Magnetic tracking for TomoTherapy systems: gradiometer based methods to filter eddy-current magnetic fields.

PURPOSE: TomoTherapy systems lack real-time, tumor tracking. A possible solution is to use electromagnetic markers; however, eddy-current magnetic fields generated in response to a magnetic source can be comparable to the signal, thus degrading the localization accuracy. Therefore, the tracking system must be designed to account for the eddy fields created along the inner bore conducting surfaces. The aim of this work is to investigate localization accuracy using magnetic field gradients to determine feasibility toward TomoTherapy applications. METHODS: Electromagnetic models are used to simulate magnetic fields created by a source and its simultaneous generation of eddy currents within a conducting cylinder. The source position is calculated using a least-squares fit of simulated sensor data using the dipole equation as the model equation. To account for field gradients across the sensor area (≈ 25 cm(2)), an iterative method is used to estimate the magnetic field at the sensor center. Spatial gradients are calculated with two arrays of uniaxial, paired sensors that form a gradiometer array, where the sensors are considered ideal. RESULTS: Experimental measurements of magnetic fields within the TomoTherapy bore are shown to be 1%-10% less than calculated with the electromagnetic model. Localization results using a 5 × 5 array of gradiometers are, in general, 2-4 times more accurate than a planar array of sensors, depending on the solenoid orientation and position. Simulation results show that the localization accuracy using a gradiometer array is within 1.3 mm over a distance of 20 cm from the array plane. In comparison, localization errors using single array are within 5 mm. CONCLUSIONS: The results indicate that the gradiometer method merits further studies and work due to the accuracy achieved with ideal sensors. Future studies should include realistic sensor models and extensive numerical studies to estimate the expected magnetic tracking accuracy within a TomoTherapy system before proceeding with prototype development.

Radiation proctopathy in the treatment of prostate cancer.

PURPOSE: To compile and review data on radiation proctopathy in the treatment of prostate cancer with respect to epidemiology, clinical manifestations, pathogenesis, risk factors, and treatment. METHODS: Medical literature databases including PubMed and Medline were screened for pertinent reports, and critically analyzed for relevance in the scope of our purpose. RESULTS: Rectal toxicity as a complication of radiotherapy has received attention over the past decade, especially with the advent of dose-escalation in prostate cancer treatment. A number of clinical criteria help to define acute and chronic radiation proctopathy, but lack of a unified grading scale makes comparing studies difficult. A variety of risk factors, related to either radiation delivery or patient, are the subject of intense study. Also, a variety of treatment options, including medical therapy, endoscopic treatments, and surgery have shown varied results, but a lack of large randomized trials evaluating their efficacy prevents forming concrete recommendations. CONCLUSION: Radiation proctopathy should be an important consideration for the clinician in the treatment of prostate cancer especially with dose escalation. With further study of possible risk factors, the advent of a standardized grading scale, and more randomized trials to evaluate treatments, patients and physicians will be better armed to make appropriate management decisions.

Tolerance of endorectal balloon in 396 patients treated with intensity-modulated radiation therapy (IMRT) for prostate cancer.

PURPOSE: To report patient tolerance and acute anorectal toxicity of an endorectal balloon used for prostate immobilization during 35 daily fractions. MATERIALS AND METHODS: The records of 396 patients treated for prostate cancer from October 1997 to November 2001 were reviewed. Patients were treated with intensity modulated radiation therapy (IMRT). Endorectal balloon catheter was inserted daily, inflated with 100 mL of air for immobilizing the prostate gland. Patient and treatment factors were analyzed. Patients received a mean dose of 77 Gy/35 fractions/7 weeks with no rectal block. RESULTS: None of the 396 patients halted treatment because of associated ano-rectal toxicity. No patient stated that he would decline to be treated again with rectal balloon. Three of 396 (0.8%) patients required a reduction in the volume of the balloon to 50 mL. Seventeen of 396 (4.3%) patients required Lidocaine jelly with the insertion of balloon. Radiation Therapy Oncology Group (RTOG) grades 1 and 2 rectal toxicity occurred in 55/396 (13.9%) and 73/396 (18.4%), respectively. No RTOG grade 3 or 4 toxicities occurred. Topical anal medications were prescribed for 46 of 396 (11.6%) patients and antidiarrhea medication for 27 of 396 (6.8%) patients. Of patients with pretreatment anorectal disease, 50% developed rectal toxicities over the 7 weeks. Rectal toxicity occurred most frequently in the third, fourth, fifth, or sixth week; 19.5%, 20.8%, 18.2%, and 16.9%, respectively. The duration of the toxicity measured lasted 1 week, 35.2%; 2 weeks, 31.0%; 3 weeks, 15.5%; 4 weeks, 11.3%; 5 weeks, 4.2%; and 6 weeks, 2.8%. CONCLUSION: Most of the patients, 393/396 (99.2%), tolerated a 100 mL endorectal immobilization balloon for IMRT. The rate of acute anorectal toxicity was acceptable with no grade 3 or 4 toxicities. Duration of the toxicities typically was 1 to 2 weeks. Patients with pre-existing anorectal disease are at higher risk of developing acute anorectal toxicity with the use of an endorectal balloon.

Rectal wall sparing by dosimetric effect of rectal balloon used during intensity-modulated radiation therapy (IMRT) for prostate cancer.

The use of an air-filled rectal balloon has been shown to decrease prostate motion during prostate radiotherapy. However, the perturbation of radiation dose near the air-tissue interfaces has raised clinical concerns of underdosing the prostate gland. The aim of this study was to investigate the dosimetric effects of an air-filled rectal balloon on the rectal wall/mucosa and prostate gland. Clinical rectal toxicity and dose-volume histogram (DVH) were also assessed to evaluate for any correlation. A film phantom was constructed to simulate the 4-cm diameter air cavity created by a rectal balloon. Kodak XV2 films were utilized to measure and compare dose distribution with and without air cavity. To study the effect in a typical clinical situation, the phantom was computed tomography (CT) scanned on a Siemens DR CT scanner for intensity-modulated radiation therapy (IMRT) treatment planning. A target object was drawn on the phantom CT images to simulate the treatment of prostate cancer. Because patients were treated in prone position, the air cavity was situated superiorly to the target. The treatment used a serial tomotherapy technique with the Multivane Intensity Modulating Collimator (MIMiC) in arc treatment mode. Rectal toxicity was assessed in 116 patients treated with IMRT to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). They were treated in the prone position, immobilized using a Vac-Loktrade mark bag and carrier-box system. Rectal balloon inflated with 100 cc of air was used for prostate gland immobilization during daily treatment. Rectal toxicity was assessed using modifications of the Radiation Therapy Oncology Group (RTOG) and late effects Normal Tissue Task Force (LENT) scales systems. DVH of the rectum was also evaluated. From film dosimetry, there was a dose reduction at the distal air-tissue interface as much as 60% compared with the same geometry without the air cavity for 15-MV photon beam and 2x2-cm field size. The dose beyond the interface recovered quickly and the dose reductions due to air cavity were 50%, 28%, 11%, and 1% at 2, 5, 10, and 15 mm, respectively, from the distal air-tissue interface. Evaluating the dose profiles of the more clinically relevant situation revealed the dose at air-tissue interface was approximately 15% lower in comparison to that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, there was only an 8% and 5% differential, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal with or without the air cavity. The median follow-up was 31.3 months. Rectal toxicity profile was very favorable: 81% (94/116) patients had no rectal complaint while 10.3% (12/116), 6.9% (8/116), and 1.7% (2/116) had grade 1, 2, and 3 toxicity, respectively. There was no grade 4 rectal toxicity. DVH analysis revealed that none of the patients had more than 25% of the rectum receiving 70 Gy or greater. Rectal balloon has rendered anterior rectal wall sparing by its dosimetric effects. In addition, it has reduced rectal volume, especially posterior and lateral rectal wall receiving high-dose radiation by rectal wall distension. Both factors may have contributed to decreased rectal toxicity achieved by IMRT despite dose escalation and higher than conventional fraction size. The findings have clinical significance for future very high-dose escalation trials whereby radiation proctitis is a major limiting factor.

Theoretical foundation for real-time prostate localization using an inductively coupled transmitter and a superconducting quantum interference device (SQUID) magnetometer system.

Real-time, 3D localization of the prostate for intensity-modulated radiotherapy can be accomplished with passively charged radio frequency transmitters and superconducting quantum interference device (SQUID) magnetometers. The overall system design consists of an external dipole antenna as a power source for charging a microchip implant transmitter and SQUID magnetometers for signal detection. An external dipole antenna charges an on-chip capacitor through inductive coupling in the near field region through a small implant inductor. The charge and discharge sequence between the external antenna and the implant circuit can be defined by half duplex, full duplex, or sequential operations. The resulting implant discharge current creates an alternating magnetic field through the inductor. The field is detected by the surrounding magnetometers, and the location of the implant transmitter can be calculated. Problems associated with this system design are interrelated with the signal strength at the detectors, detector sensitivity, and charge time of the implant capacitor. The physical parameters required for optimizing the system for real-time applications are the operating frequency, implant inductance and capacitance, the external dipole current and loop radius, the detector distance, and mutual inductance. Consequently, the sequential operating mode is the best choice for real-time localization for constraints requiring positioning within 1 s due to the mutual inductance and detector sensitivity. We present the theoretical foundation for designing a real-time, 3D prostate localization system including the associated physical parameters and demonstrate the feasibility and physical limitations for such a system.

The evolution of quality assurance for intensity- modulated radiation therapy (IMRT): sequential tomotherapy.

PURPOSE: To identify the pertinent issues to be addressed in successfully implementing IMRT using sequential tomotherapy into clinical reality and presenting the maturation of quality assurance (QA) programs for both the delivery system and patient treatments that allow routine clinical use of the system. MATERIALS AND METHODS: Initially, a cubic phantom containing silver halide film was exposed to the entire treatment before patient treatment. The processed films were digitized with a laser densitometer and the dose distributions were compared with that generated by the planning system. Later, software that calculates the dose delivered to any phantom employing the intensity patterns developed in the inverse planning system for an individual patient was implemented for point checks of dose. A measurement phantom for use with this software was developed and evaluated on a large number of patients. Invasive fixation was used for all cranial patients initially. To use sequential tomotherapy for other sites and larger targets, noninvasive immobilization systems using two types of thermoplastic masks for cranial targets and reusable, evacuated body cradles were evaluated for positional accuracy and suitability for use with port films for patient QA. RESULTS: The program for equipment validation is divided into daily, weekly, and monthly programs that add only small amounts of time to routine QA programs. For the first 15 patients treated with this modality, the maximum dose measured on the film was within 5% of that predicted by the planning computer. The prescription isodose line was measured in the anteroposterior and lateral dimensions and the average discrepancy between measured and predicted was less than 2 mm. For an isodose line between 50% and 70% of the prescribed dose, the agreement was better than 3 mm. Success with the volume QA program was followed by a point check QA program that reduced the time required for individual patient QA from days to hours. Phantom measurements compared with computer predictions for 588 data points resulted in only 8% being outside a +/-5% criterion. These cases were identified and allow a further reduction in the frequency of tests. Thermoplastic mask materials have adequate restraint characteristics for use with the system and port films on 21 patients resulted in one standard deviation = 1.3 mm. Body cradles are less accurate and require more frequent port films. A QA system that reduces the frequency of port films was developed. CONCLUSIONS: The evolution of sequential tomotherapy in our department has been from a maximum of 3 cranial patients per day with invasive fixation to 60 patients per day for treatment of cranial, head-and-neck, and prostate tumors using different immobilization techniques. With proper preparation and refinement of tools used in commissioning and validation, sequential tomotherapy IMRT can become a routine clinical treatment modality.

The use of rectal balloon during the delivery of intensity modulated radiotherapy (IMRT) for prostate cancer: more than just a prostate gland immobilization device?

PURPOSE: The purpose of this study was to investigate the role of a rectal balloon for prostate immobilization and rectal toxicity reduction in patients receiving dose-escalated intensity-modulated radiotherapy for prostate cancer. PATIENTS AND METHODS: Patients with localized prostate cancer who were undergoing intensity-modulated radiotherapy were treated in a prone position, immobilized with a customized Vac-Lok bag (MED-TEC, Orange City, IA). A rectal balloon with 100 cc of air was used to immobilize the prostate. The prostate displacements were measured using computed tomography (CT)-CT fusion on 10 patients who received radioactive seed implant before intensity-modulated radiotherapy. They were scanned twice weekly during 5 weeks of intensity-modulated radiotherapy, and breathing studies were also performed. Rectal toxicity was evaluated by use of Radiation Therapy Oncology Group scoring in 100 patients. They were treated to a mean dose of 76 Gy over 35 fractions (2.17-Gy fraction size). Dose-volume histogram of the rectum was assessed. A film phantom was constructed to simulate the 4-cm diameter air cavity that was created by the rectal balloon. Kodak XV2 films (Rochester NY) were used to measure and compare dose distribution with and without the air cavity. A fraction of 1.25 Gy was delivered to the phantom at isocenter with 15-MV photons by use of the NOMOS Peacock system and the MIMiC treatment delivery system (Sewickley, PA). RESULTS: The anterior-posterior and lateral prostate displacements were minimal, on the order of measurement uncertainty (approximately 1 mm). The standard deviation of superior-inferior displacement was 1.78 mm. Breathing studies showed no organ displacement during normal breathing when the rectal balloon was in place. The rectal toxicity profile was very favorable: 83% (83/100) patients had no rectal complaint, and 11% and 6% had grade 1 and 2 toxicity, respectively. Dose-volume histogram analysis revealed that in all of the patients, no more than 25% of the rectum received 70 Gy or greater. As visualized by film dosimetry, the dose at air-tissue interface was approximately 15% lower than that without an air cavity. The dose built up rapidly so that at 1 and 2 mm, the differential was approximately 8% and 5%, respectively. The dosimetric coverage at the depth of the posterior prostate wall was essentially equal, with or without the air cavity. DISCUSSION: The use of a rectal balloon during intensity-modulated radiotherapy significantly reduces prostate motion. Prostate immobilization thus allows a safer and smaller planning target volume margin. It has also helped spare the anterior rectal wall (by its dosimetric effects) and reduced the rectal volume that received high-dose radiation (by rectal wall distension). All these factors may have further contributed to the decreased rectal toxicity achieved by intensity-modulated radiotherapy, despite dose escalation and higher-than-conventional fraction size.

Clinical experience with intensity-modulated radiation therapy (IMRT) for prostate cancer with the use of rectal balloon for prostate immobilization.

The implementation of intensity-modulated radiation therapy (IMRT) is the result of advances in imaging, radiotherapy planning technologies, and computer-controlled linear accelerators. IMRT allows both conformal treatment of tumors and conformal avoidance of the surrounding normal structures. The first patient treated with Peacock IMRT at Baylor College of Medicine took place in March 1994. To date, more than 1500 patients have been treated with IMRT; more than 700 patients were treated for prostate cancer. Our experience in treating prostate cancer with IMRT was reviewed. Patient and prostate motions are important issues to address in delivering IMRT. The Vac-Lok bag-and-box system, as well as rectal balloon for immobilization of patient and prostate gland, respectively, are employed. Treatment planning also plays a very important role. IMRT as a boost after conventional external beam radiotherapy is not our treatment strategy. To derive maximal benefits with this new technology, all patients received full course IMRT. Three separate groups of patients receiving (1) primary IMRT, (2) combined radioactive seed implant and IMRT, and (3) post-prostatectomy IMRT were addressed. Overall, toxicity profiles in these patients were very favorable. IMRT has the potential to improve treatment outcome with dose escalation while minimizing treatment-related toxicity.

Prostate immobilization using a rectal balloon.

We use a rectal balloon for prostate immobilization during intensity modulated radiotherapy (IMRT) prostate treatment. To improve the accuracy of our prostate planning target volume, we have measured prostate displacements using computed tomography (CT)-CT fusion on patients that previously received gold seed implants. The study consists of ten patients that were scanned twice per week during the course of IMRT treatment. In addition to biweekly scans, breathing studies were performed on each patient to estimate organ motion during treatment. The prostate displacement in the anterior-posterior and the lateral direction is minimal, on the order of measurement uncertainty (~1 mm). The standard deviation of the superior-inferior (SI) displacements is 1.78 mm. The breathing studies show that no organ displacement was detected during normal breathing conditions with a rectal balloon.