Precision planning is always vital when delivering radiotherapy. With breast cancer, however, minimization of radiation overspill is especially important, given that many women receiving treatment are already free from disease.
Radiotherapy is typically offered after breast-conserving surgery to destroy residual cancer cells. It may also be offered post-mastectomy to prevent recurrence in tissue left on the chest wall. Although hitting the target should be relatively straightforward, plans must be tailored to protect nearby organs.
“Each patient is shaped differently. It can be a challenge to treat everything you want to and minimize toxicity,” said Dr. Wendy Woodward, an assistant professor of radiation oncology at the University of Texas M.D. Anderson Cancer Center in Houston. “You really don't want your therapy to cause your patients significant complications later on.”
Advanced delivery techniques used in other areas of radiation oncology may not necessarily be appropriate for the breast. Intensity-modulated radiation treatment (IMRT) systems that use multileaf collimators to shape the dose from multiple treatment beams are a case in point. IMRT plans can be constructed to avoid the heart and lungs when irradiating the breast (Figure 1). Dose will be smeared out over the rest of the patient's body, however, increasing the chance of contralateral breast cancer.
Simpler shielding solutions can often be more effective. Dr. Liesbeth Boersma, a radiation oncologist at the Maastro Clinic, Maastricht University, in the Netherlands, recommends the practice of voluntary moderately deep inspiration breath-holding. Patients take a deep breath, the diaphragm moves downward, and the heart moves out of the radiation fields.
“It is a very elegant and easy trick,” Boersma said. “We were quite surprised actually that our patients could do this reproducibly, but they can.”
Radiation oncologists must select the method that best fits his or her clinical practice and offers a good-quality and cost- effective solution to each individual patient. The particular characteristics of the breast, the anatomy position, and organ motion during the course of radiation call for a sustained effort to find the best method of precisely delivering the treatment dose to a determinate volume.
IMAGE GUIDANCE
Medical imaging forms a vital part of the radiotherapy planning process. Plans are usually constructed from CT images acquired within a week of the onset of therapy. Additional conebeam CT images are gathered on the day of treatment to check patient alignment, using megavoltage x-ray equipment incorporated in the delivery system.
“The onboard imaging evolution has impacted us a fair amount,” Woodward said. “Some of these systems we have used in more complicated breast cancer cases to check on intrafraction motion.”
The imaging-based planning process is not perfect, however. Identifying where the tumor once sat is not always easy, even if marker clips have been left. The excision cavity can also shrink over time. Radiation oncologists at the Maastro Clinic are now investigating whether an additional presurgical CT scan, acquired with the patient in the treatment position, can help improve radiotherapy targeting.
“We will only know for sure if this system is better after 10 years or so, when we see if the patients have fewer recurrences,” Boersma said. “So we are using surrogate end-points, such as interobserver variation when delineating the tumor bed.”
