Poster — Wed Eve—52: Radiobiological Modeling of a Proposed Dose Escalation in TMI

Purpose: To compare the effectiveness of different approaches to total marrow irradiation (TMI) using Helical TomoTherapy. Methods: TMI treatment was planned on a 55 year‐old male patient (87% of the GTV to receive a prescribed dose ([formula omitted]) of 20Gy). Field sizes (fs) of 25 and 50mm were examined. The normal tissue complication probability (NTCP) was calculated using Lyman‐Kutcher‐Burman model. Tumour control probability (TCP) was evaluated using the Poisson model. Dose escalation analysis was performed by linearly escalating the DVHs from the 20Gy‐TMI plan, to any [formula omitted]. Results: There was no substantial difference between TCP using 25mm (40±9%) or 50mm fs (42±9%). For organs of the torso, the difference in the prescribed dose to the GTV that would lead to a normal organ complication of 50% from the TMI ([formula omitted]) between 25 and 50mm was less than 3%. For organs in the head, the [formula omitted] for 50mm fs was consistently lower by up to 15% compared to 25 mm fs. The optimal dose given by maximizing TCP(1‐NTCP), was ∼39Gy for lungs, resulting in 95% (±3%) of tumor control and 3% (0, 16%) rate of pneumonistis. Conclusion: TCP and NTCP were estimated for a TMI patient, originally receiving 20Gy and linearly escalating the DVHs to higher doses. Tissue sparing was seen by using 25mm fs only in the organs of the head. This suggests it would be beneficial to use the small fields in the head only; since using small fields for the whole treatment would lead to long treatment times

Comparison of high dose rate, low dose rate, and high dose rate fractionated radiation for optimizing differences in radiosensitivities in vitro

Radiotherapy is administered with the assumption that all patients respond similarly to radiation although radiosensitivity does vary from patient to patient, resulting in different degrees of early and late effects. Because the dose given to a patient is limited by the response of normal tissue in the treatment field, it would be beneficial to determine the sensitivity of this normal tissue prior to therapy. Previous studies to predict radiosensitivity have used surviving fractions after a single dose given in vitro, however, differences in cell survival at this low level of kill are not easy to resolve. In this study, we set out to evaluate the use of alternative dose regimens which may better resolve differences in radiosensitivity. We have examined several radiation protocols for predictive value, including survival after high doses (6 Gy) at both high (112 cGy/min) and low (.882 cGy/min) dose rates and after fractionated doses of 2 Gy (6 fractions). A sensitive human fibroblast line (S11358) cultured from a patient showing severe effects after therapy is compared with a cell line (OMB1) cultured from an apparently normal subject. Differences between these cell lines have been compared with those between two human melanoma cell lines (SKMEL3 and HT144) which have shown resistant and sensitive response to radiation in vitro respectively. In both fibroblast and melanoma cell lines, the difference in the survival of normal and sensitive cells increased with increasing dose regardless of whether irradiation was delivered as low dose rate, high dose rate, or as fractionated doses. We propose that radiation doses which more closely mimic clinical treatment are more suitable than surviving fraction after 2 Gy (SF2) for in vitro evaluation of relative radiosensitivities of cell populations