The hot and cold spots in the junction region are one of the concerns for breast cancer RT with supraclavicular nodes. With conventional radiotherapy techniques using two opposing tangential and anterior fields, it is extremely difficult to deliver a homogeneous dose to the PTV in the junction region. In addition, this conventional radiotherapy technique is sensitive to possible misalignment errors and causes the emergence of hot and cold spots in the junction region . The effect of the matching techniques on the dose variations in the junction region was investigated using a breast phantom . Two different treatment techniques were compared under an overlap of 5 mm, a separation of 5 mm, and it was concluded that set-up difficulties influence the dose distribution. The conventional 3D-CRT treatment plan has a high dose gradient and the junction region is easily over or underdosed with the introduction of mechanical inaccuracies or possible patient movement during the treatment. The effect of uncertainty in the jaw positioning on dose distribution in the junction region was investigated . The jaw positional errors should be managed to be less than 1 mm according to the American Association of Physicists in MedicineTask Group 40 .
A combination of two tangential fields and VMAT for breast cancer has already been reported . Their strategy includes 2 tangential open fields with a 2 cm cranial slip zone delivering 85% of the chest wall area. In this study, we investigated the robustness of our proposed hybrid VMAT in breast cancer to achieve dose homogeneity in the junction region. Our proposed hybrid VMAT technique with a low dose gradient on the 3D-CRT plan could reduce the over and under dosage in the junction region. As shown in the dose profile, extreme patient motion (± 3 mm) can potentially lead to areas of high and low doses that can be up to approximately 30% different from the nominal plan. On average, the D2% to the CTV of 1.5%, 3.5%, 5.5%, and D98% to the CTV of 3.3%, 8.2%, and 12.8% for the 1 mm, 2 mm, and 3 mm, overlapped/separated shifted evaluations were improved, respectively. This is because a shifting jaw on the 3D-CRT plan will result in a low dose gradient from the VMAT also being smooth in the junction region.
Patient setup and motion errors during treatment can result in small shifts in treatment fields; typical misalignment errors may vary between 0 mm and 3 mm [23, 24]. As shown in the dose profile, our study demonstrated that a 3 mm patient motion in the SI direction resulted in a dose difference of approximately 15% in the junction region, using an even hybrid VMAT plan with a low dose gradient in the 3D-CRT plan. Daily patient setup and patient motion are random sources of errors throughout the entire treatment. Intrafraction motion is significantly limited with a translational median of 1.1 mm from the isocenter . Patients are not likely to move in the SI direction (mean = 0.09 ± 0.81 mm, median = 0.04 mm) . Patient monitoring with a surface scanning system is expected to decrease patient motion during treatment and decrease the effect of field separation or overlap. The immobilization positioning system is also essential to minimize patient motion during the time of treatment. Studies have reported significantly improved reproducibility and decreased random and systematic inter-fraction errors, with the use of an immobilization device [25, 26]. Some patients feel comfortable positioning themselves during treatment, as indicated with a simulation using an immobilization device . The beam-on-time of our hybrid VMAT approach for breast cancer patients is required for only approximately 2–3 min after image guidance. Therefore, we believe that the patient motion with the immobilization device and our proposed hybrid VMAT is minimal during treatment.
A limitation of this study was that we only examined one dose gradient with a jaw shift. A large jaw shift contributes to a shallow dose gradient and a more robust plan for patient motion. The low-dose gradient plan was able to decrease the dose differences when patient shift errors were introduced, as was also found for the dose profiles, by a maximum of 12.8% for the 3 mm shift. However, a greater dose contribution to the lung on VMAT should be considered for respiratory motion and organ deformation . A balance between the junction dose step size length and dose step size should be considered based on plan quality. The issue of dose junction associated with large treatment volumes presents problems with other sites, such as craniospinal irradiation and total marrow irradiation [28–30]. In these treatments, the couch is moved next to the treatment plan position because of the restricted field size. Therefore, a shallow dose gradient is used for plan robustness due to set-up error and patient motion at these treatment sites. It should be stressed that this study considered a maximum dose variation in the worst scenario. Intrafraction respiratory chest wall motion and the daily set-up errors have blurred the cold spots or hot spots in the junction region. Mathematical calculations of the probability distribution should be considered for the number of fractions because day-to-day setup variations were random.