Marginless Radiotherapy (MRT): Leveraging The Abscopal Effect Toward The Treatment Of Metastatic Castration Resistant Prostate Cancer

Abstract

Marginless Radiotherapy (MRT) is a new radiotherapy approach whereby only the gross tumor volume (GTV) or sub-volume (sv) is irradiated. We sought to investigate the potential of MRT in combination with in-situ delivered anti-CD40 to generate a robust abscopal effect in metastatic castration resistant prostate cancer. Using an aggressive castration resistant prostrate adenocarcinoma cell line derived from TRAMP-C1, two contralateral tumors were implanted in each mouse either subcutaneously (SQ) in two flanks or one SQ and one orthotopic to anterior prostate lobe. In both cases one SQ tumor was chosen to be treated. The mice were randomly assigned to different cohorts (n = 5-8) for image-guided radiotherapy (IGRT) of a single dose of 2, 5, 8, or 10 Gy using Small Animal Radiation Research Platform (SARRP). Following the IGRT, each cohort was treated with 20 μg of intratumoral mouse antiCD40 antibody loaded in a smart radiotherapy biomaterial (srbAntiCD40) for sustain release. These cohorts were then compared with a control group with no treatment. Tumor volume of both treated (treated site) and untreated tumors (abscopal site) were then measured and survival percent was calculated to determine the most potent RT dose which was then used for the PTV vs. GTV/sv of IGRT study. PTV of IGRT was delivered with 10x10 mm2 field size collimators whereas 1x1, 3x3, or 5x5 mm2 field size collimators were used for GTV/sv. Statistical analyses for tumor volume and Kaplan–Meier survival assay were then analyzed. A single fraction IGRT dose of 5 Gy to the PTV with srbAntiCD40 shows the highest survival fraction (****) compare with other radiation doses. Further analysis of 5 Gy of IGRT in PTV or GTV/sv along with srbAntiCD40 (intratumoral) demonstrates non-significant difference of the survival rate with PTV (80%**) compare to GTV/sv in SQ model (100%***) or orthotopic model (80%***). Similarly, fold increase of tumor volume (from day of treatment) in treated sides were 91 (+/- 11.5), 11* (+/- 6.5), 6* (+/-2.5), and 6* (+/- 2.5) respectively for Control, RT-PTV, RT-GTVsv (SQ), and RT-GTVsv (orthotopic) cohorts. Untreated, abscopal side tumors also shown similar effects (60 (+/- 17.4), 13** (+/- 2.4), 11**(+/-2.1), and 6** (+/- 1.3) respectively. Complete regression of both of the tumors were shown in 20% of mice in both SQ and orthotopic RT-GTVsv groups. Significant high infiltration of CD8+ cytotoxic T cells were also observed in both treated and untreated tumors of both PTV and GTVsv cohorts, 10 days after the treatment given. This study proffers a new MRT strategy for treating aggressive metastatic prostate cancer, leveraging the abscopal effect more efficaciously by treating only a GTV/sv which further reduces the radiation induced toxicity. This strategy may spare peripheral APCs cells, which are crucial in driving the immune-mediated abscopal effect.