| Objective: To analyze dosimetric advantages of helical tomotherapy(HT)by making dosimetric comparisons among helical tomotherapy(HT),intensity-modulated radiotherapy(IMRT)and anterior-posterior parallel-opposed(AP-PA)with involved-field(IF)and involved-site(IS)for mediastinum-involved early stage lymphoma.Methods:15 mediastinum-involved early lymphoma patients had been chose from radiotherapy planning data pool.The IF-PTVs and IS-PTVs had been created on CT images of those 15 patients according to the initially(pre-chemotherapy)involved lymphoma.Under the same criteria of requirement of dose coverage for targets and dose limits for OARs,AP-PA,IMRT and HT planning techniques had been used for both IF-PTV and IS-PTV.i.e.6 plans(IF/AP-PA,IS/AP-PA,IF-IMRT,IS-IMRT,IF-HT,IS-HT)had been created for each patient which total to 90 plans for 15 patients to be analysised.Organs at risk(OARs)such as heart(including its sub-structures),lung,female breast,spinal cord,esophagus and thyroid gland had been delineated.Dose prescription was 30 Gy in 15 fractions normalized to 95% iso-dose line.Dose comparison had been done based on the parameters embedded in their respective dose-volume histograms.Results: All the 6 plans for each case had met the criteria required for targets coverage.However,target dose conformity and homogeneity in AP-PA plans are inferior to both IMRT and HT plans(p=0.001),95% iso-dose conformity index is higher in HT plans than those in IMRT plans which is most evident when IS-PTV is applied(p=0.005).Dose homogeneity index are equal superior for IMRT and HT plans(homogeneity index close to 1).Irradiation received by the percent non-target human body volume was lower in AP-PA plans than that in HT and IMRT plans in the dose under 10 Gy but higher in the dose above 10 Gy.Body volume irradiated in all dose levels is lower in HT than in IMRT plans.The radiation dose received by the percent lung volume was the lowest in HT among 3 techniques across all dose levels.When field size decreased from IF to IS,the body volume exposed to radiation is decreased in all 3 techniques(?VIF-VIS: 28.5% to 57.5%),while the exposed volume reduction of middle to high doses in IMRT and HT was larger than that in AP-PA(?VIF-VIS: 44.7% to 57.5%),HT plans show better than IMRT in the protection of normal tissue from being irradiated.The magnitude of drop level in lungs was not significant between HT and IMRT plans(p>0.05),while the decline rate in HT plans was more evident than in IMRT plans(for instance,V20,?VIF-VIS/VIF,IMRT vs.HT: 33% vs.45.7%).The mean dose in breasts,heart and its sub-structures,esophagus and thyroid was similar in both HT and IMRT plans.Conclusion: The irradiated dose and volume in organs at risk had been decreased with field reduced from IF to IS in the radiotherapy of lymphoma with involvement of mediastinum.The HT is more powerful than AP-PA and IMRT in the reduction of lung and body radiation dose and volume hence more desirable in treating early mediastinal lymphoma. |
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