3D-CRT, IMRT and VMAT for flank irradiation due to pediatric Wilms tumor: A comparative planning study with XCAT phantoms.

PURPOSE: To compare the dosimetric parameters and radiogenic risks from 3D-CRT, IMRT and VMAT for flank irradiation due to pediatric Wilms tumor. METHODS: Two computational XCAT phantoms simulating an average 5- and 10-year-old patient were used. Four different planning target volumes (PTVs) for right flank (RF) and left flank (LF) irradiation with or without paraaortic lymph nodes (LNs) and eight surrounding organs-at-risk (OARs) were contoured on the phantoms' CT sections. Forty-eight 3D-CRT, IMRT and VMAT plans were created using 6 and 10-MV photons on the two phantoms. The target coverage index (TCI), homogeneity index (HI), conformity index (CI), conformation number (CN) and OAR exposure were determined through dose-volume histogram (DVH) analysis. Second cancer risks were estimated using a non-linear model and DVH data. RESULTS: The IMRT and VMAT for LF + LN and RF + LN irradiation reduced the radiation dose to four to six out of the eight OARs compared to 3D-CRT. Conventional treatment provided a better organ sparing for RF and LF irradiation. The IMRT and VMAT led to superior planning parameters in respect to 3D-CRT for all PTVs and both patient's ages (3D-CRT: TCI = 59.80 % - 82.26 %, CI = 0.55-0.81, CN = 0.40-0.64, HI = 1.11-1.15; IMRT: TCI = 96.04 % - 99.72 %, CI = 0.85-0.91, CN = 0.85-0.88, HI = 1.03-1.05; VMAT: TCI = 96.02 % - 99.69 %, CI = 0.86-0.91, CN = 0.85-0.89, HI = 1.03-1.06). The excess-absolute-risk for developing secondary small intestine, liver and stomach malignancies from 3D-CRT were (7.99-19.32) × 10-4, (0.29-3.83) × 10-4 and (0.37-4.50) × 10-4 persons-year, respectively. The corresponding risks from intensity modulated techniques reached to 22.26 × 10-4, 4.58 × 10-4 and 5.42 × 10-4 persons-year. CONCLUSIONS: This dataset related to plan quality, radiation dose and risks to OARs allows the selection of the proper treatment technique for flank irradiation based on the patient's age and target site.

Risk for second bladder and rectal malignancies from cervical cancer irradiation.

The objective of this study was to estimate the risk of developing second malignancies to partially in-field organs from volumetric modulated arc therapy (VMAT) of cervical cancer and to compare the above risks with those from the conventional three-dimensional conformal radiotherapy (3D-CRT). Seventeen consecutive patients with uterine cervix carcinoma were selected. VMAT and 3D-CRT plans were generated with 6 and 10 MV photons, respectively. The prescribed tumor dose was 45 Gy given in 25 fractions. Differential dose-volume histogram data from the treatment plans were obtained for the partially in-field organs such as bladder and rectum. These data were used to estimate the patient-specific lifetime attributable risk (LAR) for bladder and rectal cancer induction with a non-linear model based on a mixture of plateau and bell-shaped dose-response relationships. The estimated risks per 10000 people were compared with the baseline risks for unexposed population. The patient-specific rectal cancer risk estimates from VMAT were significantly lower than those from 3D-CRT (P = 0.0144). The LARs for developing bladder malignancies from VMAT were significantly high compared to those from conventional irradiation (P = 0.0003). The mean difference between the patient-specific LARs for radiation-induced bladder and rectal malignancies as derived from 3D-CRT and VMAT plans was 6.6% and 2.0%, respectively. The average LAR for developing bladder and rectal malignant diseases due to VMAT was 9.2 × 10-4 and 43.7 × 10-4 , respectively. The corresponding risks following 3D-CRT were 8.6 × 10-4 and 44.6 × 10-4 . These average risks showed that pelvic irradiation increases the baseline probability for cancer induction by 12.6-19.1%. The differences in the second cancer risks associated with the VMAT and 3D-CRT for cervical cancer were found to be small. Both treatment techniques resulted in considerable increased probabilities for developing bladder and rectal malignancies relative to those of unirradiated population.