Jen-San Tsai

Implementation and utility of a daily ultrasound-based localization system with intensity-modulated radiotherapy for prostate cancer

PURPOSE To evaluate the clinical feasibility of daily computer-assisted transabdominal ultrasonography for target position verification in the setting of intensity-modulated radiotherapy (IMRT) for prostate cancer. METHODS AND MATERIALS Twenty-three patients with clinically localized prostate cancer were treated using a sequential tomotherapy IMRT technique (Peacock) and daily computer-assisted transabdominal ultrasonography (BAT) for target localization. Patients were instructed to maintain a full bladder and were placed in the supine position using triangulation tattoos and a leg immobilizer to minimize pelvic rotation. The BAT ultrasound system is docked to the treatment collimator and electronically imports the CT simulation target contours and isocenter. The system is able to use the machine isocenter as a reference point to overlay the corresponding CT contours onto the ultrasound images captured in the transverse and sagittal planes. A touch screen menu is used to maneuver the CT contours in three dimensions such that they match the ultrasound images. The system then displays the three-dimensional couch shifts required to produce field alignment. Data were prospectively collected to measure the frequency by which useful ultrasound images were obtained, the amount of time required for localization/setup, and the direction/magnitude of the positional adjustments. RESULTS Of the 23 patients, the BAT ultrasound system produced images of sufficient quality to perform the overlay of the CT contours in 19 patients such that positional verification could be reliably performed. Poor image quality was associated with patient inability to maintain a full bladder, large body habitus, or other anatomic constraints. Of the 19 assessable patients, a total of 185 treatment alignments were performed (mean 8.8/patient). For all cases, the average time required for the daily ultrasound imaging and positional adjustments was 11.9 min. After the initial 5 cases, the user experience skills improved such that the time required for image verification/positional adjustments decreased to a mean of 5.6 min. The average right-left, AP, and cranial-caudal adjustment was 2.6 +/- 2.1 mm, 4.7 +/- 2.7 mm, and 4.2 +/- 2.8 mm, respectively. Positional adjustments >10 mm were infrequent and related primarily to misidentification of the target structures on the ultrasound image, patient movement, or improper registration of the triangulation tattoos. CONCLUSION Daily computer-assisted BAT ultrasound positional verification of the prostate can be successfully performed through the acquisition of high-quality images in most patients with only a modest increase in treatment setup time. Positional data obtained with this system resulted in clinically meaningful adjustments in daily setup for sequential IMRT that would not be otherwise apparent from other verification modalities.

A non-invasive immobilization system and related quality assurance for dynamic intensity modulated radiation therapy of intracranial and head and neck disease.

PURPOSE To develop and implement a non-invasive immobilization system guided by a dedicated quality assurance (QA) program for dynamic intensity-modulated radiotherapy (IMRT) of intracranial and head and neck disease, with IMRT delivered using the NOMOS Corporations Peacock System and MIMiC collimator. METHODS AND MATERIALS Thermoplastic face masks are combined with cradle-shaped polyurethane foaming agents and a dedicated quality assurance program to create a customized headholder system (CHS). Plastic shrinkage was studied to understand its effect on immobilization. Fiducial points for computerized tomography (CT) are obtained by placing multiple dabs of barium paste on mask surfaces at intersections of laser projections used for patient positioning. Fiducial lines are drawn on the cradle along laser projections aligned with nasal surfaces. Lateral CT topograms are annotated with a crosshair indicating the origin of the treatment planning and delivery coordinate system, and with lines delineating the projections of superior-inferior field borders of the linear accelerators secondary collimators, or with those of the fully open MIMiC. Port films exposed with and without the MIMIC are compared to annotated topograms to measure positional variance (PV) in superior-inferior (SI), right-left (RL), and anterior posterior (AP) directions. MIMiC vane patterns superposed on port films are applied to verify planned patterns. A 12-patient study of PV was performed by analyzing positions of 10 anatomic points on repeat CT topograms, plotting histograms of PV, and determining average PV. RESULTS AND DISCUSSION A 1.5+/-0.3 mm SD shrinkage per 70 cm of thermoplastic was observed over 24 h. Average PV of 1.0+/-0.8, 1.2+/-1.1, and 1.3+/-0.8 mm were measured in SI, AP, and RL directions, respectively. Lateral port films exposed with and without the MIMiC showed PV of 0.2+/-1.3 and 0.8+/-2.2 mm in AP and SI directions. Vane patterns superimposed on port films consistently verified the planned patterns. CONCLUSION The CHS provided adequately reproducible immobilization for dynamic IMRT, and may be applicable to decrease PV for other cranial and head and neck external beam radiation therapy.

Dosimetric comparison of stereotactic radiosurgery to intensity modulated radiotherapy

To compare the dosimetry achievable with an intensity modulated radiotherapy (IMR) system to that of stereotactic radiosurgery (SRS) for an irregularly shaped moderate size target. A treatment plan was selected from 109 single fraction SRS cases having had multiple non-coplanar arc therapy using a 6 MV linear accelerator fitted with circular tertiary collimators 1.00 to 4.00 cm in diameter at isocenter. The CT scan with delineated regions of interest was then entered into an IMR treatment planning system and optimized dose distributions, using a back projection technique for dynamic multileaf collimator delivery, were generated with a stimulated annealing algorithm. Dose volume histograms (DVH), homogeneity indices (HI), conformity indices (CI), minimum and maximum doses to surrounding highly sensitive intracranial structures, as well as the volume of tissue treated to > 80, 50, and 20% of the prescription dose from the IMR plan were then compared to those from the single isocenter SRS plan used and a hypothetical three isocenter SRS plan. For an irregularly shaped target, the IMR plan produced a HI of 1.08 and CI of 1.50 compared to 1.75 and 4.41, respectively, for the single isocenter SRS plan (SRS1) and 3.33 and 3.43 for the triple isocenter SRS plan (SRS3). The maximum and minimum doses to surrounding critical structures were less with the IMR plan in comparison to both SRS plans. However, the volume of non-target tissue treated to > 80, 50, and 20% of the prescription dose with the IMR plan was 137, 170, and 163%, respectively, of that treated with the SRS1 plan and 85, 100, and 123% of the volume when compared to SRS3 plan. The IMR system provided more conformal target doses than were provided by the single isocenter or three isocenter SRS plans. IMR delivered less dose to critical normal tissues and provided increased homogeneity within the target volume for a moderate size irregularly shaped target, at the cost of a larger penumbra.

Response of intracranial melanoma metastases to stereotactic radiosurgery

We analyzed our recent stereotactic radiosurgery (SRS) experience to determine the radiographic response of intracranial metastatic melanomas to SRS. Twelve patients with 21 intracranial melanoma metastases treated with SRS were evaluated. Fifteen (72%) metastases were hemispheric, 3 (14%) were cerebellar, and 3 (14%) were in the basal ganglion or thalamus. All lesions were 2.5 cm or less in maximum diameter. Eleven patients also had whole brain external beam radiotherapy. Mean SRS dosage was 1,800 cGy to the 85% isodose surface and median dose was 1,800 cGy to the 80% isodose surface (range 1,100-3, 100 cGy at the 80-95% isodose surface). Overall, 12 (57%) lesions showed decrease or stabilization of tumor volume (i.e., local control), while 9 (43%) showed enlargement. Division of metastases into small (< or = 1.0 cm diameter) and large (> 1.0 cm diameter) tumors showed that the small tumors were more likely to regress than the large tumors (chi-square test; P < 0.03). Only 1 of 9 (11%) large lesions regressed as opposed to 7 of 12 (58%) small lesions regressed with SRS. We conclude that SRS is suited for small melanoma brain metastases, but lesions between 1.0 and 2.5 cm in diameter, while still generally considered appropriate for SRS, may not be as responsive to SRS at currently employed dosages.

Determination of the 4 mm Gamma Knife helmet relative output factor using a variety of detectors.

Though the 4 mm Gamma Knife helmet is used routinely, there is disagreement in the Gamma Knife users community on the value of the 4 mm helmet relative output factor. A range of relative output factors is used, and this variation may impair observations of dose response and optimization of prescribed dose. To study this variation, measurements were performed using the following radiation detectors: silicon diode, diamond detector, radiographic film, radiochromic film, and TLD cubes. To facilitate positioning of the silicon diode and diamond detector, a three-dimensional translation micrometer was used to iteratively determine the position of maximum detector response. Positioning of the films and TLDs was accomplished by manufacturing custom holders for each technique. Results from all five measurement techniques indicate that the 4 mm helmet relative output factor is 0.868 +/- 0.014. Within the experimental uncertainties, this value is in good agreement with results obtained by other investigators using diverse techniques.

The measurement of linear accelerator isocenter motion using a three-micrometer device and an adjustable pointer

PURPOSE The small motions of the major axes of a linear accelerator observed during gantry and treatment table rotation were measured to improve beam-target alignment during stereotactic radiosurgery (SRS). METHODS AND MATERIALS Measurements of gantry isocenter motion and table rotational axis wobble were performed with an adjustable front pointer and a three-micrometer device. Nominal gantry and table isocenters were specified. The gantry motion path and table isocenter coordinates were then applied to offset simulated treatment target coordinates so as to compensate for gantry sag. Target simulation films were examined to document improvement of beam-target alignment. RESULTS The overall precision of the measurement of gantry and table isocenter coordinates was 0.2 mm. Over gantry rotation of 0 to 360 degrees, the gantry isocenter was found to follow a pinched loop with a maximum point to point distance of 1 mm. Table axis motion was found to be negligible relative to the reproducibility of gantry isocenter motion. Thus, a table isocenter was defined that was invariant to table rotation. CONCLUSION Results indicate that the three-micrometer device and adjustable front pointer are useful tools for three-dimensional (3D) mapping of gantry, collimator and table isocenters and their motions. It is suggested that such measurements may be useful in the quality assurance of linear accelerators, particularly to improve beam-target alignment during SRS and other high dose external beam therapy.

Quasi-independent monitor unit calculation for intensity modulated sequential tomotherapy

The number of linac monitor units (MU) from intensity modulated sequential tomotherapy (IMST) is substantially larger than the MU delivered in conventional radiation therapy, and the relation between MU and dose is obscure due to complicated variation of the beam intensities. The purpose of this work was to develop a practical method of verifying the MU and dose from IMST so that the MU of each arced beam could be double‐checked for accuracy. MU calculations for 41 arced beams from 14 IMST patients were performed using the variables of vane open fraction time, field size, target depth output factor, TMR, and derived intensity distribution. Discrepancy between planned and checked MU was quantified as 100(MUcal−MUplan)/MUplan percent. All 41 discrepancies were clustered between −5% to +5%, illustrated in a Gaussian‐shaped histogram centered at −1.0±3.5% standard deviation indicating the present MU calculations are in agreement with the planned expectations. To confirm the correctness of the present calculated MUs of the IMST plans, eight of the calculated IMST plans are performed dose verifications using their hybrid plans, which are created by transporting patients IMST plan beams onto a spherical polystyrene Phantom for dose distribution within the Phantom. The dose was measured with a 0.07 cc ionization chamber inserted in the spherical Phantom during the hybrid plan irradiation. Average discrepancy between planned and measured doses was found to be 0.6±3.4% with single standard deviation uncertainty. The spread of the discrepancies of present calculated MUs relative to their planned ones are attributed to uncertainties of effective field size, effective planned dose corresponding to each arc, and inaccuracy of quantification of scattered dose from adjacent arced beams. Overall, the present calculation of MUs is consistent with what derived from treatment plans. Since the MUs are verified by actual dose measurements, therefore the present MU calculation technique is considered adequate for double‐checking planned IMST MUs. PACS number(s): 87.53.–j, 87.66.–a

FINE TUNING OF LINEAR ACCELERATOR ACCESSORIES FOR STEREOTACTIC RADIOTHERAPY

PURPOSE Experience with the University of Wisconsins stereotactic radiotherapy (SRT) accessory system was applied to build a new system, facilitate alignment of linac photon beams with a Brown-Roberts-Wells (BRW) stereotaxy, and increase the versatility and stability of the stereotaxy. METHODS AND MATERIALS High tensile strength stainless steel was used in the floor stand to increase the range of gantry rotation relative to ranges allowed by truss-mounted stands. The collimator assembly and floor stand were each fitted with two-axis gimbal and translation adjustments in addition to the floor stands three-axis adjustments. The head ring positioning assembly was fitted with two braces to prevent the head ring from deforming with patient motion. Six MV linac photon beam characteristics were measured with a computer-controlled scanning system and a diode in water, at source to surface distances (SSD) of 80 and 100 cm, and for 13 divergent collimators ranging in diameter from 1-4 cm at 100 cm SSD. Quality assurance software was applied to screen data for questionable consistency or symmetry. Integrity of the stereotaxy was evaluated with target simulation films and repeated measurements which were part of the quality assurance of clinical treatments. A method was developed using a glass etched contact reticle to obtain average simulated target to beam center distances (delta av) from target simulation films. RESULTS AND CONCLUSION New aspects of the current system have improved the ability to fine tune and analyze stereotactic alignment. Beam characteristics met stringent output criteria and penumbral widths were the same or narrower than penumbral widths reported elsewhere. The precision of measuring delta av was 0.1 mm, and delta av averaged over 50 target simulation films was 0.7 +/- 0.1 mm. Results suggest that it may be useful to determine delta av from target simulation films with the method described here.

Use of a 1 mm collimator to test the accuracy of stereotactic radiotherapy

PURPOSE To develop a method of measuring locations of the center of dose in stereotactic radiotherapy relative to the center of the target, and thereby obtain a test of the accuracy of stereotactic radiotherapy (SRT). METHODS AND MATERIALS An insert was mounted in an SRT collimator on a 6 MV linear accelerator to provide a photon beam approximately 1 mm in diameter at isocenter, and a method of measuring radiation center coordinates of arced SRT beams. To simulate a small intracranial target, two halves of a Barium paste column were embedded in two adjacent slabs of a humanoid phantom. A film was placed between the slabs to image the radiation relative to the target center. A surgical head ring and computerized tomography (CT) localizer were attached to the phantom and CT scans were obtained. The scans were entered in a three-dimensional computerized treatment-planning system and radiation isocenter coordinates determined by iteratively moving the 90% isodose surface center of arced beam dose distributions to coincide with the target center. The phantom was bolted to an SRT floorstand with isocenter coordinates obtained from the treatment plan, and then irradiated in two sets of experiments. The first set applied five 1 mm noncoplanar arced beams with and without offsets of the planned coordinates in the transverse plane. The second set applied one large transverse arc coplanar to the film with and without offsets in the craniocaudal direction. Irradiations with coordinate offsets tested the sensitivity of the method. Films were developed and digitized with a high resolution film scanner to measure the location of the radiation relative to the target center. RESULTS AND CONCLUSION The radiation center was found from 0.0 to 0.3 mm of the target center, within requirements of our clinical quality assurance program. The measurement and evaluation of coincidence of radiation and target centers are, thus, proposed as elements of radiosurgery facility acceptance and annual quality assurance.

Comparison of radiobiologic modeling for one- and two-isocenter dose distributions applied to ellipsoidal radiosurgery targets

PURPOSE RTOG protocol 90-05 determined the maximum acutely tolerated dose of single-fraction radiosurgery in patients receiving prior fractionated large volume cranial irradiation. Data from 90-05 have suggested that patients treated with a gamma unit, compared to linac-based therapy, have a tumor control advantage and lower rates of severe complications. This study was performed to investigate the radiobiologic effect of using one vs. two isocenters in single-fraction radiosurgery of ellipsoidal targets. METHODS AND MATERIALS For a series of ellipsoidal targets that varied by volume and radiosensitivity, single and two-isocenter treatment plans were generated to approximate those typically employed for gamma unit and linac radiosurgery. Tumor control probabilities (TCP) and normal tissue complication probabilities (NTCP) were generated automatically by the treatment planning system based on established parameter values. RESULTS The modeling data showed that multiple-isocenter plans resulted in improved TCP with equivalent or lesser NTCP, particularly for larger, radioresistant targets. Multiple-isocenter plans reduce the amount of normal tissue that receives high dose. Also, areas within the tumor receive significantly higher doses than the prescription dose, which contributes to increased tumor cell inactivation. CONCLUSION For ellipsoidal targets, radiobiologic modeling data are consistent with the clinical findings of the RTOG 90-05 trial, as they predict improved outcome with a multiple-isocenter plan relative to a single-isocenter plan. The benefit is most apparent with increasing target volume and decreasing tumor radiosensitivity.