OBJECTIVE: To estimate the combined effect of setup uncertainty on optimal dosimetric margin by analyzing the dose distribution and biological effect in LINAC-based stereotactic radiosurgery (SRS) with dynamic conformal arc (DCA) technique.
METHODS: SRS treatment plans were generated from CT scans of the Rando head phantom using four non-coplanar DCA\'s with total 480-degrees of arc. A single spherical planning target volume (PTV) of 4 different diameters was placed at the center of the phantom to simulate brain lesions. For each PTV, 5 treatment plans were created using identical dose calculation parameters, each with 5 different dosimetric margins. To simulate the effect of setup uncertainty, the isocenter for each plan was shifted to 13 different positions. A marginal dose of 20Gy in a single fraction with 6MV photon beam was prescribed to 49 different percentage isodose surfaces (%IDS). The plan quality was evaluated using Conformity Index (CI), Gradient Index (GI), EUD-based Tumor Control Probability (TCP), Normal Tissue Complication Probability (NTCP), and uncomplicated biological objective function (TCP x (1-NTCP) =p+).
RESULTS: A +1mm dosimetric margin could result in a much higher p+ compared to 0mm and 1mm dosimetric margins and a smaller GI while achieving an equivalent p+ in a certain range of %IDS compared to +2mm and +3mm dosimetric margins. With 2mm setup error and +1mm dosimetric margin, the %IDS range optimized for each PTV is: around 80%IDS (10mm diameter); 63~70%IDS (20mm diameter); 66~79%IDS (30mm diameter).
CONCLUSIONS: This simulation study identified the preferred prescription %IDS for a given setup error and dosimetric margin to achieve an optimal dose distribution and favorable biological effect.

OBJECTIVE: Our purpose was to explore which immobilization is more suitable for clinical practice in postmastectomy intensity modulation radiotherapy, the single-pole position or the double-pole position?
METHODS: Patients treated with postmastectomy intensity modulation radiotherapy were eligible. They were selected randomly for single-pole position or double-pole position. Dose-volume histogram (DVH) was used to evaluate plans. After their first radiotherapy, the physicians asked a question about the comfort level of their position. The dosimetric parameters, comfort levels, and reproducibility of the two immobilization techniques were collected and analyzed after all patients had finished the whole radiotherapy.
RESULTS: Totally, 94 patients were enrolled. Of these, 54 patients were treated with the single-pole position, 28 (51.9%)had left-sided lesions. While 40 patients were treated with the double-pole position, 20 (50%) had left-sided lesions. Patients\' characteristics in two groups were comparable. The single-pole and double-pole immobilizations had similar conformity (0.60 ± 0.05 vs 0.60 ± 0.06, P = 0.887) and homogeneity index (0.14 ± 0.03 vs 0.13 ± 0.03, P = 0.407). Compared to single-pole position, double-pole position typically increased the mean dose, V20 , and V30 of heart (P < 0.05). Moreover, patients in the single-pole group felt more comfortable than another group (P < 0.05). There was no difference in reproducibility between the two groups (P > 0.05).
CONCLUSIONS: Single-pole position seems to be more comfortable and can reduce dose coverage to heart. Both devices allow for reproducible setup and acceptable dosimetry.