Potential Gender Differences in a Normal Tissue Complication Probability Model for Heart Toxicity During Radiation Therapy for Esophageal Cancer

Differences in radiation-induced heart dysfunction in male versus female rats

PURPOSE

Radiation therapy remains part of the standard of care for breast, lung, and esophageal cancers. While radiotherapy improves local control and survival, radiation-induced heart dysfunction is a common side effect of thoracic radiotherapy. Cardiovascular dysfunction can also result from non-therapeutic total body radiation exposures. Numerous studies have evaluated the relationship between radiation dose to the heart and cardiotoxicity, but relatively little is known about whether there are differences based on biological sex in radiation-induced heart dysfunction (RIHD).

MATERIALS AND METHODS

We evaluated whether male and female inbred Dahl SS rats display differences in RIHD following delivery of 24 Gy in a single fraction to the whole heart using a 1.5 cm beam size (collimater). We also compared the 2.0 cm vs. 1.5 cm collimator in males. Pleural and pericardial effusions and normalized heart weights were measured, and echocardiograms were performed.

RESULTS

Female SS rats displayed more severe RIHD relative to age-matched SS male rats. Normalized heart weight was significantly increased in females, but not in males. A total of 94% (15/16) of males and 55% (6/11) of females survived 5 months after completion of radiotherapy (p < .01). Among surviving rats, 100% of females and 14% of males developed moderate-to-severe pericardial effusions at 5 months. Females demonstrated increased pleural effusions, with the mean normalized pleural fluid volume for females and males being 56.6 mL/kg ± 12.1 and 10.96 mL/kg ± 6.4 in males (p = .001), respectively. Echocardiogram findings showed evidence of heart failure, which was more pronounced in females. Because age-matched female rats have smaller lungs, a higher percentage of the total lung was treated with radiation in females than males using the same beam size. After using a larger 2 cm beam in males which results in higher lung exposure, there was not a significant difference between males and females in terms of the development of moderate-to-severe pericardial effusions or pleural effusions. Treatment of males with a 2 cm beam resulted in comparable increases in LV mass and reductions in stroke volume to female rats treated with a 1.5 cm beam.

CONCLUSION

Together, these results illustrate that there are differences in radiation-induced cardiotoxicity between male and female SS rats and add to the data that lung radiation doses, in addition to other factors, may play an important role in cardiac dysfunction following heart radiation exposure. These factors may be important to factor into future mitigation studies of radiation-induced cardiotoxicity.

Normal tissue complication probability modeling to guide individual treatment planning in pediatric cranial proton and photon radiotherapy

PURPOSE

Proton therapy (PT) is broadly accepted as the gold standard of care for pediatric patients with cranial cancer. The superior dose distribution of PT compared to photon radiotherapy reduces normal tissue complication probability (NTCP) for organs at risk. As NTCPs for pediatric organs are not well understood, clinics generally base radiation response on adult data. However, there is evidence that radiation response strongly depends on the age and even sex of a patient. Furthermore, questions surround the influence of individual intrinsic radiosensitivity (α/β ratio) on pediatric NTCP. While the clinical pediatric NTCP data is scarce, radiobiological modeling and sensitivity analyses can be used to investigate the NTCP trends and its dependence on individual modeling parameters. The purpose of this study was to perform sensitivity analyses of NTCP models to ascertain the dependence of radiosensitivity, sex, and age of a child and predict cranial side-effects following intensity-modulated proton therapy (IMPT) and intensity-modulated radiotherapy (IMRT).

METHODS

Previously, six sex-matched pediatric cranial datasets (5, 9, and 13 years old) were planned in Varian Eclipse treatment planning system (13.7). Up to 108 scanning beam IMPT plans and 108 IMRT plans were retrospectively optimized for a range of simulated target volumes and locations. In this work, dose-volume histograms were extracted and imported into BioSuite Software for radiobiological modeling. Relative-Seriality and Lyman-Kutcher-Burman models were used to calculate NTCP values for toxicity endpoints, where TD50, (based on reported adult clinical data) was varied to simulate sex dependence of NTCP. Plausible parameter ranges, based on published literature for adults, were used in modeling. In addition to sensitivity analyses, a 20% difference in TD50 was used to represent the radiosensitivity between the sexes (with females considered more radiosensitive) for ease of data comparison as a function of parameters such as α/β ratio.

RESULTS

IMPT plans resulted in lower NTCP compared to IMRT across all models (p < 0.0001). For medulloblastoma treatment, the risk of brainstem necrosis (> 10%) and cochlea tinnitus (> 20%) among females could potentially be underestimated considering a lower TD50 value for females. Sensitivity analyses show that the difference in NTCP between sexes was significant (p < 0.0001). Similarly, both brainstem necrosis and cochlea tinnitus NTCP varied significantly (p < 0.0001) across tested α/β as a function of TD50 values (assumption being that TD50 values are 20% lower in females). If the true α/β of these pediatric tissues is higher than expected (α/β ∼ 3), the risk of tinnitus for IMRT can significantly increase (p < 0.0001).

CONCLUSION

Due to the scarcity of pediatric NTCP data available, sensitivity analyses were performed using plausible ranges based on published adult data. In the clinical scenario where, if female pediatric patients were 20% more radiosensitive (lower TD50 value), they could be up to twice as likely to experience side-effects of brainstem necrosis and cochlea tinnitus compared to males, highlighting the need for considering the sex in NTCP models. Based on our sensitivity analyses, age and sex of a pediatric patient could significantly affect the resultant NTCP from cranial radiotherapy, especially at higher α/β values.

Gender-dependent radiotherapy: The next step in personalised medicine?

Individuals do not react to radiation in a homogeneous manner. Recent radiogenomic research has proven that individual polymorphisms can correlate with treatment response most likely due to variation in the ability to recognise and repair DNA breaks. The difference in radiosensitivity between genders has been well documented, yet most radiotherapeutic guidelines are based solely on population averages rather than demographic subgroups such as age, race and gender. This paper is a review of the burgeoning literature available on the differences in efficacy and outcome of radiotherapy between genders. The work examines the effect of radiation on gender both from a tumour control as well as normal tissue toxicity perspective. While the literature reporting such findings is limited, the results show a small but significant difference in response to radiotherapy between sexes. Prospective and retrospective studies for evaluating these gender-specific differences are encouraged as a next step in personalised medicine.