WASHINGTON -- Low-dose adjuvant radiation therapy clears thyroid cancer
remnants as effectively as high-dose therapy, according to preliminary data from
an ongoing study.
All patients treated with a 30-mCi radiation dose had stimulated thyroglobulin
levels <2 pmol/L, compared with 88% of patients who received a 100-mCi radiation
dose, Shoba Sujana Kumar, M.D., of the University of Toronto, reported here at
the Endocrine Society meeting.
The higher dose has achieved slightly better disease control as defined by the
more stringent criterion of undetectable stimulated thyroglobulin levels (<0.3
pmol/L).
"Our early, single-center data suggest that the lower-dose of radiation is as
effective as the higher dose for achieving post-ablation thyroglobulin
negativity in low-risk patients," Dr. Sujana Kumar said.
"Ongoing follow-up with a larger group of patients will be required to confirm
these findings and assess the effectiveness of low-dose radiation therapy in
reducing long-term recurrence rates in low-risk patients."
Patients with differentiated thyroid cancer usually receive postoperative
irradiation with 131I, which can prevent recurrence, even in low-risk patients.
Although the "lowest effective dose" is the standard recommendation for adjuvant
irradiation, the optimal dose has yet to be determined, and physicians often
administer a 100-mCi dose, said Dr. Sujana Kumar.
At
Women's College Hospital in Toronto, thyroid cancer patients have received a
30-mCi dose of adjuvant radiation on the basis of data demonstrating effective
disease control at that level.
Since the fall of 2007, Dr. Sujana Kumar and colleagues have prospectively
collected data on low-risk patients treated with 30 mCi or 100 mCi of adjuvant
131I.
Follow-up has included whole-body scan and thyroglobulin stimulation with
thyrotropin alfa (Thyrogen) or levothyroxine withdrawal six to 24 months after
treatment.
Patients in the study have papillary cancer or follicular variant papillary
cancer, stage T1N0M0 or T2N0M0 (also T1NxM0 for patients less than 45 years
old).
The primary outcome was stimulated thyroglobulin level (<2.0 pmol/L) from six to
24 months after irradiation. The secondary outcome was undetectable stimulated
thyroglobulin (<0.3 pmol/L) within the same post-irradiation time period.
Dr. Sujana Kumar presented data for 17 patients treated with low-dose adjuvant
radiation therapy and 42 patients treated with 100 mCi. Time to follow-up
stimulated thyroglobulin assessment averaged 14 to 15 months in both groups.
All 17 patients in the low-dose group had stimulated thyroglobulin levels <2
pmol/L, as did 37 of 42 (88.1%) in the high-dose group. Eleven of 17 (64.7%)
low-dose and 32 of 42 (76.2%) high-dose patients had stimulated thyroglobulin
levels <0.3 pmol/L. Neither difference was statistically significant.
Primary source: The Endocrine Society
Source reference: Kumar S "Comparison of the use of low-dose (30 mCI) 131I
versus standard dose (100 mCI) 131I for thyroid remnant ablation in low-risk
papillary thyroid cancer" ENDO 2009; Abstract P2-533).
