In this study, we aimed to solve the problem of insufficient clinical target dose due to respiratory motion during radiotherapy for postoperative left breast cancer patients. Robust optimization module was introduced to limit the minimum dose of PTV-CHEST in patient's planned target area. After shifting, we found that the Robust optimization plans provided better target coverage, HI, PTVD98, D95 and D2 than those without Robust optimization plans (P<0.05). The dose distribution in these Robust optimization plans was less affected by perturbations. The PTV-CHEST with the greatest activity had Robust optimization slightly better than Robust optimization (P=0.000060). The target coverage rate of CTVV50 without Robust optimization was 79.12±9.51%, and the target coverage rate with Robust optimization was 90.58±4.42%. There was a significant difference in the target coverage of CTVV50 between the two treatment plans (P=0.000069, P< 0.05). It could be clearly seen from Fig.3 that shifted center of field leads to changes in CTV coverage.
In terms of OARs, it was mainly statistically different between V5 of patients' ipsilateral lung and Dmax of spinal cord. The treatment plan of V5 without Robust optimization in patients' ipsilateral lung was lower than that with Robust optimization, and Dmax of spinal cord was significantly higher than those were enrolled in Robust optimization. In terms of machine execution efficiency, there was no statistical difference between them in terms of total beam time and total MU.
In order to reduce the errors caused by patients' respiratory movement in actual treatment, the Robust optimization technology was added to radiotherapy plan, and clinical studies had shown that it could not only improve the coverage rate of the clinical target area, but also reduce the dose of OARs.The Mahmoudzadeh team’s experiment proved that the introduction of Robust optimization could potentially reduce the need for deep inspiration breath-hold technology, allowing patients to reduce the exposure dose to heart with breathe freely, and improve the dose coverage of tumor area 21. Fredriksson proposed that adding Robust optimization could significantly increase the patient’s skin dose, and to some extent, it could replace the efficacy of VB 22. The Alex Dunlop team added Robust optimization to study breast cancer radiotherapy based on organ motion, found that D98, D95, D50 and D2 of CTV that added Robust optimization were significantly different from treatments without Robust optimization (P<0.01), they proposed that the use of robust optimization based on organ motion to generate VMAT plan is clinically acceptable for the typical and extreme target area changes during treatment 14. Hideharu Miura et al. verified the advantages of introducing Robust in other tumor types with greater CTV activity, such as Larynx cancer,Robust-optimized plan had better CTV coverage rate than that without Robust-optimized, and less carotid artery exposure dose 23. This was slightly different from actual exposure dose of OARs dose parameters in this study, the exposure dose of ipsilateral lung V5 was higher than that without Robust optimization, because the improved CTV coverage in our study was at the expense of a low dose bath to healthy tissue, thereby delivering a higher volume of low dose to the ipsilateral lungs than that without Robust optimization plans.
Designed PTV was aim to reduce the error impact of a series of radiotherapy procedures on CTV, while the clinical target area of postoperative left breast cancer patient is long and narrow, and close to the surface of skin, CTV can only expand to the chest cavity, but not to the outer surface of skin, compared with other tumor target areas, respiratory movement is more likely to affect actual radiation dose of breast cancer’s CTV, introduced of Robust optimization method significantly improved this situation.As could be seen from Figure 4, the blade width of multi-blade collimator in the Robust optimization plan increased by about 0.81cm compared with that without Robust optimization. When the center of field moved a same distance, the coverage rate of CTVV50 dose with Robust optimized treatment plan increased by about 14.49%, while V5 of the ipsilateral lung only increased by about 1.00%,the reason for the increase was the introduction of Robust optimization, which optimized the movement of target area to the left and front due to respiratory movement, when seeking the best solution under the condition of increasing uncertain factors, the actual volume of the illuminated target area increases and the scattering amount increased in the same proportion. Although Robust optimization could ensure the dose coverage of target area to a certain extent, it was still recommended if patient's respiratory movement amplitude caused the difference between outer contour of target area and the planned target area to exceed 0.50 cm during CBCT verification before implemented the treatment plan, re-check the patient's position or use deep inspiration breath-hold techniques as appropriate.
In summary, under the premise that with or without Robust optimization met clinical requirements before displacement, the deviation of CTV target area of human respiratory motion was simulated by changing the position of center point of radiation field. VMAT plans using the robustness feature of RayStation are less affected in average than that without Robust optimization, it could be seen after the shift that the treatment plan optimized by Robust was better for the D98, D95, D2, CI, HI of PTV and the most active PTV-CHEST dose parameters, and in most important CTV coverage rate, the Robust optimized plan was significantly higher than the without Robust optimization plan, meanwhile better CTV prescription dose coverage usually means more effective tumor control rates. Therefore, FFF-VMAT Robust optimization treatment plan is better than the plan without Robust optimization. If more Robust is needed to optimize the follow-up radiotherapy effect and evidence of clinical use, the sample size can be increased for research.