ESTRO 2024 - Abstract Book

S5160

Radiobiology - Immuno-radiobiology

ESTRO 2024

607

Poster Discussion

Radiation-induced lymphopenia in proton therapy for prostate cancer

Sarah F. A. Al-Hamami 1 , Samuel Kurucz 2 , Vladimír Vondráček 2 , Vladimír Pekar 1 , Jiří Kubeš 1

1 Proton Therapy Center, Radiation Oncology - Medical, Prague, Czech Republic. 2 Proton Therapy Center, Radiation Oncology - Physics, Prague, Czech Republic

Purpose/Objective:

Radiotherapy exhibits an interesting duality— it can be both immunosuppressive and immunostimulatory. The occurrence of radiation-induced lymphopenia (RIL) is an ongoing challenge in cancer treatment. This issue becomes particularly pressing with the increasing prominence of immunotherapy, which when combined with the immunostimulatory effects of radiotherapy, has the potential to produce enhanced outcomes. Our aim was to explore the change in absolute lymphocyte count (ALC) during proton radiotherapy, assessing the effects of different dosage, fractionation schedules, and lymph node irradiation. Prostate cancer patients were chosen for this study, due to their relatively homogenous treatment plans

Material/Methods:

Prostate cancer patients treated with proton radiotherapy at a single center were prospectively selected. Treatment protocols were categorised into three groups: Group A received 36.25 Gy in 5-fractions, Group B were treated with 63 Gy in 21-fractions and group C received 63 Gy in 21-fractions with an additional 48.3 Gy for lymph node irradiation. Blood counts were collected before, during (weekly) and post-treatment, and analysed along with clinical status and dosimetric data. To account for individual characteristic differences and differing baseline values, samples were also evaluated according to the change in ALC relative to the baseline. This was divided into six levels (L): L+ was defined as any increase in ALC above 5% from the baseline; L0 was used as a buffer, accounting for changes due to natural fluctuation in lymphocyte counts, with values between -5%

Results:

170 patients were analysed, with 53 in group A, 55 in group B and 62 in group C. Some degree of lymphopenia developed in 0%, 24% and 80.6% of patients in groups A, B, and C, respectively. A nadir of ALC < 500 cells/μL (grade 3 RIL) occurred in 2% and 8% of patients in groups B and C, with no incidences below <200 cells/μL (grade 4 RIL). Results for ALC are displayed in figure 1. An initial increase in ALC, placing patients in L+, occurred in 45% of patients in group A, 47% of group B and 31% of group C, after a median dose of 12 Gy, 9.5 Gy and 10.4 Gy for groups A, B and C, respectively. At the end of treatment, the majority of group A (52/53) were distributed between L+ to L1, the majority of group B (45/55) between L1-L2, and 51 out of 62 patients in group C had over a 50% decline in their ALC, as shown in figure 2.