ABSTRACT
Objective To commission a Mobetron intra-operative mobile accelerator and analyze the characteristics of its electron beams. Methods The dosimetrie characteristics of the electron beams genera-ted by Mobetron accelerator were measured and compared with those generated by conventional accelerator (Primus, Siemens). M oberton accelerator can generate electron beams of nominal energies of 4,6,9 and 12 MeV. The measurement items were as followings : percentage depth dose perpendicular to water phantom sur-face and beam profiles parallel to water phantom surface, output factors, applicator leakage, electron beam at-tenuation made by lead blocks,and machine output calibration. The measurement devices included a three-dimensional ( 3 D) water scanning phantom, an electrometer, a 0.6 cm3 Farmer ionization chamber, a parallel-plate ionization chamber and solid water slabs. During measurement, all applicators of different tilt angles and diameters were attached to the machine head,and their ends were adjusted to be tangent to the phantom surface. Results Except for the 12 MeV,skin dose for all energies was no more than 90%. The skin dose was higher for Mobetron accelerator electron beams than for regular electron beams. The Dmax depth in water for a 10 cm flat applicator were 0.7,1.3,2.0 and 2.2 cm for the 4 energies,respectively. The depths of 90% dose were 1.0,1.8,2.7 and 3.6 cm, respectively. The selected flat applicator was just 1 cm larger than the tumor bed. But for the beveled applicators,the field flatness and symmetry became worse,and con-sequently,the applicator size had to be selected based on the isodose distribution. The leakage dose at 1 cm outside the applicator was 1.2% ,5.1%, 10.0% and 9.1%, respectively. The lead thickness for full block was 1.5,3.0,4.5 and 6.0 mm,respectively. Conclusions Through the commissioning of Mobetron accel-erator, the machine characteristics are understood, and the data for clinical implementation and routine quality assurance are acquired.
ABSTRACT
Objective To evaluate the dose distribution of target volume and normal tissues with different treatment planning such as three dimensional conformal radiotherapy(3DCRT) ,simplified intensity modulated radiotherapy(sIMRT) ,and intensity modulated radiotherapy (IMRT) for patients with radically resected rectal cancer. Methods Ten male patients with stage Ⅱ and Ⅲ rectal cancer after radical resection (Dixon surgery) were enrolled in this study. 3-field or 5-field 3DCRT,slMRT and 5-field or 7-field IMRT plans were performed for each patient. The dose distributions of target volume and normal tissues,conformal index(CI) and heterogeneous index(HI) were analyzed using the dose-volume histogram(Dvit). The prescription dose was 50 Gy in 25 fractions. Results The CI for PTV of IMRT and sIMRT was superior to 3DCRT. Conversely,the HI for PTV of 3DCRT was superior to sIMRT and IMRT. sIMRT and IMRT can protect the organs at risk better than 3DCRT. The mean of total MU for 3DCRT3f,3DCRT5f, sIMRT, IMRT5f and IMRTT7f was 482±13,504±11,455±42,841±36 and 884±46, respectively. Conclusions Comparing with 3DCRT plans and IMRT plans, sIMRT plan was the optimal plan for clinical practice. All of the three radiotherapy techniques can protect the rectal stump and anal canal well with the prescription dose of 50 Gy.
ABSTRACT
Objective Studies on IMRT technique demonstrates that the improved dose homogeneities throughout the breast as well as reduction in dose to the heart and the ipsi-and control-lateral lungs. Methods Based on the physical principle of rolling tangential technique, a treatment plan for a patient has been planned by using Pinnacle aperture-based inversion software. The rolling tangential IM-beam was pre-determined at each pre-selected gantry angle with normally angle separation of 5?, 10? or 15?. Each beam's width was so chosen that the inner-and outer-edge of the beam should be tangential to the inner chest wall and target outer surface respectively. For the starting and ending tangential beam, a pair of opposing beam was designed. The beam's orientation was suggested to be reversed to its opposite angle when the tangential beam's gantry angle reached 90? or 270?. Each beam's shape and size were adjusted on its BEV map. For each tangential beam, 7 sub-segment beams were chosen for each tangential beam. Then, planning system was allowed to do optimization according to the clinical dose prescriptions. Results The calculated dose distributions in target(CTV), left & right lungs, and heart showed that the volume received the prescribed dose 50?Gy V_ 50 was 0.95; The volume in target enclosed with 95%, 105%, 110%, 115% isodose lines V_ 95% , V_ 105% , V_ 110% , V_ 115% was 0.98, 0.84, 0.53, 0.18, respectively. The volume of V_ 10 ,V_ 20 , V_ 30 , V_ 40 V_ 50 , in the lungs and heart were significantly decreased when compared to the current IMRT techniques published in the literature(Table 1).Conclusions The rolling tangential IM-segment irradiation technique for the breast developed by the author is able to offer not only a better dose distribution and better dose conformity throughout the target, but also reduce significantly the dose to the lungs and heart, with the patient's set up and localization process being very simple. The treatment planning for individual patient is optimal in any treatment planning system with Aperture-Optimization inverse software. This technique is certainly to offer more benefit to the patient if some measures are taken to limit or to follow the movement of the chest wall during irradiation.