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This work is aimed at investigating treatment planning strategies to optimally com-bine stereotactic body radiation therapy (SBRT) with intracavitary brachytherapy (ICBT) for the treatment of locally advanced cervical cancer. Forty patients (stage IIB - IIIB) previously treated with combined SBRT and ICBT were randomly selected for this retrospective study. All patients were CT- and MR-scanned with a ring applicator in situ. HR-CTV and OARs were contoured according to fused CT and MR images. Several ICBT plans were generated for each patient based on different dose prescription points, and then a matching SBRT plan was generated for each ICBT plan. The dose distribution of each composite plan was analyzed with a focus on the doses received by 90% and 100% of the target volume (D90 and D100), the target volume receiving 100% of the prescription dose (V100%), and the doses received by 2 cc and 40% of the OARs (D2 cc and D40). As the distance, d, between the prescription point and the tandem varied within 1.0 and 1.9 cm, the D90, D100 and V100% for the target, as well as D2 cc and D40 for the bladder and rectum approached their optimal values for d value between 1.0 and 1.4 cm. When design-ing a combined ICBT + SBRT plan, one should measure the size of the cervix and set the prescription isodose line 1.0 to 1.4 cm away from the tandem for the ICBT plan first and then optimize the SBRT plan based on the ICBT dose distribution to achieve the best target coverage and critical structures sparing.

This paper investigates the clinical significance of real-time monitoring of intrafractional prostate motion during external beam radiotherapy using a commercial 4D localization system. Intrafractional prostate motion was tracked during 8,660 treatment fractions for 236 patients. The following statistics were analyzed: 1) the percentage of fractions in which the prostate shifted 2-7 mm for a certain duration; 2) the proportion of the entire tracking time during which the prostate shifted 2-7mm; and 3) the proportion of each minute in which the shift exceeded 2-7 mm. The ten patients exhibiting maximum intrafractional-motion patterns were analyzed separately. Our results showed that the percentage of fractions in which the prostate shifted by > 2, 3, 5, and 7 mm off the baseline in any direction for > 30 s was 56.8%, 27.2%, 4.6%, and 0.7% for intact prostate and 68.7%, 35.6%, 10.1%, and 1.8% for postprostatectomy patients, respectively. For the ten patients, these percentages were 91.3%, 72.4%, 36.3%, and 6%, respectively. The percentage of tracking time during which the prostate shifted > 2, 3, 5, and 7 mm was 27.8%, 10.7%, 1.6%, and 0.3%, respectively, and it was 56.2%, 33.7%, 11.2%, and 2.1%, respectively, for the ten patients. The percentage of tracking time for a > 3 mm posterior motion was four to five times higher than that in other directions. For treatments completed in 5 min (VMAT) and 10 min (IMRT), the proportion for the prostate to shift by > 3mm was 4% and 12%, respectively. Although intrafractional prostate motion was generally small, caution should be taken for patients who exhibit frequent large intrafractional motion. For those patients, adjustment of patient positioning may be necessary or a larger treatment margin may be used. After the initial alignment, the likelihood of prostate motion increases with time. Therefore, it is favorable to use advanced techniques (e.g., VMAT) that require less delivery time in order to reduce the treatment uncertainty resulting from intrafractional prostate motion.

Pulsed low-dose-rate radiotherapy (PLDR) has been proven to be a valid method of reirradiation. Previous studies of recurrent cancer radiotherapy were mainly based on conventional 3D CRT and VMAT delivery techniques. There are difficulties in IMRT planning using existing commercial treatment planning systems (TPS) to meet the PLDR protocol. This work focuses on PLDR using ten-field IMRT and a commercial TPS for two specific sites: recurrent lung cancers and head and neck cancers. Our PLDR protocol requires that the maximum dose to the PTV be less than 0.4 Gy and the mean dose to be 0.2 Gy per field. We investigated various planning strategies to meet the PLDR requirements for 20 lung and head and neck patients. The PTV volume for lung cases ranged from 101.7 to 919.4 cm3 and the maximum dose to the PTV ranged from 0.22 to 0.39 Gy. The PTV volume for head and neck cases ranged from 66.2 to 282.1 cm3 and the maximum dose to the PTV ranged from 0.21 to 0.39 Gy. With special beam arrangements and dosimetry parameters, it is feasible to use a commercial TPS to generate quality PLDR IMRT plans for lung and head and neck reirradiation.

The dosimetric advantage of modulated electron radiotherapy (MERT) has been explored by many investigators and is considered to be an advanced radiation therapy technique in the utilization of electrons. A computer-controlled electron multileaf collimator (MLC) prototype, newly designed to be added onto a Varian linac to deliver MERT, was investigated both experimentally and by Monte Carlo simulations. Four different electron energies, 6, 9, 12, and 15 MeV, were employed for this investigation. To ensure that this device was capable of delivering the electron beams properly, measurements were performed to examine the electron MLC (eMLC) leaf leakage and to determine the appropriate jaw positioning for an eMLC-shaped field in order to eliminate a secondary radiation peak that could otherwise appear outside of an intended radiation field in the case of inappropriate jaw positioning due to insufficient radiation blockage from the jaws. Phase space data were obtained by Monte Carlo (MC) simulation and recorded at the plane just above the jaws for each of the energies (6, 9, 12, and 15 MeV). As an input source, phase space data were used in MC dose calculations for various sizes of the eMLC shaped field (10 × 10 cm2, 3.4 × 3.4 cm2, and 2 × 2 cm2) with respect to a water phantom at source-to-surface distance (SSD) = 94 cm, while the jaws, eMLC leaves, and some accessories associated with the eMLC assembly as well were modeled as modifiers in the calculations. The calculated results were then compared with measurements from a water scanning system. The results showed that jaw settings with 5 mm margins beyond the field shaped by the eMLC were appropriate to eliminate the secondary radiation peak while not widening the beam penumbra; the eMLC leaf leakage measurements ranged from 0.3% to 1.8% for different energies based on in-phantom measurements, which should be quite acceptable for MERT. Comparisons between MC dose calculations and measurements showed agreement within 1%/1 mm based on percentage depth doses (PDDs) and off-axis dose profiles for a range of field sizes for each of the electron energies. Our current work has demonstrated that the eMLC and other relevant components in the linac were correctly modeled and simulated via our in-house MC codes, and the eMLC is capable of accurately delivering electron beams for various eMLC-shaped field sizes with appropriate jaw settings. In the next stage, patient-specific verification with a full MERT plan should be performed.

The objective of this study is to validate the capabilities of a cylindrical diode array system for volumetric-modulated arc therapy (VMAT) treatment quality assurance (QA). The VMAT plans were generated by the Eclipse treatment planning system (TPS) with the analytical anisotropic algorithm (AAA) for dose calculation. An in-house Monte Carlo (MC) code was utilized as a validation tool for the TPS calculations and the ArcCHECK measurements. The megavoltage computed tomography (MVCT) of the ArcCHECK system was adopted for the geometry reconstruction in the TPS and for MC simulations. A 10 × 10 cm2 open field validation was performed for both the 6 and 10 MV photon beams to validate the absolute dose calibration of the ArcCHECK system and also the TPS dose calculations for this system. The impact of the angular dependency on noncoplanar deliveries was investigated with a series of 10 × 10 cm2 fields delivered with couch rotation 0° to 40°. The sensitivity of detecting the translational (1 to 10 mm) and the rotational (1° to 3°) misalignments was tested with a breast VMAT case. Ten VMAT plans (six prostate, H&N, pelvis, liver, and breast) were investigated to evaluate the agreement of the target dose and the peripheral dose among ArcCHECK measurements, and TPS and MC dose calculations. A customized acrylic plug holding an ion chamber was used to measure the dose at the center of the ArcCHECK phantom. Both the entrance and the exit doses measured by the ArcCHECK system with and without the plug agreed with the MC simulation to 1.0%. The TPS dose calculation with a 2.5 mm grid overestimated the exit dose by up to 7.2% when the plug was removed. The agreement between the MC and TPS calculations for the ArcCHECK without the plug improved significantly when a 1 mm dose calculation grid was used in the TPS. The noncoplanar delivery test demonstrated that the angular dependency has limited impact on the gamma passing rate (< 1.2% drop) for the 2%-3% dose and 2mm-3 mm DTA criteria. A 1° rotational misalignment introduces 11.3% (3%/3mm) to 21.3% (1%/1 mm) and 0.2% (3%/3 mm) to 0.8% (1%/1 mm) Gamma passing rate drop for ArcCHECK system and MatriXX system, respectively. Both systems have comparable sensitivity to the AP misalignments. However, a 2 mm RL misalignment introduces gamma passing rate drop ranging from 0.9% (3%/3 mm) to 4.0% (1%/1 mm) and 5.0% (3%/3 mm) to 12.0% (1%/1 mm) for ArcCHECK and MatriXX measurements, respectively. For VMAT plan QA, the gamma analysis passing rates ranged from 96.1% (H&N case) to 99.9% (prostate case), when using the 3%/3 mm DTA criteria for the peripheral dose validation between the TPS and ArcCHCEK measurements. The peripheral dose validation between the MC simulation and ArcCHECK measurements showed at least 97.9% gamma passing rates. The central dose validation also showed an agreement within 2.2% between TPS/MC calculations and ArcCHECK measurements. The worst discrepancy was found in the H&N case, which is the most complex VMAT case. The ArcCHECK system is suitable for VMAT QA evaluation based on the sensitivity to detecting misalignments, the clinical impact of the angular dependency, and the correlation between the dose agreements in the peripheral region and the central region. This work also demonstrated the importance of carrying out a thorough validation of both the TPS and the dosimetry system prior to utilizing it for QA, and the value of having an independent dose calculation tool, such as the MC method, in clinical practice.

The purpose of this study was to assess target repositional accuracy with respect to the bony structures using daily CBCT, and to validate the planning target volume (PTV) margin used in the lung SBRT. All patients underwent 4D CT scanning in preparation for lung SBRT. The internal target volume (ITV) was outlined from the reconstructed 4D data using the maximum-intensity projection (MIP) algorithm. A 6 mm margin was added to the ITV to create the PTV. Conformal treatment planning was performed on the helical images, to which the MIP images were fused. Prior to each treatment, CBCT was taken after a patient was set up in the treatment position. The CBCT images were fused with the simulation CT based on the bony anatomy, in order to derive setup errors and separate them from the tumor repositional errors. The treating physician then checked and modified the alignment based on target relocalization within the PTV. The shifts determined in such a method were recorded and the subtractions of these shifts with respect to the corresponding setup errors were defined as the target relocalization accuracy. Our study of 36 consecutive patients, treating 38 targets for a total of 153 fractions shows that, after setup error correction, the target repositional accuracy followed a normal distribution with the mean values close to 0 in all directions, and standard deviations of 0.25 cm in A-P, 0.24 cm in Lat, and 0.28 cm in S-I directions, respectively. The probability of having the shifts ? 0.6 cm is less than 0.8% in A-P, 0.6% in Lat, and 1.7 % in S-I directions. For the patient population studied, the target centroid position relative to the bony structures changed minimally from day to day. This demonstrated that the PTV margin that is designed on the MIP image-based ITV was adequate for lung SBRT.

Due to the angulation of the breast board used for tangential breast irradiation, additional normal lung tissues are included in the supraclavicular field. This work investigates a method to reduce the lung volume and dose delivered during supraclavicular irradiation for breast cancer. Ten patients included for this retrospective study received chest wall and supraclavicular irradiation following radical surgery or breast-conserving surgery. Three-dimensional conformal radiation therapy plans were generated using the CMS XiO treatment planning system. The clinical target volume (CTV) of the supraclavicular irradiation is defined as the subcutaneous tissues from 0.5 cm under the anterior skin surface to a 3 cm depth. Only the ipsilateral lung is defined as the organ at risk. In the new method, the couch is rotated 90° and the supraclavicular field is tilted to maintain a normal incident angle to the breast board rather than the couch surface to spare more normal lung tissues. The absolute volume of the ipsilateral lung irradiated, and the volumes of lung tissues receiving 5 Gy and 20 Gy (V5 and V20) are analyzed. The new method can reduce the lung volume irradiated by the supraclavicular field significantly. For the ten patients investigated, only 5.3% of the ipsilateral lung is irradiated with the new method, while 14.9% of the ipsilateral lung is irradiated using the conventional method. Compared with the conventional method, the new method reduces V5 by 53.6% and V20 by 59.0%. Our new method does not alter the patient positioning for breast treatment but rotates the couch to deliver a tilted supraclavicular field to maintain adequate CTV coverage and spare more normal lung tissues. The results of this study demonstrated that our new method is effective, and that the reduction of normal lung tissue volume in the field is significant.

A formalism is developed for evaluating the shielding in an existing vault to be used for IMRT. Existing exposure rate measurements are utilized as well as a newly developed effective modulation scaling factor. Examples are given for vaults housing 6, 10 and 18 MV linear accelerators. The use of an 18 MV Siemens linear accelerator is evaluated for IMRT delivery with respect to neutron production and the effects on individual patients. A modified modulation scaling factor is developed and the risk of the incurrence of fatal secondary malignancies is estimated. The difference in neutron production between 18 MV Varian and Siemens accelerators is estimated using Monte Carlo results. The neutron production from the Siemens accelerator is found to be approximately 4 times less than that of the Varian accelerator resulting in a risk of fatal secondary malignancy occurrence of approximately 1.6% when using the SMLC delivery technique and our measured modulation scaling factors. This compares with a previously published value of 1.6% for routine 3D CRT delivery on the Varian accelerator., ((c) 2003 American College of Medical Physics.)

Therapeutic treatment plan evaluation is often based on examining the radiotherapy treatment planning (RTP) system dose distributions in the target and surrounding normal structures. To study the effects of tissue inhomogeneities on photon dose distributions, we compared FOCUS RTP system dose distributions from the measurement-based Clarkson and model-based MultiGrid Superposition (MGS) algorithms with those from the BEAM Monte Carlo code system in a set of heterogeneous phantoms. The phantom inhomogeneities mimic relevant clinical treatment sites, which include lung slab, lung-bone slab, bone-lung slab, mediastinum, and tumor geometries. The benchmark comparisons were performed in lung densities of 0.20 and 0.31 g/cm3, and a bone density of 2.40 g/cm3 for 5x5 cm2 and 10x10 cm2, 6- and 15-MV photon beams. Benchmark comparison results show that the MGS model and BEAM doses match better than 3% or 3 mm, and the MGS model is more accurate than the Clarkson model in all phantoms. The MGS model, unlike the Clarkson model, predicts the build-down and build-up of dose near tissue interfaces and penumbra broadening in lung associated with high energy beams. The Clarkson model overestimates the dose in lung by a maximum of 10% compared to BEAM. Dose comparisons suggest turning-off the effective path length inhomogeneity correction in the Clarkson model for lung treatments.