Adaptive radiation therapy monitors tumor change

2009 01 15 16 03 49 510 Med Phys Web Logo 100 Imaging techniques used to generate radiotherapy treatment plans are being continually developed and enhanced, enabling the creation of increasingly accurate plans. But what these plans can't do is account for changes that occur once treatment has begun, such as tumor shrinkage and morphing. At the recent American Society for Radiation Oncology (ASTRO) annual meeting held in Chicago, researchers presented their latest work in the application of adaptive radiation therapy to address this issue.

At M. D. Anderson Cancer Center at the University of Texas in Houston, for instance, researchers are undertaking a prospective clinical trial to examine the clinical feasibility and impact of adaptive radiotherapy during intensity-modulated radiation therapy (IMRT) of head and neck cancers. Dr. David Schwartz, assistant professor of radiation oncology, presented an update on the study's progress to date.

"A single plan designed with the assistance of pretreatment imaging cannot accurately deliver dose throughout," he told ASTRO delegates. Participants in the study (22 so far) received daily in-room CT-on-rails imaging and weekly offline plan re-evaluation. Dosimetric evaluations were performed by deformably mapping the daily doses back to the original plan.

The researchers examined four planning scenarios: the original IMRT plan aligned to the marked isocenter; the original plan aligned according to daily bone alignment (image-guided radiation therapy [IGRT]); IGRT with one adaptive replan; and IGRT with two adaptive replans.

In the 17 cases analyzed to date, all planning scenarios provided acceptable minimum dose coverage to the target. In terms of normal tissue sparing, Schwartz noted that IGRT increased the parotid dose compared with the original IMRT plan. Adaptive replanning reduced the mean parotid dose relative to IGRT alone and significantly reduced the integral body dose. The second replan had a smaller impact.

"This is the first prospective clinical evidence supporting the feasibility of head and neck adaptive radiotherapy," Schwartz concluded. He noted that IGRT alone did not provide meaningful dosimetric benefit if conventional planning target volume margins are used, and that one properly timed adaptive replan provided all relevant dosimetric improvements in this study. The trial is ongoing, with a target cohort of 30 patients.

Lung sparing

Tumor shrinkage is a common occurrence during fractionated chemoradiotherapy for advanced non-small cell lung cancer (NSCLC). As such, adaptive radiotherapy could potentially increase sparing of normal lung tissue as therapy progresses. Dr. Matthias Guckenberger, a radiation oncologist at the University of Würzburg in Germany, described a retrospective planning study evaluating this proposition.

The Würzburg team examined 13 NSCLC patients receiving conventional fractionated radiotherapy to a dose of 66 Gy, with initial treatment planning based on 4D CT and FDG-PET imaging. Guckenberger and colleagues acquired weekly 3D CT images throughout treatment, and calculated the time-accumulated doses with consideration of anatomic changes.

The researchers observed a continuous regression of tumor during treatment, with an average decrease in gross tumor volume of 1.2% per day, and a median regression of gross tumor volume to 50% after six weeks. They also noted that the morphological pattern of regression was heterogeneous, with some patients exhibiting decompression of surrounding pulmonary tissue, some showing release of surrounding tissue, and some having a mixture of the two patterns.

Adapting the treatment plans either once or twice to account for the observed tumor shrinkage did indeed reduce the mean lung dose. One adaptation reduced the mean lung dose by an average of 5%, while two reduced it by 8%. Guckenberger noted that performing two plan adaptations allowed escalation of the mean gross tumor volume dose from 67 to 73 Gy.

CT-guided PBI

Lumpectomy cavities being treated using accelerated partial-breast irradiation (APBI) often exhibit significant day-to-day changes in position, shape, and volume. To account for such interfractional variations during APBI treatment, a team from the Medical College of Wisconsin in Milwaukee has proposed an online adaptive radiotherapy scheme based on aperture morphing.

The replanning technique involves the following steps: daily CT scanning with in-room CT prior to treatment; modifying contours of target and normal structures to agree with the anatomy of the day, either manually or using a deformable registration of planning and daily CTs; adjusting the beam/multileaf collimator apertures; computing dose distributions and optimizing weights of the new apertures; and transferring the new apertures and weights for delivery.

The Wisconsin researchers evaluated the scheme using a series of daily CT sets from patients treated with either 3D conformal radiation therapy or IMRT. According to X Allen Li, chief of physics in the department of radiation oncology, both target coverage and normal tissue sparing were improved with the online replanning scheme, compared with patient repositioning. The larger the breast deformation, the larger the benefits from online replanning.

The online replanning process, which took between seven and 10 minutes to perform, was seen to be comparable to full-scale reoptimization based on daily CT. Li proposed that this scheme will permit shrinking of margins, enabling more patients to receive APBI treatment, as well as facilitating hypofractionated partial-breast irradiation.

By Tami Freeman
Medicalphysicsweb editor
December 25, 2009

Related Reading

MRI techniques show early tumor response to radiation, September 30, 2008

Linac/MRI prototype allows real-time tumor tracking during rad therapy, September 14, 2007

© IOP Publishing Limited. Republished with permission from medicalphysicsweb, a community Web site covering fundamental research and emerging technologies in medical imaging and radiation therapy.

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