Impact of Salvage Versus Palliative Dose Bridging Radiation Therapy on Local Control and Overall Survival in Patients with DLBCL Receiving CD19 CAR T Cell Therapy

PURPOSE/OBJECTIVE(S)

In patients with relapsed/refractory diffuse large B-cell lymphoma (DLBCL), radiation therapy (RT) can be used to "bridge" patients during the period of chimeric antigen receptor T cell (CAR T) manufacturing. Although RT has been shown to improve local control (LC) in patients who receive RT versus do not, it is unknown whether there is an optimal dose.

MATERIALS/METHODS

This was a retrospective study of patients with DLBCL who received bridging RT and CAR T cell therapy between 8/2021 and 12/2022 at a single institution (IRB #202103122). Patient, disease, and treatment characteristics were abstracted. PET/CT and laboratory data were collected at various time points. RT data including dose, planning target volume (PTV), and body volume receiving 2Gy and 4 Gy were tested for associations with LC, overall survival (OS), cytokine release syndrome (CRS) and immune effect cell-associated neurotoxicity syndrome (ICANS). Statistical tests were performed using Python libraries.

RESULTS

Twenty-two patients with DLBCL were included for analysis. The median patient age was 68 (range 35-82) and the median prior lines of failed therapies was two. All patients had successful apheresis and subsequent CAR T manufacturing. No patients died before CAR T infusion. All patients had a pre-radiation PET/CT scan that showed Deauville 5 disease. Patients with limited stage disease received salvage dose RT (EQD2∼40Gy, n = 8) while patients with advanced disease received palliative dose RT (n = 14). The median PTV volume was 1332 cm3. With a median follow-up of 156 days (IQR 63 - 252 days) from CAR T infusion, 10 patients had died and six progressed at an RT target. A higher salvage RT dose was not associated with LC at the day 30 PET or day 90 PET time points. In a Cox regression model, no independent variable, including RT dose, was significantly associated with LC or OS, but post-RT residual target SUV trended toward an association with OS (HR 6.2, 95% CI 0.9 - 471.0, p = 0.06). Post-RT LDH was associated with CRS grade (ρ = -0.39, p = 0.05). Other non-significant associative trends were noted between CRS grade and post-RT absolute lymphocyte count (ALC) (ρ = - 0.33, p = 0.09) and residual target SUV max (ρ = - 0.37, p = 0.08) and between ICANS grade and pre- to post-RT LDH change (ρ = -0.38, p = 0.06) and post-RT lymphocyte count (ρ = 0.36, p = 0.07). In an exploratory analysis of body volume receiving integral doses of 4Gy or 2Gy and pre- and post-RT ALC change, it appeared that higher integral dose did lead to larger decreases in ALC (ρ = -0.32, p = 0.15 and ρ = -0.3, p = 0.18, respectively).

CONCLUSION

In this small series of patients with r/r DLBCL treated with bridging RT prior to CAR T therapy, RT dose was not associated with LC. However, higher post-RT target residual SUV was associated with worse OS. The impact of post-RT LDH on CRS and ICANS grades should be explored further.

The Impact of Intra-Fraction Bowel Motion on Luminal Gastrointestinal Organ at Risk Dosimetry When Using Stereotactic Adaptive Radiotherapy for Abdominal Malignancies

PURPOSE/OBJECTIVE(S)

Daily online adaptive radiotherapy (ART) provides inter-fraction motion management of the luminal gastrointestinal (GI) structures when delivering abdominal SBRT. One potential drawback of ART is the time-consuming process, and intra-fraction GI changes from completion of the ART process to the end of treatment delivery have not been thoroughly evaluated. We explored intra-fraction bowel motion for patients receiving abdominal stereotactic adaptive radiotherapy (sART) MATERIALS/METHODS: Six patients with abdominal malignancies treated with CT-guided sART on a prospective feasibility trial had additional CBCT's acquired post-treatment (pTx-CBCT). All patients were prescribed to 50 Gy/5 fractions (fx), and the constraint for all GI OARs was V33≤0.5 cc. Time from initial CBCT (I-CBCT) used for adaptive planning to pTx-CBCT was collected. The luminal GI OAR (stomach (S), duodenum (D), small bowel (SB), and large bowel (LB)) were retrospectively contoured on pTx-CBCT. The OAR doses were compared between the I-CBCT and pTx-CBCT. The adaptive plan (PA) and initial plan (PI) doses were overlayed on the pTx-CBCT contours. The PA pTx-CBCT OAR doses were then compared to the PI pTx-CBCT OAR doses. A Boolean OAR structure of all GI OARs was evaluated to remove potential differences in structure definitions between providers. The T-test was used to compare differences in instances of D0.5cc ≥ 33 and 50 Gy. Patient charts were reviewed for grades (G) ≥ 3 toxicity.

RESULTS

Thirty fractions (fx) of sART were delivered and pTx-CBCT were acquired in 26 fx. Mean time from I-CBCT to pTx-CBCT acquisition was 66 min (38-98 min). On average at 0.5 cc the PA overdosed the S by 1.74 Gy based on pTx-CBCT anatomy compared to 2.35 Gy by the PI, the D by 0.47 Gy (PA) vs .84 Gy (PI), the SB by 1.14 Gy (PA) vs 1.43 Gy (PI), and the LB by 0.13 Gy (PA) vs 0.60 Gy (PI). The dose to the Boolean OAR structure was on average 2.51 Gy/fx higher than expected when overlaying the PA on the pTx-CBCT compared to 3.38 Gy/fx higher when overlaying the PI on the pTx-CBCT. There was no significant difference in the instance of the PA exceeding D0.5 cc ≥33 Gy vs the PI (p = 0.083), but the PA significantly reduced the instances of D0.5cc≥50 Gy (p = 0.001) compared to the PI. No patient experienced G≥3 toxicity at a median follow-up of 8 months (3-12).

CONCLUSION

These data demonstrate sART led to a significant decrease in dose to GI OARs, particularly for prescription dose or greater, even after accounting for intra-fractional bowel motion. While both the PI and the PA violated the V33 luminal GI OAR constraint in approximately ½ of pTx-CBCTs, the fraction of OARs receiving at least 50 Gy was significantly higher when overlaying the PI compared to the PA. While no G3 toxicities were reported in this small cohort, further studies are needed to characterize if the increased dose to GI OARs over the expected dose is clinically significant.