Patients with localized prostate cancer who receive carbon ion radiotherapy have a lower risk for subsequent primary cancers than recipients of photon radiotherapy, according to results from a recent study by Osama Mohamad, MD, National Institute of Radiological Sciences, National Institutes for Quantum and Radiological Science and Technology, Chiba, Japan, and colleagues (Lancet Oncol. 2019 Mar 15. Epub ahead of print).

“The risk of subsequent primary cancers in patients with prostate cancer after treatment with photon radiotherapy is small in absolute numbers, but it is higher than that after surgical treatment,” said Dr Mohamad and colleagues.

“Carbon ion radiotherapy has a theoretically lower risk of inducing secondary malignancies than photon radiotherapy, but this risk has not been investigated in practice because of the low number of facilities offering such therapy worldwide and the limited data on long-term follow-up because the therapy has only been available since 1994,” they continued.

Seeking to examine the risk for subsequent primary cancers in patients with localized prostate cancer who have undergone carbon ion versus photon radiotherapy or surgery, the investigators conducted a retrospective cohort study of patients who received carbon ion radiotherapy for prostate cancer between June 27, 1995, and July 10, 2012, at the National Institute of Radiological Sciences (NIRS) in Japan. They also reviewed the records of patients treated for prostate cancer between January 1, 1994, and December 31, 2012, in the Osaka Cancer Registry.

Patients were excluded from the analysis if they had metastasis, node-positive, or locally invasive disease; previous or synchronous malignancies; or received previous radiotherapy or chemotherapy.

“We did a multivariable analysis to estimate predictors of subsequent cancers after carbon ion radiotherapy treatment. We also used propensity score inverse probability weighting to retrospectively compare the incidence of subsequent cancers in patients with localised prostate cancer treated with carbon beams, photon radiotherapy, or surgery,” explained Dr Mohamad and colleagues.

In total, 1455 (92%) of 1580 patients who received carbon radiotherapy for prostate cancer at the NIRS met the study eligibility criteria. The same was determined for 1983 (5%) patients treated with photon radiotherapy and 5948 (15%) who underwent surgery of 38,594 patients with prostate cancer identified in the Osaka registry.

After limiting patient data to those with a 10-year maximum follow-up duration, Dr Mohamad and colleagues found that patients who underwent carbon ion radiotherapy, photon radiotherapy, and surgery were followed-up with for a median of 7.9 years, 5.7 years, and 6.0 years, respectively.

In the carbon ion radiotherapy cohort, patients were diagnosed with 234 subsequent primary cancers; some patients had multiple tumors.

According to results of a multivariable analysis, patient age (P = .0021 for 71-75 years vs ≤60 years; P = .012 for >75 years vs ≤60 years) and status as a smoker (P = .0005) were associated with a higher risk for subsequent primary cancers in those treated with carbon ion radiotherapy versus photon radiotherapy or surgery.

Carbon ion radiotherapy was tied to a lower risk for subsequent primary cancers than photon radiotherapy (hazard ratio [HR], 0.81; 95% confidence interval [CI], 0.66-0.99]; P = .038) or surgery (HR, 0.80; 95% CI, 0.68-0.95; P = .0088) in the propensity score–weighted analyses. Photon radiotherapy, however, was associated with a higher risk for primary cancers later in life than surgery (HR, 1.18; 95% CI, 1.02-1.36; P = .029).

“Our analysis suggests that patients with localised prostate cancer treated with carbon ion radiotherapy appear to have a lower risk of subsequent primary cancers than those treated with photon radiotherapy,” Dr Mohamad and colleagues said.

“Although prospective evaluation with longer follow-up is warranted to support these results, our data supports a wider adoption of carbon ion radiotherapy for patients with expected long-term overall survival or those with poor outcomes after receiving conventional treatments,” they concluded.—Hina Khaliq