The Fraction Size Sensitivity of Late Genitourinary Toxicity: Analysis of Alpha/Beta (α/ß) Ratios in the CHHiP Trial.

PURPOSE: Moderately hypofractionated external beam intensity modulated radiation therapy (RT) for prostate cancer is now standard-of-care. Normal tissue toxicity responses to fraction size alteration are nonlinear: the linear-quadratic model is a widely used framework accounting for this, through the α/ß ratio. Few α/ß ratio estimates exist for human late genitourinary endpoints; here we provide estimates derived from a hypofractionation trial. METHODS AND MATERIALS: The CHHiP trial randomized 3216 men with localized prostate cancer 1:1:1 between conventionally fractionated intensity modulated RT (74 Gy/37 fractions (Fr)) and 2 moderately hypofractionated regimens (60 Gy/20 Fr and 57 Gy/19 Fr). RT plan and suitable follow-up assessment was available for 2206 men. Three prospectively assessed clinician-reported toxicity scales were amalgamated for common genitourinary endpoints: dysuria, hematuria, incontinence, reduced flow/stricture, and urine frequency. Per endpoint, only patients with baseline zero toxicity were included. Three models for endpoint grade ≥1 (G1+) and G2+ toxicity were fitted: Lyman Kutcher-Burman (LKB) without equivalent dose in 2 Gy/Fr (EQD2) correction [LKB-NoEQD2]; LKB with EQD2-correction [LKB-EQD2]; LKB-EQD2 with dose-modifying-factor (DMF) inclusion [LKB-EQD2-DMF]. DMFs were age, diabetes, hypertension, pelvic surgery, prior transurethral resection of prostate (TURP), overall treatment time and acute genitourinary toxicity (G2+). Bootstrapping generated 95% confidence intervals and unbiased performance estimates. Models were compared by likelihood ratio test. RESULTS: The LKB-EQD2 model significantly improved performance over LKB-NoEQD2 for just 3 endpoints: dysuria G1+ (α/ß = 2.0 Gy; 95% confidence interval [CI], 1.2-3.2 Gy), hematuria G1+ (α/ß = 0.9 Gy; 95% CI, 0.1-2.2 Gy) and hematuria G2+ (α/ß = 0.6 Gy; 95% CI, 0.1-1.7 Gy). For these 3 endpoints, further incorporation of 2 DMFs improved on LKB-EQD2: acute genitourinary toxicity and prior TURP (hematuria G1+ only), but α/ß ratio estimates remained stable. CONCLUSIONS: Inclusion of EQD2-correction significantly improved model fitting for dysuria and hematuria endpoints, where fitted α/ß ratio estimates were low: 0.6 to 2 Gy. This suggests therapeutic gain for clinician-reported GU toxicity, through hypofractionation, might be lower than expected by typical late α/ß ratio assumptions of 3 to 5 Gy.

3D tumour spheroids for the prediction of the effects of radiation and hyperthermia treatments.

For multimodality therapies such as the combination of hyperthermia and radiation, quantification of biological effects is key for dose prescription and response prediction. Tumour spheroids have a microenvironment that more closely resembles that of tumours in vivo and may thus be a superior in vitro cancer model than monolayer cultures. Here, the response of tumour spheroids formed from two established human cancer cell lines (HCT116 and CAL27) to single and combination treatments of radiation (0-20 Gy), and hyperthermia at 47 °C (0-780 CEM43) has been evaluated. Response was analysed in terms of spheroid growth, cell viability and the distribution of live/dead cells. Time-lapse imaging was used to evaluate mechanisms of cell death and cell detachment. It was found that sensitivity to heat in spheroids was significantly less than that seen in monolayer cultures. Spheroids showed different patterns of shrinkage and regrowth when exposed to heat or radiation: heated spheroids shed dead cells within four days of heating and displayed faster growth post-exposure than samples that received radiation or no treatment. Irradiated spheroids maintained a dense structure and exhibited a longer growth delay than spheroids receiving hyperthermia or combination treatment at (thermal) doses that yielded equivalent levels of clonogenic cell survival. We suggest that, unlike radiation, which kills dividing cells, hyperthermia-induced cell death affects cells independent of their proliferation status. This induces microenvironmental changes that promote spheroid growth. In conclusion, 3D tumour spheroid growth studies reveal differences in response to heat and/or radiation that were not apparent in 2D clonogenic assays but that may significantly influence treatment efficacy.

A cellular automaton model for spheroid response to radiation and hyperthermia treatments.

Thermo-radiosensitisation is a promising approach for treatment of radio-resistant tumours such as those containing hypoxic subregions. Response prediction and treatment planning should account for tumour response heterogeneity, e.g. due to microenvironmental factors, and quantification of the biological effects induced. 3D tumour spheroids provide a physiological in vitro model of tumour response and a systems oncology framework for simulating spheroid response to radiation and hyperthermia is presented. Using a cellular automaton model, 3D oxygen diffusion, delivery of radiation and/or hyperthermia were simulated for many ([Formula: see text]) individual cells forming a spheroid. The iterative oxygen diffusion model was compared to an analytical oxygenation model and simulations were calibrated and validated against experimental data for irradiated (0-10 Gy) and/or heated (0-240 CEM43) HCT116 spheroids. Despite comparable clonogenic survival, spheroid growth differed significantly following radiation or hyperthermia. This dynamic response was described well by the simulation ([Formula: see text] > 0.85). Heat-induced cell death was implemented as a fast, proliferation-independent process, allowing reoxygenation and repopulation, whereas radiation was modelled as proliferation-dependent mitotic catastrophe. This framework stands out both through its experimental validation and its novel ability to predict spheroid response to multimodality treatment. It provides a good description of response where biological dose-weighting based on clonogenic survival alone was insufficient.

Focused Ultrasound-Mediated Hyperthermia in Vitro: An Experimental Arrangement for Treating Cells under Tissue-Mimicking Conditions.

An experimental arrangement that allows in vitro exposure of cells to focused ultrasound-mediated hyperthermia (43°C-55°C) in a tissue-mimicking phantom with biological, acoustic and thermal properties comparable to those of human soft tissue is described. Cells were embedded in a compressed collagen gel, which was sandwiched between 6-mm-thick slices of biocompatible, acoustically absorbing and thermally tissue mimicking poly(vinyl alcohol) cryo-gel. To illustrate the system's potential, cells were exposed using a 1.66-MHz focused ultrasound beam (spatial-peak temporal-average intensities (ISPTA) = 900-1400 W/cm2) that traced out a circular trajectory (5-8 mm in diameter). Real-time temperature monitoring allowed cells to be exposed reproducibly to a pre-determined thermal dose. An experimental planning tool that estimates the thermal dose distribution throughout the sample and allows spatial correlation with cell position has been developed. Treatment response was evaluated qualitatively using microscopy and cell viability testing. This experimental arrangement has significant potential for future, biologically relevant, in vitro focused ultrasound-mediated hyperthermia studies.