Hematopoietic stem cell transplantation (HSCT) may offer the chance of a cure for patients with leukemia and other blood cancers, but the process of preparing the body to receive such a transplant can be brutal, involving whole body irradiation as well as chemotherapy conditioning. New results show that both steps are needed: a trial that omitted whole body irradiation in young patients with acute lymphoblastic leukemia (ALL) was stopped early because of significantly poorer outcomes.

The multicenter, global FORUM (For Omitting Radiation Under Majority Age) trial involved 75 centers in 17 countries between 2013 and 2018.

"Our study shows significantly better outcomes for total body irradiation compared to myeloablative chemo-conditioning arms, with no differences between the [two]chemo-conditioning groups," concluded Christina Peters, MD, professor of pediatrics in the Department of Stem Cell Transplantation at St Anna Children's Hospital in Vienna, Austria.

The findings in favor of total body irradiation were pronounced enough that the study was halted early by a safety committee, she added.

Peters presented the findings as part of the virtual European Hematology Association (EHA) 25th Annual Congress 2020.

Describing the results as "sobering," session co-moderator Shai Izraeli, MD, director of the Department of Hematology-Oncology at Schneider Children's Medical Center, in Petah Tikva, Israel, said an online comment from the virtual meeting audience reflected the reaction to these unwelcome results: "So we are still stuck with total body irradiation?"

Peters responded the good news is that the number of patients needing to undergo stem cell transplants is low, and with research advances, may hopefully drop even further.

"Only 10% of patients under the age of 18 nowadays undergo allogeneic HSCT, and perhaps in the future that will become even less if we are able to rescue some of the groups with other immunological measures such as CAR-T cells and antibodies," she said.

"I think it is very important to better identify those who really need total body irradiation in the future," she added.

Commenting for Medscape Medical News, Izraeli agreed.

"The prognosis of children after bone marrow transplantation is excellent the majority are cured from their leukemia," he said. "And we have to remember that those who undergo bone marrow transplant have the worst leukemias."

He pointed out that, in fact, contemporary chemotherapy alone is effective in the treatment of more than 90% of patients with ALL younger than aged 18.

For the 10% of patients who do not respond to chemotherapy alone and undergo allogeneic HSCT, about 50% to 80% of pediatric patients who have resistant leukemia are cured. However, the total body irradiation used to prepare the body to receive the transplant is linked to potentially serious consequences later in life, including sterility, lung problems, growth retardation, and secondary cancer.

To determine if the irradiation component could be safely replaced with a chemotherapy-based conditioning approach, Peters and colleagues conducted the FORUM trial.

In total 413 patients undergoing HSCT were enrolled and randomized to pretransplant conditioning with total body irradiation and etoposide (n = 202) or a chemotherapy-only approach with fludarabine/thiotepa/busulfan (flu/thio/bu; n = 99) or fludarabine/thiotepa/treosulfan (treo; n = 93).

Most patients (72%) had B-cell precursor ALL and 23% had T-cell ALL. Just over half (54%) were transplanted in first complete remission (CR1), 40% in CR2, and 4% in CR3.

The source of stem cells was bone marrow for most patients (82%); peripheral blood stem cell for 12%, and cord blood for 4%.

The aim of the study was to demonstrate noninferiority with the chemotherapy approach.

However, the significantly inferior outcome observed in the chemotherapy-only group led to randomization being halted in March 2019.

The 2-year overall survival in the intent-to-treat (ITT) analysis, with a mean observation time of 2.1 years, was 0.75 0.04 for chemo-conditioning versus 0.91 0.02 for total body irradiation/etoposide (ITT P < .001).

The ITT analysis showed relapses were significantly higher in the chemo-conditioning group (2-year cumulative incidence of relapse [CIR], 0.33) compared with the total body irradiation group (CIR, 0.12; P < .001).

The 2-year event-free survival (EFS) rate was also significantly higher in the total body irradiation group (0.86 vs 0.58; P < .001), and transplant-related mortality over 2 years was lower with total body irradiation (0.02 vs 0.09; P = .02).

A per-protocol analysis showed the 2-year overall survival to be the same in the two chemotherapy groups (both 0.77 0.05) compared with 0.91 0.02 in the total body irradiation group (P = .003).

"In this cohort [the 91% overall survival rate] may even be lower than contemporary intensive frontline therapy results that are achieved nowadays," Peters said.

In looking at subgroups, there were no significant differences according to age group or cancer phenotype, while MLL rearrangement was associated with higher relapse incidence.

Remission status was found to notably influence EFS, dropping from 0.91 in CR1 patients with total body irradiation to 0.76 in CR2 patients. However, total body irradiation remained significantly higher compared with the chemo-conditioning groups in CR1 (P = .004) and CR2 (P < .001).

Transplant-related mortality was not significantly different between the total body irradiation and chemo-conditioning groups in the CR1 or CR2 groups (P = .09 and P = .18, respectively), despite the significant difference when remission status was not included.

Overall, "we tried to identify subgroups in which total body irradiation might be eliminated, however in all analyses, total body irradiation was better than chemo-conditioning in all arms," Peters said.

Meanwhile, the findings underscore that even when patients cannot receive total body irradiation, the alternative chemo-conditioning therapy in fact shows favorable efficacy on its own, Izraeli said.

"The prognosis of the chemotherapy group is also quite remarkably good, although less than the total body irradiation arm. This means that if for some reason total body irradiation cannot be given, the chemotherapy is a very reasonable alternative."

Peters has reported relationships with Amgen, Novartis, Pfizer, Medac, Jazz, and Neovii. Izraeli has reported no relevant financial relationships.

EHA 2020 Congress. Presented June 12, 2020. Abstract S102.

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Trial Omitting Total Body Irradiation Before HSCT Stopped Early - Medscape