.72)%(-49.54%-42.96%) (P=0.039), respectively (P=0.039), and the difference in the optic chiasm was the greatest ((5.85±19.63)%:(1.36±4.64)%, P=0.042). The mean dose differences (Dmean) of OAR in the junior and senior physicians were (3.10±8.07)%(-46.76%-59.76%) and (-0.93±2.03)% (-45.54%-35.69%), respectively (P=0.021), and the difference in the parotid gland was the greatest ((13.23±13.39)%:(3.20±6.71)%, P=0.002). In the secondary delineation after training, the Dmax of the optic chiasm in the junior and senior physicians was (1.68±3.34)%and (1.50±1.87)%, respectively (P=0.841), and the difference in junior physicians was reduced significantly compared with before training ((1.68±3.34)%:(5.85±19.63)%, P=0.048); the Dmeaa of the parotid gland in the junior and senior physicians was (2.46±3.06)% and (1.35±3.00)%, respectively (P=0.274), significantly reduced compared with before training ((2.46±3.06)%:(13.23±13.39)%, P=0.002; (1.35±3.00)%:(3.20±6.71)%, P=0.033).On LCNs dose, the two target definition methods based (RTOG and China) Dmax of the left, right sides was (7450±273)cGy, (7294±309)cGy and (7361±160)cGy, (7190±395)cGy and the Dmax from RTOG method was larger than form China method (P=0.018,0.042). The Dmean of left, right sides was (6735±285)cGy, (6660±333)cGy and (6446±429)cGy, (6299±467)cGy, and the Dmean from RTOG method was larger than from China method (P= 0.000,0.000). The LCNs NTCP of the left, right sides was (60±10)%, (57±13)% and (51±15)%、(45±17)%, and China method based NTCP was much lower than RTOG based (P= 0.000,0.000).The Dmax,Dmean and NTCP in T1-2 group were significantly less than those in T3-4 group, ((7433±427) cGy Vs (7516 ±339) cGy, (6495±498) cGy Vs (6784±352) cGy, (53.00±12.58)% Vs (59.63±15.14)%,p=0.046)。Conclusions:During IMRT of nasopharynx cancer, the delineation difference of OAR may result in dose uncertainties and the special training could improve the delineation accuracy. And it is feasible to precisely delineate the LCNs which can be served as a routine OARs and to further assess its dose and NTCP in IMRT planning for T1-2 NPC. Of the two most common target definition methods in IMRT, the China method based LCNs dose and NTCP was much lower than which RTOG based. Thedose and NTCP of LCNs in T1-2 group were significantly less than those in T3-4 group.