Effects of Heat Therapy on Prostate Cancer Cells Subjected to 4 Gy Radiation

Background Despite many advancements in treatment options, radiation therapy remains a common treatment for individuals diagnosed with prostate cancer. However, radiation resistance presents a significant challenge for many patients. Due to its numerous effects on cancer cell growth and survival, heat therapy has been studied as a potential mechanism to improve radiation efficacy. However, the modality of heat therapy varies widely between studies, and the majority of heat therapy research relies on non-physiological temperatures (>43º C). We hypothesized that mild temperature hyperthermia would improve the efficacy of radiation of prostate cancer cells in vitro. Methods Clonogenic survival was assessed using human prostate cancer (PC-3) cells in vitro. Cells were grown in tissue culture flasks maintained in a 37º C incubator with 5% CO2 and randomized into 3 groups: normothermic (NT; n=8), acute hyperthermia (HTA; n=8), and chronic hyperthermia (HTC; n=8). NT cells were maintained in a normal incubator until radiation. For heat treatments, HTA cells were heated to 41.0º C in a separate incubator for 1 h immediately prior to radiation. HTC cells were heated to 41.0º C in a separate incubator for 1 h each session, for a total of 3 heat treatments, each separated by 48 h, with the final heat treatment occurring 24 h prior to radiation. Immediately following a single dose of 4 Gy radiation, each group of cells were counted and plated into 60 mm culture dishes at densities of both 500 cells/plate and 1000 cells/plate, with 5 replicates plated per density. Following 8 days of incubation, cells were fixed and stained and colonies of greater than 50 cells were counted. Results The survival fractions were determined from the average of the two plating densities. Clonogenic cell survival was significantly reduced in HTA vs. NT (11.6% ± 2% vs. 19.3% ± 1%, p<0.05) and in HTC vs. NT (8.3% ± 2% vs. 19.3% ± 1%, p<0.05). Cell survival between HTA and HTC was not different (p>0.05). Conclusions These results suggest that both acute and chronic mild hyperthermia improve the efficacy of 4 Gy radiation in PC-3 cells. While exact mechanisms are yet to be elucidated, it is thought that hyperthermia at lower temperature induces DNA damage, inhibits DNA damage repair, or alters cell cycle progression, which individually or in combination with one another, may potentiate the effects of radiation. Mild temperature hyperthermia prior to radiation may be an effective neo-adjuvant to increase the radiosensitivity of prostate cancer cells.