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IMPROVED TARGETING
Larger tumors require higher radiation doses at both isocenters and peripheries, making it more difficult to minimize dose to healthy tissues around target volume margins.7 Strategies for precise targeting of tumor tissues to limit “collateral” irradiation of nontarget tissue were presented at the European Lung Cancer Conference.
Advances in respiratory gating Respiratory motion of lung tissue moves tumors as well, complicating radiotherapy targeting. Two tools were presented for more precisely targeting moving tumor tissue.
Dr Udrescu presented evidence that real-time x-ray imaging with ExacTrac SV during radiotherapy allows adjustment for position and shape changes in lung and liver tumors during irradiation sessions and, hence, better targeting of tumors.
Software improvements for breathing-adapted radiotherapy (BART) were described and reviewed by Dr Nicolas Peguret (University Hospital, Geneva). BART delivers “gated” radiation duringspecified phases of the respiratory cycle to minimize irradiation of healthy lung tissue. But identifying the optimal treatment phase of respiration for a particular patient has proven challenging, Dr Peguret said. He presented radiotherapy planning software that uses CT to help visualize tumor position in different phases of respiration.
Cold spots Dr Siedschlag presented preliminary data from the Netherlands suggesting that metabolic imaging of tumors with positron emission tomography (PET) using the radioactively-labeled sugar fluorodeoxyglucose-18 (FDG-PET) allows identification of fast- and slow-growing portions of a tumor and, hence, improved precision in radiation dose planning. Because glucose is consumed more rapidly by metabolically active tumor cells (and because tumor cells are growing at a faster rate than healthy cells), the fastest-growing regions of a tumor show up in FDG-PET scans as bright spots—typically, as a bright sphere that is brightest in the center. But when tumors appear “donut-shaped” with a cold spot in their center, or “boomerang-shaped” with a cold spot on the periphery, that usually indicates that the cold spots are slower growing or dead regions of the tumor, Dr Siedschlag reasoned. If true, radiation could be more precisely targeted with intensified irradiation of the fastest-growing regions, helping to minimize irradiation of healthy adjacent tissue.
The data presented by Dr Siedschlag suggested this might be possible. Tumors for 7 of 61 patients in a preliminary study had cold spots, and subsequent surgical examination confirmed that 5 of the 7 cold spots were regions of dead cancer cells, Dr Siedschlag reported.
“By decreasing the doses given to cold spots, one might be able to increase the dose given to the rest of the tumor, while keeping the normal tissue dose constant,” Dr Siedschlag said. “Or one could keep the dose given to the rest of the tumor constant, which would lead to less side effects with an identical therapeutic result.”
However, 2 of 7 (or 28.5% of) cold spots in the preliminary study were not composed of dead tumor cells, raising questions about whether FDG-PET data will prove reliable in identifying which cold spots are appropriate candidates for receiving diminished radiation doses. ONA
Bryant Furlow is a medical writer in Albuquerque, New Mexico.
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