Room: Exhibit Hall
Purpose: To evaluate portal imaging (PI) dose to organs at risk (OARs) in head and neck (H&N) patient IMRT radiotherapy.
Methods: This study included 28 patient treatment data. Through the treatment course, patients received an average 33.04Â±2.17 fraction treatments, 11.25Â±3.08 times PI, and 122.96Â±47.35 MU delivered for PI. Varian EPID was used with 6MV beam for PI. Eclipse (v11) was used as planning system. The dose calculation grid size was 0.25 cm. Orthogonal PI fields were set to 10x10 cm2 with delta 4. A same field size / same gantry angle PI field was generated as one PI sub-plan with its total delivered MUs. Final treatment organ doses were calculated by summed all the treatment plans and all the PI sub-plans together. A maximum dose (Dmax) for cord, cord+5mm, mandible and mean dose (Dmean) for parotid were evaluated.
Results: The results showed that Dmax increased between 2.90 and 181.00, 3.20 and 221.30, 0.90 and 191.30 cGy for spinal cord, cord+0.5cm, and mandible, Dmean increased between 12.60 and 114.60, 8.80 and 86.80 cGy for ipsilateral and contralateral parotid because PI during treatment course. An additional average 7.62Â±2.53, 6.85Â±2.55, 7.87Â±2.92 cGy doses were deposited to cord, cord+0.5cm and mandible Dmax, 5.23Â±2.01, 4.24Â±1.56 cGy doses were deposited to ipsilateral and contralateral parotid Dmean per PI. An additional average 0.71Â±0.22, 0.65Â±0.24, 0.73Â±0.25 cGy doses were deposited to cord, cord+0.5cm, mandible Dmax, 0.49Â±0.20, 0.40Â±0.14 cGy doses were deposited to ipsilateral and contralateral parotid Dmean per MU used for PI.
Conclusion: Amount of PI dose to OARs effected by number of factors, such as (1) patient size; (2) device; (3) department policies; (4) MUs needed for each PI field; (5) number of patient positioning adjustment; Final PI dose to OARs was unpredictable before a treatment course completed, but it could be tracked from used MUs.