Helical tomotherapy planning
Patient will dress the diving suit (3 mm thick) to create bolus effect. AccuFix™ Cantilever Board™ with shoulder depression and thermoplastic fixation were used for head and shoulder immobilization. BlueBagTM immobilization system (Medical Intelligence) was used to fix main trunk and extremities. Both immobilization systems provide a consistent treatment position from the initial computed tomography (CT) scanning, pretreatment megavoltage CT (MVCT) imaging to final treatment delivery. For tomotherapy treatment planning, a CT image set of the whole body was required. The patients were scanned in a large bore (75 cm) CT scanner (Siemens, SOMATOM Definition, Dual source computed tomography system). Because most critical organs are located in the central part of the body, therefore two image sets were scanned with 2.5 mm and 5 mm for upper and lower part, respectively. The level at 15 cm above knee was used as a reference point to separate the upper and lower set. The geometric edges of both fields were abutted at the HT treatment's 50% isodose plane.
Both image sets were restored on a Pinnacle workstation using the Pinnacle Launch Pad Restore utility then using the Philips Pinnacle3 treatment planning system for contouring because the HT planning system has no such capability. After that, the plan was transferred to the Tomotherapy Hi Art Planning system (Tomotherapy, Inc., Madison, Wisconsin, USA). The clinical target volume (CTV) included the entire body surface system with subcutaneous 0.5 cm. To account for set-up variability and breathing motion, a planning target volume (PTV) was generated with a 0.5 cm margin The prescription dose was 75 cGy/fraction in 4 times/wk for total skin area and for tumor area. Total doses of 30 Gy to 95% of the PTV are delivered to the total skin area and tumor part, respectively. The normal tissue dose constraints utilized were based on the results of the survey of the clinical outcome of the target dose and dose limits to various organs at risk (OARs) such as the brain, optic chiasm, optic nerves, lenses, eyes, parotid glands, oral cavity, thyroid gland, bilateral lungs, esophagus, heart, liver, spleen, pancreases, kidneys, bowel, bladder, uterus and vagina. Maximum importance was given to target dose coverage. The constraints on dose and penalty were adjusted accordingly during optimization. The field width, pitch, and modulation factor (MF) used for the treatment planning optimization were 2.5, 0.287 cm, and 3.5, respectively. The dose volume histograms (DVHs) were calculated for the target and individual OARs. Toxicity of treatment was scored according to the Common Terminology Criteria for Adverse Events v4.0 (CTCAE v4.0).
Image guidance:
Daily check of patient positioning was performed by the MVCT system integrated in the tomotherapy machine. Briefly, before every treatment, MVCT scans were taken in selected regions using the normal mode (4 mm slice thickness) of the body and fused with the treatment planning CT scan using bony anatomy and soft tissue images such as the lungs. Three sets of MVCT scan (scan 1, orbits to T4; scan 2, T10 to the ischial tuberosities; scan 3, 15 cm above knee to 15 cm below knee) were performed to the check the patient's whole body alignment. MVCT scans were obtained. Image fusions were evaluated by the attending physician and physicist. Any translational shifts suggested by the image fusion results were applied to the final patient setup before treatment delivery. The tolerance of setup error allowed only a 5-mm difference between the three scans in any of the three translation directions and 1° of difference in roll. Additional selected MVCT scans were performed after treatment to verify patient immobilization.
