Objective To analyze the differences of intensity-modulated radiotherapy (IMRT) plan verification results obtained from various verification methods with PTW 729 ionization chamber matrix for postoperative cervical cancer patients.
Methods The radiotherapy data of 10 postoperative cervical cancer patients, aged 44–69 years old, with a median age of 59 years old were analyzed retrospectively. The patients were underwent IMRT in the Second Affiliated Hospital of Nantong University from August 2020 to December 2020. The IMRT plan was designed with seven fields. Using the treatment planning system, four verification plans, namely, V1–V4, with different phantoms were designed. Three 2D isocenter plane verification plans (V1−V3) and one 3D stereoscopic verification plan (V4) were designed. In V1, all field angles were converted to 0°, and PTW 729 RW3 solid water was used which composed of 4.2 cm-thick solid water + ionization chamber matrix + 5.0 cm-thick solid water. In V2, all field angles were converted to 0°, and the PTW Octavius 4D phantom was used. The actual angle and the Octavius 4D phantom were used to obtain one-plane verification (V3) and stereoscopic verification (V4). Then, all of the above mentioned four verification plans were actually measured in an accelerator, and the differences of the measurement results between the verification plans in which different verification phantoms were used (V1 vs. V2), between two 2D plans in which all field angles were converted to 0° or actual angle were used (V2 vs. V3), and between 2D and 3D plans(V1, V2, V3 vs. V4), were analyzed with paired t test respectively.
Results (1) The plan validation pass rate of V1 was significantly higher than that of V2 ((99.72±0.44)% vs. (94.95±6.13)%, t=2.621, P<0.05). The evaluation point of V1 were lower than that of V2 (311±50 vs. 392±61, t=−6.992, P<0.05). (2) The single-field validation pass rates of 180° and 232° fields in V2 were higher than that in V3 ((96.86±3.79)% vs. (95.72±3.56)%, (98.50±2.28)% vs. (92.98±5.04)%; t=2.294, 4.052; both P<0.05). (3) The plan validation pass rates of V1, V2, and V3 plans were significantly higher than that of V4 ((99.72±0.44)%, (94.95±6.13)%, (94.72±6.43)% vs. (86.91±2.63)%; t=17.912, 6.645, 5.962; all P<0.05). The evaluation points of V1, V2, and V3 were significantly lower than that of V4 (311±50, 392±61, 391±60 vs. 726 034±61 656; t=−37.244, −37.253, −37.252; all P<0.05).
Conclusions Differences were found in the verification results of the IMRT plan obtained from different verification methods with PTW 729 ionization chamber matrix, especially the single field verification results for 180° and 232°. Therefore, to judge whether the IMRT plan for postoperative cervical cancer passed the verification, the verification methods and the location of the evaluation point with significant dose difference must be considered.
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