RESUMO
PURPOSE: The goal of this work was to analyse small bowel (SB) dose-volume following the Quantitative Analyses of Normal Tissue Effects in the Clinic (QUANTEC) guidelines for rectal cancer patients treated using a couch top inclined belly board (iBB). As part of this, the consistency in SB displacement was evaluated using on-treatment cone-beam computed tomographic (CBCT) imaging. METHODS: Twenty-four patients with rectal cancer were treated on a commercially available iBB. All patients went through the standard radiochemotherapy protocol in either a pre- or postoperative setup. All patients underwent weekly CBCT imaging during the course of radiation treatment. The planning computed tomographic data sets were used to analyze the quality of SB displacement, and the CBCT data sets were used to assess the reproducibility in SB displacement during treatment. The SB dose volume was evaluated and compared with QUANTEC-recommended dose limitations. Similarly, the impact of body mass index on dose volume and SB displacement was evaluated. RESULTS: The SB displacement was assessed respectively as "good" and "very good" by both independent evaluating radiation oncologists. The consistency of SB displacement through the course of radiation treatment was scored as "excellent" for 22 of 24 and 23 of 24 patients by both radiation oncologists, respectively. The QUANTEC recommendation was met for all patients without bowel adhesions; however, the most benefit was observed for patients with body mass index > 23 kg/m2. CONCLUSIONS: Our study has shown that QUANTEC recommendations for SB dose during rectal cancer treatment can easily be met by treating patients on a couch top iBB. This technique is robust and produces consistent SB displacement.
RESUMO
AIMS: To calculate a planning target volume (PTV) margin that would account for inter-fractional systematic and random clinical target volume positional errors for patients treated prone on a recently available couch top bellyboard and to evaluate potential critical structure dose reduction using intensity-modulated radiotherapy (IMRT) techniques. MATERIALS AND METHODS: Twenty-four patients (12 men and 12 women) were included in this study, all treated on a commercial bellyboard. Cone beam computed tomography (CBCT) data were acquired once every five fractions for a total of five images per patient. A three-dimensional-three-dimensional bony anatomy auto-match was carried out off-line and the residual difference in position used as a surrogate for clinical target volume inter-fractional positional errors. Systematic (Σ) and random (σ) variations were evaluated and used in PTV(margin)=1.96Σ+0.7σ. The influence of intra-fractional positional errors was evaluated in the margin analysis by introducing published values. Critical structure sparing, as a function of PTV(margin) size, was investigated through the evaluation of three-dimensional conformal radiation therapy (3DCRT) and IMRT treatment plans developed using the margin derived from this work, the American Society for Radiation Oncology Contouring Atlas and the Radiation Therapy Oncology Group 0822 trial specifications. RESULTS: The PTV(margin) that accounts for only the inter-fractional positional errors was calculated to be (anterior-posterior (AP), superior-inferior (SI), left-right (LR))=(5.2mm, 3.1mm, 2.8mm). If we assumed a combined intra-fractional motion up to 3.0mm then the required PTV(margin) increased to (AP, SI, LR)=(7.0mm, 5.0mm, 5.0mm). Treatment plan evaluation showed that the bellyboard provides excellent small bowel sparing regardless of planning technique. In most cases, IMRT reduced the average femoral head, bladder and small bowel dose by 20, 15 and 40% with respect to 3DCRT planning. CONCLUSION: A PTV(margin) expansion of (AP, SI, LR)=(7.0mm, 5.0mm, 5.0mm) is required to account for all positional uncertainties. The use of a bellyboard with IMRT provides better critical structure sparing when compared with a bellyboard with 3DCRT.
Assuntos
Planejamento da Radioterapia Assistida por Computador/métodos , Neoplasias Retais/radioterapia , Terapia Combinada , Tomografia Computadorizada de Feixe Cônico/instrumentação , Tomografia Computadorizada de Feixe Cônico/métodos , Fracionamento da Dose de Radiação , Feminino , Humanos , Masculino , Decúbito Ventral , Dosagem Radioterapêutica , Planejamento da Radioterapia Assistida por Computador/instrumentação , Radioterapia de Intensidade Modulada , Neoplasias Retais/diagnóstico por imagem , Neoplasias Retais/patologia , Neoplasias Retais/cirurgiaRESUMO
The goal of this work was to use daily kV-kV imaging and weekly cone-beam CT (CBCT) to evaluate rectal cancer patient position when treated on a new couch top belly board (BB). Quality assurance (QA) of the imaging system was conducted weekly to ensure proper performance. The positional uncertainty of the combined kV-kV image match and subsequent couch move was found to be no more than ± 1.0 mm. The average (1 SD) CBCT QA phantom match was anterior-posterior (AP) = -0.8 ± 0.2 mm, superior-inferior (SI) = 0.9 ± 0.2 mm, and left-right (LR) = -0.1 ± 0.1 mm. For treatment, a set of orthogonal kV-kV images were taken and a bony anatomy match performed online. Moves were made along each axis (AP, SI, and LR) and recorded for analysis. CBCT data were acquired once every 5 fractions for a total of 5 images per patient. The images were all taken after the couch move but before treatment. A 3-dimensional (3D-3D) bony anatomy auto-match was performed offline and the residual difference in position recorded for analysis. The average (± 1 SD) move required from skin marks, calculated over all 375 fractions (15 patients × 25 fractions/patient), were AP = -2.6 ± 3.7 mm, SI = -0.3 ± 4.9 mm, and LR = 1.8 ± 4.5 mm. The average residual difference in patient position calculated from the weekly CBCT data (75 total) were AP = -1.7 ± 0.4 mm, SI = 1.1 ± 0.6 mm, and LR = -0.5 ± 0.2 mm. These results show that the BB does provide simple patient positioning that is accurate to within ± 2.0 mm when using online orthogonal kV-kV image matching of the pelvic bony anatomy.