Does the fluence map editing in electronic tissue compensator improve dose homogeneity in bilateral field plan of head and neck patients?

The purpose of this study was to evaluate the effect of fluence map editing in electronic tissue compensator (ETC) on the dose homogeneity for head and neck cancer patients. Treatment planning using 6-MV X-rays and bilateral field arrangement employing ETC was carried out on the computed tomography (CT) datasets of 20 patients with head and neck cancer. All the patients were planned in Varian Eclipse three-dimensional treatment planning system (3DTPS) with dynamic multileaf collimator (DMLC). The treatment plans, with and without fluence editing, was compared and the effect of pre-editing and post-editing the fluence maps in the treatment field was evaluated. The skin dose was measured with thermoluminescent dosimeters (TLDs) and was compared with the skin dose estimated by TPS. The mean percentage volume of the tissue receiving at least 107% of the prescription dose was 5.4 (range 1.5-10; SD 2.4). Post-editing fluence map showed that the mean percentage volume of the tissue receiving at least 107% of the prescription dose was 0.47 (range 0.1-0.9; SD 0.3). The mean skin dose measured with TLD was found to be 74% (range 71-80%) of the prescribed dose while the TPS showed the mean skin dose as 85% (range 80-90%). The TPS overestimated the skin dose by 11%. Fluence map editing thus proved to be a potential tool for improving dose homogeneity in head and neck cancer patients planned with ETC, thus reducing the hot spots in the treatment region as well. The treatment with ETC is feasible with DMLC and does not take any additional time for setup or delivery. The method used to edit the fluence maps is simple and time efficient. Manual control over a plan is essential to create the best treatment plan possible.

Comparison of geometric uncertainties using electronic portal imaging device in focal three-dimensional conformal radiation therapy using different head supports.

AIMS AND OBJECTIVES: To study the geometric uncertainties in the treatment and evaluate the adequacy of the margins employed for planning target volume (PTV) generation in the treatment of focal conformal radiotherapy (CRT) for patients with brain tumors treated with different head support systems. MATERIALS AND METHODS: The study population included 11 patients with brain tumors who were to be treated with CRT. Contrast-enhanced planning CT scan (5-mm spacing and reconstructed to 2 mm) of brain were performed. Five patients were immobilized using neck support only (NR-only) and six patients had neck support with flexion (NRF), the form of immobilization being decided by the likely beam arrangements to be employed for that particular patient. The data was transferred to the planning system (CadPlan) where three-dimensional conformal radiation therapy was planned. Digitally reconstructed radiographs (DRRs) were created for the orthogonal portals with the fixed field sizes of 10 x 10 taken at the isocenter. Treatment verification was done using an amorphous silicon detector portal imaging device for using orthogonal portals and the DRR was used as a reference image. An image matching software was used to match the anatomical landmarks in the DRR and the portal imaging and the displacement of the portals in x, y axis and rotation were noted in the anteroposterior (AP) and lateral images. Electronic portal imaging was repeated twice weekly and an average of 8-14 images per patient was recorded. The mean deviation in all the directions was calculated for the each patient. Comparison of setup errors between the two head support systems was done. RESULTS: A total 224 images were studied in anterior and lateral portals. The patient group with NR-only had 100 images, while the NRF group had 124 images. The mean total error in all patients, NR-only group, and NRF group was 0.33 mm, 0.24 mm, and 0.79 mm in the mediolateral (ML) direction; 1.16 mm, 0.14 mm, and 2.22 mm in the AP direction; and 0.67 mm, 0.31 mm, and 0.96 mm in the superoinferior (SI) direction, respectively. The systematic error (S) in all patients, NR-only group, and NRF group in the ML direction was 0.31 mm, 0.28 mm, and 0.78 mm; 1.29 mm, 0.1 mm, and 2.24 mm in the AP direction; and 0.75 mm, 0.52 mm, and 0.94 mm in the SI direction, respectively. Random error (s) in all patients, NR-only group, and NRF group in the ML direction was 1.25 mm, 1.04 mm, and 1.41 mm; 1.31 mm, 1.36 mm, and 1.28 mm in the AP direction; 1.38 mm, 1.37 mm, and 1.39 mm in the SI direction, respectively. In all patients, the PTV margin with Stroom's formula in the NR-only and NRF group was 1.29 mm and 2.55 mm in the ML, 1.15 mm and 5.38 mm in the AP, and 2.0 mm and 2.85 mm in the SI directions, respectively. CONCLUSION: A PTV margin of 5 mm appears to be adequate; further reduction to 3 mm may be considered based on our results. Errors were significantly higher in the AP direction with NRF when compared to NR-only. Differential PTV margin may therefore be considered, with more margin in the AP and less in other directions, especially with the use of flexion devices.