Semie Hong

Performance evaluation of field-in-field technique for tangential breast irradiation.

Conventional hard or dynamic wedge systems are commonly applied to reduce the dose inhomogeneity associated with whole breast irradiation. We evaluated the dosimetric benefits of the field-in-field (FIF) technique by comparing it with the electronic compensator (EC), Varian enhanced dynamic wedge (EW) and conventional hard wedge (HW) techniques. Data were obtained from 12 patients who had undergone breast-conserving surgery (six left-sided and six right-sided). For these patients, the average breast planning target volume (PTV) was 447.4 cm(3) (range, 211.6-711.8 cm(3)). For the experiments, a 6 MV photon beam from a Varian 21 EX was used, the HW and EW angles were applied from 15 to 45 degrees, while 40-50% isodose values were chosen to achieve the best dose distribution for electronic compensation. In applying the FIF technique, we used two or three subfields for each portal. To evaluate the performance for each planning technique, we analysed a dose-volume histogram (DVH) for the PTV and organs-at-risk (OARs). To evaluate the effects of these techniques on dose inhomogeneity, we defined the PTV Dose Improvement (PDI) index, which was derived from a PTV volume between 97-103% of the differential DVHs. In addition, we compared the average monitor units (MUs) for each technique. The average PDI index with FIF is 76.4%, while the PDI indices for other treatments were 65.8, 41.8 and 50.9% for EC, EW and HW, respectively. This study demonstrated an improved performance using the FIF technique compared with the conventional HW/EW system, as well as a new modality for EC. We demonstrated that FIF is a very useful technique for improving PTV conformity, while protecting the OARs from breast tangential irradiation.

Effects of static dosimetric leaf gap on MLC-based small-beam dose distribution for intensity-modulated radiosurgery.

The aim of the present study was to evaluate the effect of various specific dosimetric leaf gaps on the multileaf collimator (MLC)–based small‐beam dose distribution. The dosimetric static leaf gap was determined by comparing the profiles of small MLC‐based beams with those of small collimated fields (square fields of 1, 2, 3, and 4cm). The results showed that an approximately 2‐mm gap was optimal with the Millennium 120‐leaf MLC (Varian Medical Systems, Palo Alto, CA) and a Varian 21EX 6‐MV photon beam. We also investigated how much the leaf gap affects the planning results and the actual dose distribution. A doughnut‐shaped planning target volume (PTV, 6.1 cm3) and inner organ at risk (OAR, 0.3 cm3) were delineated for delicate intensity‐modulated radiosurgery test planning. The applied leaf gaps were 0, 1, and 2 mm. The measured dose distributions were compared with the dose distribution in the treatment planning system. The maximum dose differences at inside PTV, outside PTV, and inner OAR were, respectively, 22.3%, 20.2%, and 35.2% for the 0‐mm leaf gap; 17.8%, 22.8%, and 30.8% for the 1‐mm leaf gap; and 5.5%, 8.5%, and 6.3% for the 2‐mm leaf gap. In a human head phantom (model 605: CIRS, Norfolk, VA) study, large dose differences of 1.3% – 12.7% were noted for the measurements made using the MLC files generated by the three different leaf gaps. The planned results were similar, and measurements showed a large dose difference associated with the various leaf gaps. These results strongly suggest that plans generated by a commercial inverse planning system commissioned using general collimated field data will probably demonstrate discrepancies between the planned treatments and the measured results. PACS number: 87.53.Dq

Radiobiological model-based bio-anatomical quality assurance in intensity-modulated radiation therapy for prostate cancer

A bio-anatomical quality assurance (QA) method employing tumor control probability (TCP) and normal tissue complication probability (NTCP) is described that can integrate radiobiological effects into intensity-modulated radiation therapy (IMRT). We evaluated the variations in the radiobiological effects caused by random errors (r-errors) and systematic errors (s-errors) by evaluating TCP and NTCP in two groups: patients with an intact prostate (Gintact) and those who have undergone prostatectomy (Gtectomy). The r-errors were generated using an isocenter shift of ±1 mm to simulate a misaligned patient set-up. The s-errors were generated using individual leaves that were displaced inwardly and outwardly by 1 mm on multileaf collimator field files. Subvolume-based TCP and NTCP were visualized on computed tomography (CT) images to determine the radiobiological effects on the principal structures. The bio-anatomical QA using the TCP and NTCP maps differentiated the critical radiobiological effects on specific volumes, particularly at the anterior rectal walls and planning target volumes. The s-errors showed a TCP variation of –40–25% in Gtectomy and –30–10% in Gintact, while the r-errors were less than 1.5% in both groups. The r-errors for the rectum and bladder showed higher NTCP variations at ±20% and ±10%, respectively, and the s-errors were greater than ±65% for both. This bio-anatomical method, as a patient-specific IMRT QA, can provide distinct indications of clinically significant radiobiological effects beyond the minimization of probable physical dose errors in phantoms.

Development of a novel quality assurance system based on rolled-up and rolled-out radiochromic films in volumetric modulated arc therapy.

PURPOSE To develop a cylindrical phantom with rolled-up radiochromic films and dose analysis software in the rolled-out plane for quality assurance (QA) in volumetric modulated arc therapy (VMAT). METHODS The phantom consists of an acrylic cylindrical body wrapped with radiochromic film inserted into an outer cylindrical shell of 5 cm thickness. The rolled-up films with high spatial resolution enable detection of specific dose errors along the arc trajectory of continuously irradiated and modulated beams in VMAT. The developed dose analysis software facilitates dosimetric evaluation in the rolled-up and rolled-out planes of the film; the calculated doses on the corresponding points where the rolled-up film was placed were reconstructed into a rectangular dose matrix equivalent to that of the rolled-out plane of the film. The VMAT QA system was implemented in 3 clinical cases of prostate, nasopharynx, and pelvic metastasis. Each calculated dose on the rolled-out plane was compared with measurement values by modified gamma evaluation. Detected positions of dose disagreement on the rolled-out plane were also distinguished in cylindrical coordinates. The frequency of error occurrence and error distribution were summarized in a histogram and in an axial view of rolled-up plane to intuitively identify the corresponding positions of detected errors according to the gantry angle. RESULTS The dose matrix reconstructed from the developed VMAT QA system was used to verify the measured dose distribution along the arc trajectory. Dose discrepancies were detected on the rolled-out plane and visualized on the calculated dose matrix in cylindrical coordinates. The error histogram obtained by gamma evaluation enabled identification of the specific error frequency at each gantry angular position. The total dose error occurring on the cylindrical surface was in the range of 5%-8% for the 3 cases. CONCLUSIONS The developed system provides a practical and reliable QA method to detect dosimetric errors according to the gantry angle. Film dosimetry based on rolled-up and rolled-out techniques leads to dose verification in the subspaces of the 3D dose volume. The system can be employed as an alternative tool to detect the pitfalls of planar dose verification.

Dosimetric Effects of Magnetic Resonance Imaging-assisted Radiotherapy Planning: Dose Optimization for Target Volumes at High Risk and Analytic Radiobiological Dose Evaluation

Based on the assumption that apparent diffusion coefficients (ADCs) define high-risk clinical target volume (aCTVHR) in high-grade glioma in a cellularity-dependent manner, the dosimetric effects of aCTVHR-targeted dose optimization were evaluated in two intensity-modulated radiation therapy (IMRT) plans. Diffusion-weighted magnetic resonance (MR) images and ADC maps were analyzed qualitatively and quantitatively to determine aCTVHR in a high-grade glioma with high cellularity. After confirming tumor malignancy using the average and minimum ADCs and ADC ratios, the aCTVHR with double- or triple-restricted water diffusion was defined on computed tomography images through image registration. Doses to the aCTVHR and CTV defined on T1-weighted MR images were optimized using a simultaneous integrated boost technique. The dosimetric benefits for CTVs and organs at risk (OARs) were compared using dose volume histograms and various biophysical indices in an ADC map-based IMRT (IMRTADC) plan and a conventional IMRT (IMRTconv) plan. The IMRTADC plan improved dose conformity up to 15 times, compared to the IMRTconv plan. It reduced the equivalent uniform doses in the visual system and brain stem by more than 10% and 16%, respectively. The ADC-based target differentiation and dose optimization may facilitate conformal dose distribution to the aCTVHR and OAR sparing in an IMRT plan. Graphical Abstract

Analysis and comparison of the dose distribution of RTP and experimental measurement during IMRT for a moving organ

IMRT quality assurance (QA) is performed to confirm the intensity modulated radiation beams, which are determined by inverse planning using the computed optimization procedure before the patient is irradiated by IMRT. IMRT QA does not consider the motion of a moving organ whilst performing IMRT. Therefore, the purpose of this study was to improve the efficiency of treatment by not only accurately determining the dosimetry with respect to a moving organ, but to correct the tumor and normal structure on the radiation treatment planning (RTP) when a moving organ was treated. A moving phantom system (moving device, cork, and acryl phantom) was designed to simulate the motion of a moving organ. The diaphragmatic motions of 5 patients were analyzed by fluoroscopy to acquire the motion of an internal organ. The CT scan images were obtained simultaneously during the operation of the moving phantom system such as diaphragm motion (static state, 10, 15, and 20 mm). The phantom system was irradiated using the sliding window method, after Gafchromic EBT films had been inserted within the space (40, 50, and 60 mm depth) of the phantom. The results, in which the dose distribution of the films were compared with those from planning, showed a decrease in dose on the PTV region with increasing motion of the moving phantom The difference of planning and me ose are maximum 39.6–43.2% at 20 mm asurement dmoving and minimum 0.1–3.4% at static state. The dose on the penumbra region increased with increasing the motion of the moving phantom. Overall, the efficiency of radiotherapy would be improved if radiotherapy QA considers the motion of an internal organ and the PTV, CTV, and OAR are corrected accordingly before a patient is treated with the planning data.