Dose perturbations by two carbon fiber treatment couches and the ability of a commercial treatment planning system to predict these effects.

PURPOSE: To measure the effect of the treatment couch on dose distributions and to investigate the ability of a modern planning system to accurately model these effects. METHODS: This work measured the dose perturbation at depth and in the dose buildup region when one of two treatment couches, CIVCO (formerly MED-TEC) or Medical Intelligence, was placed between a photon beam source (6, 10, and 18 MV) and the phantom. Beam attenuation was measured in the center of a cylindrical acrylic phantom with a Farmer type ion chamber at multiple gantry angles. Dose buildup was measured in Solid Water with plane parallel ion chambers (NACP-02 and PTW Markus) with the beam normal to both the phantom and couch surfaces. The effective point of measurement method as described [M. R. McEwen et al. "The effective point of measurement of ionization chambers and the build-up anomaly in MV x-ray beams," Med. Phys. 35(3), 950-958 (2008)] was employed to calculate dose in the buildup region. Both experiments were modeled in XiO. Images of the treatment couches were merged with images of the phantoms such that they were included as part of the "patient" image. Dose distributions calculated with superposition and fast superposition algorithms were compared to measurement. RESULTS: The two treatment couches have different radiological signatures and dissimilar water equivalent thicknesses (4.2 vs 6.3 mm.) Maximum attenuation was 7%. Both couches caused significant loss of skin sparing, the worst case showing an increase in surface dose from 17% (no couch) to 88% (with couch). The TPS accurately predicted the surface dose (+/-3%) and the attenuation at depth when the phantom was in contact with the couch. For the open beam the TPS was less successful in the buildup region. CONCLUSIONS: The treatment couch is not radio-transparent. Its presence between the patient and beam source significantly alters dose in the patient. For the most part, a modern treatment planning system can adequately predict the altered dose distribution.

The emerging role of IG-IMRT for palliative radiotherapy: a single-institution experience.

Many modern radiotherapy centers now have image-guided intensity-modulated radiotherapy (IG-IMRT) tools available for clinical use, and the technique offers many options for patients requiring palliative radiotherapy. We describe a single-institution experience with IG-IMRT for short-course palliative radiotherapy, highlighting the unique situations in which the technique can be most effectively used.

Intraarterial cisplatin and concurrent radiation for locally advanced bladder cancer.

Between 1983 and 1987 25 patients with invasive bladder cancer (16 stage tumor 3 (T3) and nine stage T4) were treated with intraarterial cisplatin and concurrent radical radiation (20/25) or intraarterial cisplatin, concurrent preoperative radiation, and cystectomy (5/25). One patient died from treatment-related toxicity. Other toxicities have been what one would expect from the individual treatment modalities except for a sensory sacral root neuropathy in 11 of 24 (46%) patients. Twenty-three of 24 (96%) patients achieved a complete response (CR) and the projected actuarial 2-year survival is 90%. Only one of the 23 complete responders has had an invasive local recurrence. The excellent complete local response and survival rates achieved warrant further study of the combination of intraarterial cisplatin and radiation as a bladder-preserving strategy.

Effects of urethrography on prostate position: considerations for radiotherapy treatment planning of prostate carcinoma.

PURPOSE: Retrograde urethrography is commonly used to define the prostate apex at simulation. This study evaluated the hypothesis that urethrography causes prostate displacement, resulting in an error in treatment planning. METHODS AND MATERIALS: Forty-five patients with carcinoma of the prostate were evaluated. Gold seeds were placed in the apex, posterior wall, and base of the gland. In the first 20 patients, the position of the seed-defined apex was compared at simulation (with urethrogram) and on day 1 of treatment (without urethrogram). In the second cohort of 25 patients, the effects of urethrography on prostate position were evaluated directly at simulation by comparing the position of apex pre- and post-urethrography. An analysis was performed to estimate the possible impact of urethrogram-induced prostate motion on target coverage. RESULTS: The mean superior displacement in the first and second cohort was 5.2 mm and 6.8 mm, respectively (combined mean shift 6.1 mm). With a 10-mm field margin below the tip of the urethrogram cone, 56% of patients in this study would have inadequate planning target volume (PTV) coverage. CONCLUSION: Retrograde urethrography causes a significant superior shift of the prostate. Strict reliance on urethrography in determining the inferior field margin could result in inadequate treatment.

A comparison of heat and radiation sensitivity of three human glioma cell lines.

Three human glioma cell lines were tested for radiation and hyperthermia sensitivity and compared to the responses of a normal human fibroblast cell line. The radiation response of the glioma cell lines exhibited a large shoulder on the radiation survival curve indicating radioresistance when compared to the more radiosensitive fibroblast cell line. The hyperthermia response for the glioma cell lines was qualitatively similar to responses reported for other cell lines. When compared to normal human fibroblasts the glioma cells were found to be more sensitive to hyperthermia than the normal fibroblasts indicating hyperthermia may be a promising method or adjunct to radiotherapy in the treatment of resistant glioma cells or tumors. The results also show that both the radiation and thermal response is influenced by cell culture conditions and growth status. Two of the cell lines grown to confluency and treated in confluency showed an increased radiation resistance at low doses and the cell lines showed decreased resistance at high doses compared to cells plated to confluency (see Methods and Materials). An increased thermal resistance, especially at the lower heating temperatures, was also observed for cells grown to confluency. Measurements of residual glucose in the culture medium at the time of irradiation was about the same for the two culture methods (55%-65%). Cell cycle analysis showed that the differences were not related to changes in cell cycle distribution.

A prospective comparison of three systems of patient immobilization for prostate radiotherapy.

PURPOSE: The study compared the setup reliability of 3 patient immobilization systems, a rubber leg cushion, the alpha cradle, and the thermoplastic Hipfix device, in 77 patients with cT1-T3, N0, M0 prostate cancer receiving conformal radiotherapy. METHODS AND MATERIALS: Port films were analyzed and compared to simulation films to estimate the setup errors in the three coordinate axes (anterior-posterior, cranial-caudal, medial-lateral). A total vector error was calculated from these shifts. RESULTS: The Hipfix was found significantly superior to the other two devices in reducing mean setup errors in all axes (p < 0.005). The average field-positioning error with the Hipfix ranged from 1.9 mm to 2.6 mm for all axes, whereas the deviation for the other two systems ranged from 2.7 to 3. 4 mm. Errors greater than 10 mm were virtually eliminated with the Hipfix system. There was a reduction in the mean total vector error in the alpha cradle and Hipfix patient cohorts over time, reflecting improved efficacy as a result of experience. CONCLUSION: There was a significant difference in the performance of each immobilization device. The Hipfix was consistently more reliable in reducing setup errors than the other devices.

Asymmetric arc technique for posterior pharyngeal wall and retropharyngeal space tumors.

PURPOSE: Tumors of the posterior pharyngeal wall and nasopharyngeal cancer with retropharyngeal extension can partly encircle the cervical vertebrae. Treating the patient within spinal cord tolerance can cause a geographic miss. A simple technique has been developed to avoid this problem. METHODS AND MATERIALS: The standard fields for posterior pharyngeal wall and nasopharyngeal tumors are used up to 36-40 Gy. A planning computed tomography (CT) scan is taken during the second or third week of treatment with the patient fitted in a new shell ensuring that the cord is straight and parallel to the treatment couch. The asymmetric arc technique consists of two posterior arcs with closure of one jaw beyond the central axis. Each arc delivers the total dose to each ipsilateral side, while the median region of the U-shaped volume is treated by the summation of both arcs. RESULTS: We have treated 10 patients using asymmetric arcs in the last 3 years. This technique proved to be a versatile way of treating targets wrapped around the spine. The technique allows better individualization for target volume irregularities than the partial rotation with a central bar.

Impact of local radiation in the management of salivary gland carcinomas.

Seventy patients with salivary gland carcinoma (63% major gland and 37% minor gland) are reviewed. Histologies included adenoid cystic (54%), mucoepidermoid (16%), and adenocarcinoma (14%). Patients were analyzed according to extent of surgery and whether or not adjuvant postoperative radiotherapy was given. There is no difference in survival in patients who had complete excision of gross tumor with or without adjuvant radiotherapy. Patients who did not undergo radiotherapy had a 62% actuarial risk of locoregional failure at 5 years, with a 20% risk in the adjuvantly irradiated group (P less than 0.001). A failure analysis demonstrates that among the 44% of patients with recurrence 71% (22/31) failed locoregionally and 69% (21/31) had distant metastases. Twenty-seven percent (19/70) died of disease, with 31% (6/19) dying of locoregional disease and 26% (5/19) of distant disease. Implications for management are discussed.

On the spatial resolution of clinical thermometers.

In this work the spatial resolution of type T (copper-Constantan) and type K (Chromel-Alumel) multipoint thermocouple thermometers was investigated. For clinical use, thermocouples are usually inserted within catheters to provide easy access, reproducibility, and a sterile environment. The effect of several types of catheter on the ability of these thermometers to accurately resolve thermal gradients was studied. The influence of the number of wire pairs within the multipoint thermometer was also investigated. A mathematical model has been developed to determine the spatial resolving power of these thermometers. Results indicate that type K thermocouples are generally superior to type T and the use of catheters plays a significant role in spatial resolution. Moreover, the use of mineral oil as a thermal coupling agent between the catheter and the thermometer was also found to have some effect.

Lateral electron equilibrium and electron contamination in measurements of head-scatter factors using miniphantoms and brass caps.

The head-scatter factor (Sh) can be measured with a narrow miniphantom or a metal cap provided it is completely covered by the photon beam and its lateral size is thick enough to prevent electron contamination contributions. The effects of lateral electron equilibrium (LEE) and electron contamination on the Sh values were studied. The EGS4 Monte Carlo technique was used to calculate the minimum beam radii (rLEE) required to achieve complete LEE for photon beams ranging from 60Co to 24 MV. The measurement shows that the error introduced to the Sh value due to lateral electron disequilibrium is negligible. The radii of the miniphantoms or the sidewall thicknesses of the caps can be reduced below rLEE provided they are thick enough to prevent the effect of electron contamination.

The effect of dose rate dependence of p-type silicon detectors on linac relative dosimetry.

Cumulative radiation damage to silicon semiconductor diode detectors can induce dose rate dependent sensitivity, a concern in the pulsed beam of a linac. Two p-Si diode photon detectors were used in this study, diodes A and B. Both were preirradiated by the supplier to 5 kGy, with diode A receiving an estimated 8 kGy from measurements, and diode B, 25 kGy. At 6 MV, the PDD measured with diode B was lower (by 4.4% at a depth of 25 cm) than diode A. Using SSD to vary the dose per pulse from 0.02 to 0.64 mGy/pulse, diode A was dose rate independent (within 2%), while the sensitivity of diode B changed by 13%. Silicon diode detectors should be checked regularly against ionization chambers in the pulsed beam of a linac, especially older high-resistivity diodes that have accumulated dose from high-energy photon beams.

Beam characteristics of the therapax DXT300 orthovoltage therapy unit.

The operating performance and beam characteristics of a new orthovoltage unit, the Therapax DXT300, have been evaluated. Percentage depth-dose and backscatter tables are presented for several applicator sizes, at 30 cm and 50 cm focal skin distances (FSDS) and for multiple x-ray beam qualities with the tube operating between 100 and 300 kVp accelerating potential. The unit has been found to provide beam characteristics similar to those reported for other orthovoltage therapy machines. The linearity and short- and long-term stability/reproducibility of the unit's internal dosimetry system have also been studied, and results indicate a very stable beam output of better than 1% standard deviation. The data presented in this work should provide the basis for comparison with other units and act as a reference for clinics commissioning the Therapax DXT300 in the future.

A translating-bed technique for total-body irradiation.

Total-body irradiation (TBI) is a therapy modality that is being used with increasing frequency, in conjunction with chemotherapy, for patients undergoing bone-marrow transplantation. At the Ottawa Regional Cancer Centre a technique has been developed for the delivery of TBI to patients prior to bone-marrow transplantation. In this technique patients are treated on a mobile couch at approximately 195 cm SSD with a field size of 66.5 cm wide by 57 cm long. A computer-controlled stepping motor drives the patient couch at a user-selectable speed. The total dose delivered to the patient is a function of couch velocity, field size and patient separation. Treatment times are of the order of 10 min for each of the anterior and posterior fields for a 400 cGy fraction. It has been found that the conventional central axis tissue maximum ratio (TMR) and percentage depth dose (PDD) functions are not appropriate for describing dose delivered during dynamic treatment. To this end we have developed dynamic TMR and PDD functions. Extensive measurements have been performed in an anthropomorphic water phantom to determine the dose distributions in three dimensions and the efficacy of polymethyl methacrylate (PMMA) beam spoilers as a replacement for anterior and lateral bolus. It has been found that 2.4 cm PMMA spoilers do provide full skin dose and negate the requirement for lateral bolus. This TBI procedure is simple, rapid and appears to be well tolerated by the patients. 55 patients have been treated since the introduction of this technique in 1991.

Prognostic factors in early carcinoma of the endometrium.

Two hundred forty-one patients with clinical-pathological Stage I and 58 patients with clinical-pathological Stage II carcinoma of the endometrium treated between January 1959 and December 1983 at the Ottawa General Hospital were analyzed. The adjusted survival rate at 5 years was 92% in patients with Stage I and 66% in patients with Stage II. In patients with Stage I, the most important prognostic factors were the histological grade of the tumor and the depth of myometrial invasion. In patients with Stage II, the single most important prognostic factor was the clinical extent of the disease. Grade and depth of myometrial invasion were also significant prognostic factors, particularly in patients with pathological Stage II. Combined surgery and radiation therapy was clearly superior to surgery alone in patients with Stage II but not in patients with Stage I, although, with long-term follow-up, our results may suggest improved survival in these patients as well.

Sci-Fri PM: Delivery - 06: A generalized solution to the wide field array calibration method.

Multi detector arrays are commonly used in radiation oncology for IMRT and Linac QA. The calibration of detector arrays is usually based on the wide field calibration technique. Unfortunately small beam changes between measurements will result in large error propagation. The present work introduces a generalized modified version of the wide field calibration method, robust against measurement to measurement variation. Our generalized framework uses an unlimited number of measurement pairs, n geometric positions providing n(n-1)/2 pairs. We solve this large over determined linear system using least squares with gradient method. Measurements were made on an Elekta synergy 6 MV beam with two IBA Matrixx detectors, each containing a 32 × 32 array (1024) of vented pixel ionization chambers. Data acquisition was by the IBA Omnipro Advance software, version 1.2 running in the "ONLINE" cine mode with a 10 sec integration time. Continuous beam sampling (10 seconds long) measured over 10 minutes demonstrated why consistent calibration using the conventional wide field calibration is a challenge. Overall signal changes of 1.6%, flatness changes of 0.3% and the beam symmetry changes of 0.2% over the full 10 minute beam-on time were observed. For the purpose of testing and demonstration of our method, we have chosen to make measurements in 5 geometric orientations relative to the beam, including 1 reference position, 2 rotations and 2 translations. With our method we were able to calibrate all 1024 detectors to better than 0.6% total uncertainty as demonstrated against inter and intra MatriXX comparison.

Poster - Thur Eve - 32: Water tank referenced calibration method for detector array devices.

INTRODUCTION: Detector array devices, such as the I'mRT Matrixx (IBA Dosimetry), provide a means of evaluating beam profiles with respect to gantry for a range of dose rates and monitor units. The relative calibration of these devices is typically highly susceptible to even relatively small variations in beam output. An alternative method is proposed here, which directly references the device detector response to water tank data. METHODS: The Matrixx response was measured at the four cardinal angles for three devices. A calibration factor was determined for each orientation of the Matrixx device by dividing a water tank measured profile by the Matrixx response for the in-plane and cross-plane detectors. A geometric mean of each orientation was used as the estimate of the calibration coefficient. RESULTS: Before calibration, the three-detector average of the deviation from the profile measured in the water tank centered on each of the horns was 0.4% (SD 0.2%); applying the calibration procedure reduced this to 0.1% (SD 0.1%). The energy independence of the proposed relative calibration was also confirmed. A comparison of the linac output for relatively short Matrixx acquisitions to the longer water tank acquisition suggested some difference. This difference was mitigated by averaging. CONCLUSIONS: The proposed water tank reference calibration procedure is an effective means of determining the relative calibration of a detector array and mitigates the effect of compound error by avoiding the recursive algorithm of typical calibration methods. In addition it has the benefit of being directly relatable to commissioning beam data.

Total scalp radiation using image-guided IMRT for progressive cutaneous T cell lymphoma.

Modern radiotherapy has advanced dramatically over the past decade and it is now possible to focus radiotherapy with extreme precision. This allows the radiation dose to be targeted to the area(s) of tumour while sparing adjacent normal tissues even in seemingly complicated and difficult parts of the body. The case report presented here will illustrate how it is possible to irradiate the entire scalp for extensive cutaneous T cell lymphoma while minimising radiotherapy to the underlying brain, orbits and other critical structures.

Poster - Thurs Eve-32: Dose errors related to the treatment couch.

Modern radiotherapy linacs often use carbon fibre for their couch tops due to its radio translucent properties. Beam attenuation by the couches is often ignored during planning and MU calculation. This work examines beam attenuation and loss of "skin sparing" (dose build up region) when various photon beams transit either the MedTec (Siemens) or Medical Intelligence (Elekta) couches. Additionally, measured doses were compared to CMS treatment planning system (XiO version 4.33.02) predictions. We found the two couches to have different structures, resulting in different attenuation signatures as a function of gantry angle. For normal beam incidence the Siemens and Elekta couches had radiological thicknesses of 4.5 mm and 6.0 mm, respectively. For a normal incidence 10×10 cm2 6MV beam the surface dose after couch transmission was 93% vs. 83% for Elekta and Siemens, respectively. Conversely, the increased mass on the lateral edge of the Siemens couch resulted in a maximum attenuation (6 MV 5×5 cm2 beams) of 8% compared to 5% by the Elekta couch. Incorporating the treatment couch as part of the patient planning CT allowed the CMS TPS model to calculate couch attenuation within 1% of measurement, except at the very edge of the Siemens couch, where the attenuation is strongly gantry angle dependent. The CMS beam model was also able to predict the loss of skin sparing within 1%. In conclusion, the two patient couches are different, but both can significantly affect patient dose which can be accounted for in the CMS TPS.