A megavoltage CT (MVCT) scan was performed before each daily treatment to verify that the patient was positioned precisely the same way as it was intended by the treatment plan. Once the position was checked and the location and size of the tumor were verified, the treatment was delivered.
A Special Report for Physicians Regarding Tomotherapy
By Niko Papanikolaou, Ph.D
Image Guided Radiation Therapy (IGRT) is the latest invention in a series of developments radiation oncology has seen over the last five years.
Historically, a typical patient receiving radiotherapy (RT) is scanned in a computed tomography (CT) scanner and the scan is used as a reference to build a custom radiation treatment plan. This plan will be executed for the subsequent five to eight weeks of treatment, assuming that the patient’s anatomy will not change during the treatment; an assumption that is frequently false, as the tumors often change size in response to radiation or the combination of chemotherapy and radiation treatments.
With IGRT physicians can image a patient daily by obtaining CT images prior to treatment delivery. In doing so, they can verify the alignment of the radiation beams to the tumor and adapt the treatment plan if the target location or size has changed.
Helical tomotherapy is a revolutionary form of IGRT that combines the imaging and treatment aspects of a treatment in one unit called HiArt TomoTherapyAA. Tomotherapy, or “slice therapy,” gets its name from tomography, or cross-sectional imaging. A helical tomotherapy system looks much like a CT system: the patient lies on a treatment couch that moves continuously through a rotating ring gantry. The gantry houses a linear accelerator, which delivers photon radiation in the shape of a fan beam as the ring is turning. With the couch moving at the same time the gantry is rotating, the radiation beam makes a spiral (or helical) pattern around the patient.
The design of the TomoTherapyTM unit is such to deliver intensity modulated radiotherapy (IMRT) using a multileaf collimator. One of the most important recent advances in radiation treatment delivery, IMRT involves changing the size, shape and intensity of the radiation beam to conform to the size, shape, and location of the patient's tumor.
Experiencing odynophagia, dysphagia, and an ear ache, “Sue,” a 45-year-old woman with a chronic history of smoking and alcohol consumption, sought medical treatment from an ENT specialist. A biopsy identified a mass in the base of tongue (BOT) as squamous cell carcinoma, and further examination revealed metastatic squamous cell carcinoma in the neck.
After diagnosing the patient with local-regional spread of squamous cell carcinoma of the BOT, oncologists at the Cancer Therapy and Research Center (CTRC) and the University of Texas Health Sciences Center in San Antonio (UTHSCSA) prescribed concomitant chemotherapy and radiation treatment to the patient.
Instead of standard radiation treatment, this patient was selected to receive intensity modulated radiation therapy, or IMRT, using the HiArt TomoTherapyTM unit.
The HiArt Tomo-TherapyTM machine is an engineering marriage between a CT and a linear accelerator, bringing together in one unit the capability and the benefit of imaging and treating a patient. The system was specifically designed to deliver precise, high treatment doses to the tumor while sparing healthy tissue, potentially improving control and survival rates. Tomotherapy is well-suited to provide both conformal radiotherapy (targeting tumor structures) and conformal avoidance radiotherapy (sparing sensitive structures).
Conformal RT delivers a high dose to a volume that closely conforms to the shape of the patient’s tumor volume. The CT imaging capabilities of the TomoTherapyTM unit helps clinicians to accurately identify both the exact shape and location of the tumor prior to each daily treatment so as to distribute the dose exactly at the target.
Conformal avoidance RT allows clinicians to take an “everything but” approach when planning treatment for patients with complex or ill-defined tumor volumes. Rather than trying to map the precise area to be treated, clinicians can map out the sensitive structures that must be avoided, leaving generous margins when the tumor boundary is not well defined.
The tomotherapy concept allows clinicians to use modern 3D computed tomography to verify the precise position of the tumor and any sensitive organs before each treatment. While conventional radiotherapy delivers radiation from only a few directions, a helical tomotherapy system delivers pencil beams of radiation from every point on a spiral (or helix), focusing a precise dose of radiation on the tumor while reducing the dose sustained by surrounding healthy tissue.
The ability to precisely direct the radiation under image guidance to the tumor, enables the radiation oncologist to escalate the tumor dose while maintaining a much lower dose to critical structures.
Our current understanding of how a tumor cell responds to radiation, based on published data for both head and neck and prostate cancer, suggest that an increase in radiation dose to the tumor will improve local control of the disease and can increase a patient’s long-term chance of survival.
The process Once it was decided that “Sue” would be treated with radiation at CTRC, a treatment planning CT was ordered. The patient was placed in the treatment position and an immobilization device was made (a face mask in this case) to ensure that the patient does not move during the CT scan.
The CT images were then transferred to the dedicated treatment planning computer where an IMRT plan was developed with the objective of treating the tumor and lymphnodes while sparing the cord and the parotid glands. The approved IMRT plan was transferred to the TomoTherapyTM unit for delivery over a period of eight weeks.
Although there are other systems that can provide some form of daily image guidance or verification, such as ultrasound and portal imaging, they can be more cumbersome to use, more time consuming; they are not integrated with the delivery unit, and they are not as precise. With tomotherapy, the physician can obtain true three-dimensional data through the MVCT scan and reference it against the planning CT to evaluate the progress of the treatment and the need for intervention.
Head and neck tumors are not the only forms of cancer that would benefit from an IGRT form of treatment delivery. Prostate, lung, liver and other abdominal cancers are excellent candidates for tomotherapy-type treatments as the location of those organs and the associated tumors can vary daily. With a TomoTherapy unit’s image guidance and daily MVCT capabilities, the radiation can be adaptively repositioned to treat the tumor at its current location.
The treatment and results
As a patient with BOT cancer, “Sue” was an excellent candidate for tomotherapy treatment and began an eight week treatment program at CTRC. Under daily image guided treatment delivery, the primary tumor received a high dose of radiation, while irradiation to the spinal cord and salivary glands was kept well below tolerance levels.
“Sue” has successfully completed her treatment with very good response to the therapy and with minimal side effects to the adjacent healthy tissues.
It is estimated that about 30 percent of patients either cannot be safely or optimally treated without a form of IMRT. Currently available at only a limited number of medical facilities in the U.S., the TomoTherapyTM unit at CTRC is one of the few image guided radiation therapy programs in Texas.
Niko Papanikolaou, Ph.D., DABR is a Professor and Director at the Cancer Therapy and Research and University of Texas Health Science Center in San Antonio.