Mobile linear accelerators for intraoperative radiation therapy

Evaluation of RADIANCE Monte Carlo algorithm for treatment planning in electron based Intraoperative Radiotherapy (IOERT)

PURPOSE

To perform experimental as well as independent Monte Carlo (MC) evaluation of the MC algorithm implemented in RADIANCE version 4.0.8, a dedicated treatment planning system (TPS) for 3D electron dose calculations in intraoperative radiation therapy (IOERT).

METHODS AND MATERIALS

The MOBETRON 2000 (IntraOp Medical Corporation, Sunnyvale, CA) IOERT accelerator was employed. PDD and profiles for five cylindrical plastic applicators with 50-90 mm diameter and 0°, 30° beveling were measured in a water phantom, at nominal energies of 6, 9 and 12 MeV. Additional PDD measurements were performed for all the energies without applicator. MC modeling of the MOBETRON was performed with the user code BEAMnrc and egs_chamber of the MC simulation toolkit EGSnrc. The generated phase space files of the two 0°-bevel applicators (50 mm, 80 mm) and three energies in both RADIANCE and BEAMnrc, were used to determine PDD and profiles in various set-ups of virtual water phantoms with air and bone inhomogeneities. 3D dose distributions were also calculated in image data sets of an anthropomorphic tissue-equivalent pelvis phantom. Image acquisitions were realized with a CT scanner (Philips Big Bore CT, Netherlands). Gamma analysis was applied to quantify the deviations of the RADIANCE calculations to the measurements and EGSnrc calculations. Gamma criteria normalized to the global maximum were investigated between 2%, 2 mm and 3%, 3 mm.

RESULTS

RADIANCE MC calculations satisfied the gamma criteria of 3%, 3 mm with a tolerance limit of 85% passing rate compared to in- water phantom measurements, except for the dose profiles of the 30° beveled applicators. Mismatches lay in surface doses, in umbra regions and in the beveled end of the 30° applicators. A very good agreement to the EGSnrc calculations in heterogeneous media was observed. Deviations were more pronounced for the larger applicator diameter and higher electron energy. In 3D dose comparisons in the anthropomorphic phantom, gamma passing rates were higher than 96 % for both simulated applicators.

CONCLUSIONS

RADIANCE MC algorithm agrees within 3%, 3 mm criteria with in-water phantom measurements and EGSnrc MC dose distributions in heterogeneous media for 0°-bevel applicators. The user should be aware of missing scattering components and the 30° beveled applicators should be used with attention.

Future Directions of Intraoperative Radiation Therapy: A Brief Review

The use of intraoperative radiation therapy (IORT) is increasing with the development of new devices for patient treatment that allow irradiation without the need to move the patient from the surgical table. At the moment, ionizing radiation in the course of IORT is supported most often by the use of mobile devices that produce electrons, kilo voltage X-rays, and electronic brachytherapy and the development of applicators suitable for delivery of radionuclides for short-term brachytherapy. The establishment of new treatment devices and protocols that can be foreseen in the future, e.g., the development of proton or heavy ion sources suitable for IORT or the establishment of new treatment protocols such as the use of IORT in combination with immune system modulators or radiosensitizing nanoparticles, could lead to a significant increase in the use of IORT in the future. This review discusses the still limited use of IORT at this point in time and hypothesizes about possible future approaches to radiotherapy.

Dose consumption for quality assurance and maintenance at a dedicated IORT accelerator

Dedicated accelerators for intra‐operative radiation therapy (IORT) are operated at high dose rates in order to achieve short treatment times within which the anaesthetisized patient must be remotely monitored (e.g. via video cameras and telemetric anesthesia instruments). Due to these high dose rates, large doses accumulate from the irradiations necessary for quality assurance (QA) and maintenance. In practice, the dose load for QA, maintenance and repairs will probably far exceed the patient dose. The total dose consumption for all of these actions must be considered in facility licensing, in radiation protection assessments, and in the shielding calculations. Dose consumption for QA and maintenance was assessed for the dedicated IORT facility at Heidelberg University for the operation period between June 1991 and December 2007 (15.5 years). Average doses per year of 5847 Gy for maintenance and repairs and 3686 Gy for QA were needed during this period. The causes and composition of these high doses are analyzed and discussed separately for irradiations that need to be performed in the operation room and which, with a mobile accelerator, may be performed in a separate QA vault.

PACS number: 87.56.bd

Clinical results of intraoperative radiation therapy for patients with locally recurrent and advanced tumors having colorectal involvement

BACKGROUND

Intraoperative radiation therapy (IORT) may be useful in the treatment of patients who have a locally advanced primary and recurrent abdominopelvic neoplasm with colorectal involvement.

METHODS

A retrospective review of colorectal cancer patients treated since 1999 with IORT using the Mobetron device.

RESULTS

Forty patients underwent colectomy or proctectomy with IORT. All patients had evidence of local extension to contiguous structures and based on preoperative staging were deemed by the operating surgeon as being likely to have incomplete resection. IORT was selected as an alternative to sacrectomy or exenteration for an expected close margin in 10 patients. Mean survival was 35 +/- 26 months, and 1 patient had local recurrence.

CONCLUSIONS

The introduction of IORT has allowed a selective treatment approach to locally advanced primary and recurrent neoplasms, which traditionally would have been deemed unresectable. Using IORT, extended resections may be avoided in selected high-risk patients with low risk of local recurrence and minimal morbidity.

High-dose-rate remote afterloaders for intraoperative radiation therapy

Intraoperative radiation therapy (IORT) is a treatment option that directly irradiates a surgically exposed tumor or tumor bed while preventing radiation exposure of normal tissues. This article discusses the high-dose-rate intraoperative radiation therapy (HDR-IORT) technique by reviewing the roles of IORT team members, discussing needed equipment and supplies, describing quality assurance processes, explaining the HDR-IORT treatment delivery procedure, and reviewing the post-treatment phase.

Intraoperative radiation therapy using a mobile electron linear accelerator: field matching for large-field electron irradiation

Intraoperative radiation therapy (IORT) consists of delivering a large, single-fraction dose of radiation to a surgically exposed tumour or tumour bed at the time of surgery. With the availability of a mobile linear accelerator in the OR, IORT procedures have become more feasible for medical centres and more accessible to cancer patients. Often the area requiring irradiation is larger than what the treatment applicators will allow, and therefore, two or more adjoining fields are used. Unfortunately, the divergence and scattering of the electron beams may cause significant dose variations in the region of the field junction. Furthermore, because IORT treatments are delivered in a large single fraction, the effects of underdosing or overdosing could be more critical when compared to fractionated external beam therapy. Proper matching of the fields is therefore an important technical aspect of treatment delivery. We have studied the matching region using the largest flat applicator available for three different possibilities: abutting the fields, leaving a small gap or creating an overlap. Measurements were done using film dosimetry for the available energies of 4, 6, 9 and 12 MeV. Our results show the presence of clinically significant cold spots for the low-energy beams when the fields are either gapped or abutted, suggesting that the fields should be overlapped. No fields should be gapped. The results suggest that an optimal dose distribution may be obtained by overlapping the fields at 4 and 6 MeV and simply abutting the fields at 9 and 12 MeV. However, due to uncertainties in the placement of lead shields during treatment delivery, one may wish to consider overlapping the higher energy fields as well.

Intraoperative radiation therapy using mobile electron linear accelerators: report of AAPM Radiation Therapy Committee Task Group No. 72

Intraoperative radiation therapy (IORT) has been customarily performed either in a shielded operating suite located in the operating room (OR) or in a shielded treatment room located within the Department of Radiation Oncology. In both cases, this cancer treatment modality uses stationary linear accelerators. With the development of new technology, mobile linear accelerators have recently become available for IORT. Mobility offers flexibility in treatment location and is leading to a renewed interest in IORT. These mobile accelerator units, which can be transported any day of use to almost any location within a hospital setting, are assembled in a nondedicated environment and used to deliver IORT. Numerous aspects of the design of these new units differ from that of conventional linear accelerators. The scope of this Task Group (TG-72) will focus on items that particularly apply to mobile IORT electron systems. More specifically, the charges to this Task Group are to (i) identify the key differences between stationary and mobile electron linear accelerators used for IORT, (ii) describe and recommend the implementation of an IORT program within the OR environment, (iii) present and discuss radiation protection issues and consequences of working within a nondedicated radiotherapy environment, (iv) describe and recommend the acceptance and machine commissioning of items that are specific to mobile electron linear accelerators, and (v) design and recommend an efficient quality assurance program for mobile systems.

Stability of a mobile electron linear accelerator system for intraoperative radiation therapy

The flexibility of mobile electron accelerators, which are designed to be transported to an operating room and plugged into a normal 3-phase outlet, make them ideal for use in intraoperative radiation therapy. However, their transportability may cause trepidation among potential users, who may question the stability of such an accelerator over a period of use. In order to address this issue, we have studied the short-term stability of the Mobetron system over 20 daily quality assurance trials. Variations in output generally varied within +/-2% for the four energies produced by the unit (4, 6, 9, and 12 MeV) and changes in energy produced an equivalent shift of less than 1 mm on the depth-dose curve. Hours of inactivity, with the Mobetron powered on for use either throughout the day or overnight, led to variations in output of about 1%. Finally, we have tested the long-term stability of the absolute dose output of the Mobetron, which showed a change of about 1% per year.

Intraoperative radiotherapy using a mobile electron LINAC: a retroperitoneal sarcoma case

The advent of mobile LINACs for use in intraoperative radiation therapy (IORT) promises to make IORT more accessible than before and easier to deliver to patients undergoing surgery. Although mobile IORT systems have been available since 1999, few treatment centers currently use them. Here, we present the case of a typical patient undergoing IORT for retroperitoneal sarcoma to show how easy these mobile systems are to use and how adaptable they are within the operating room (OR) environment. We also discuss the roles and coordination of multidisciplinary team members during IORT and the feasibility of using mobile LINACs for IORT.

PACS number(s):

Calibration of low-energy electron beams from a mobile linear accelerator with plane-parallel chambers using both TG-51 and TG-21 protocols

A new approach to intraoperative radiation therapy led to the development of mobile linear electron accelerators that provide lower electron energy beams than the usual conventional accelerators commonly encountered in radiotherapy. Such mobile electron accelerators produce electron beams that have nominal energies of 4, 6, 9 and 12 MeV. This work compares the absorbed dose output calibrations using both the AAPM TG-51 and TG-21 dose calibration protocols for two types of ion chambers: a plane-parallel (PP) ionization chamber and a cylindrical ionization chamber. Our results indicate that the use of a 'Markus' PP chamber causes 2-3% overestimation in dose-output determination if accredited dosimetry-calibration laboratory based chamber factors (N(60Co)(D,w,) Nx) are used. However, if the ionization chamber factors are derived using a cross-comparison at a high-energy electron beam, then a good agreement is obtained (within 1%) with a calibrated cylindrical chamber over the entire energy range down to 4 MeV. Furthermore, even though the TG-51 does not recommend using cylindrical chambers at the low energies, our results show that the cylindrical chamber has a good agreement with the PP chamber not only at 6 MeV but also down to 4 MeV electron beams.

The role of intraoperative radiotherapy in pancreatic cancer

In single-institution studies, IORT at appropriate doses seems to safely improve local control in patients who have locally advanced pancreatic cancer, compared with historical controls. IORT also has been a component of adjuvant treatment programs that have led to excellent local control in resected patients. When considering the use of IORT, it is essential to have an understanding of the physical characteristics of the electron beam and how it can differ with the use of flat and beveled applicators. Although apparent improvement in local control with the use of IORT seems to have produced some improvement in median survival rates, high rates of distant failure continues to prevent a significant improvement in long-term survival. Because effective local control in patients with unresectable pancreatic cancer is a prerequisite to the development of curative therapies, the development of improved systemic therapies in patients with locally advanced pancreatic cancer will likely make local therapies such as the use of IORT even more important in the future.

Potential of mobile intraoperative radiotherapy technology

Mobile IORT units have the potential to change the way patients who have cancer are treated. The integration of IORT into cancer treatment programs, made possible by the new technologies of mobile linear accelerators that can be used in unshielded operating rooms, makes IORT significantly less time-consuming, less costly, and less risky to administer. It is now practical for IORT to be used in early-stage disease, in addition to advanced disease, and in sites for which patient transportation in the middle of surgery is considered too risky. Preliminary results of trials for early-stage breast and rectal cancer indicate benefits of IORT. Pediatric patients and patients who have lung cancer, previously underserved by IORT therapies, can be offered potential gains when patient transport issues do not limit IORT. Furthermore, because many of these mobile systems require no shielding, it is now practical for mobile units to be shared between hospitals, making this new mobile technology much more widely available.

Using intraoperative radiation therapy--a case study

The introduction of a mobile linear accelerator in the OR has made intraoperative radiation therapy (IORT) more plausible. An IORT treatment can deliver a single high dose of radiation to a tumor or tumor bed after surgical resection or surgical exposure of high risk areas. This article details a case study in which IORT was used on a patient with sigmoid carcinoma and the procedure outcomes.