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Abstract
We read with great interest the article by Kil et al
1
on symptom relief after quad shot (QS) for 2 patients with neglected breast cancer
(NBC). Palliative radiation therapy is effective for various symptoms caused by NBC,
and as the authors described, it is desirable to use a short-course radiation therapy
with a high rate of symptom relief to avoid delaying systemic cancer therapy. However,
we believe that further consideration may be necessary to decide the eligibility for
QS and single fraction (SF) radiation therapy for NBC.
The authors noted that a disadvantage of SF was the high rate of recurring breast
symptoms, requiring repeat radiation therapy in 57% of cases.
2
Conversely, the other 43% of cases did not require reirradiation. In QS, up to 3 or
4 courses of radiation therapy can be repeated, and the advantage of QS may be that
systemic chemotherapy can be started immediately after the first QS, and the decision
to perform a second QS can be made based on the subsequent outcome during the systemic
cancer therapy. However, in the 2 cases presented by the authors, 3 repeated courses
of radiation therapy were needed, which may not differ from the need for reirradiation
in SF.
Another concern in selecting QS for NBC is whether QS contributes to maintaining cancer-control
effects and reducing side effects. The concept of QS first reported by William was
intending to maintain the high level of tumor control assuming an α:β ratio of 10
and reduce the expected late complications assuming an α:β ration of 4.
3
In radiation therapy of NBC, as noted by the authors, the α:β ratios of the breast
tissue and breast cancer are low at 2 to 4 and 3, respectively.
4
,
5
Therefore, sufficient tumor control effects might not be expected in NBC as in tumors
with high α:β ratios, such as pelvic malignancies and advanced head and neck cancers,
for which the efficacy of QS has been reported.
3
,
6
We again appreciate the authors’ great effort toward revealing the usefulness of QS
for NBC. Although the rate of repeat radiation therapy is high, we believe that SF
is also a good option considering the convenience of a one-time procedure. Further
studies regarding the proper use of SF and QS in NBC will be needed in the future.
Disclosures
The authors declare that they have no known competing financial interests or personal
relationships that could have appeared to influence the work reported in this paper.
Background Prediction of radiobiological response is a major challenge in radiotherapy. Of several radiobiological models, the linear-quadratic (LQ) model has been best validated by experimental and clinical data. Clinically, the LQ model is mainly used to estimate equivalent radiotherapy schedules (e.g. calculate the equivalent dose in 2 Gy fractions, EQD2), but increasingly also to predict tumour control probability (TCP) and normal tissue complication probability (NTCP) using logistic models. The selection of accurate LQ parameters α, β and α/β is pivotal for a reliable estimate of radiation response. The aim of this review is to provide an overview of published values for the LQ parameters of human tumours as a guideline for radiation oncologists and radiation researchers to select appropriate radiobiological parameter values for LQ modelling in clinical radiotherapy. Methods and materials We performed a systematic literature search and found sixty-four clinical studies reporting α, β and α/β for tumours. Tumour site, histology, stage, number of patients, type of LQ model, radiation type, TCP model, clinical endpoint and radiobiological parameter estimates were extracted. Next, we stratified by tumour site and by tumour histology. Study heterogeneity was expressed by the I2 statistic, i.e. the percentage of variance in reported values not explained by chance. Results A large heterogeneity in LQ parameters was found within and between studies (I2 > 75%). For the same tumour site, differences in histology partially explain differences in the LQ parameters: epithelial tumours have higher α/β values than adenocarcinomas. For tumour sites with different histologies, such as in oesophageal cancer, the α/β estimates correlate well with histology. However, many other factors contribute to the study heterogeneity of LQ parameters, e.g. tumour stage, type of LQ model, TCP model and clinical endpoint (i.e. survival, tumour control and biochemical control). Conclusions The value of LQ parameters for tumours as published in clinical radiotherapy studies depends on many clinical and methodological factors. Therefore, for clinical use of the LQ model, LQ parameters for tumour should be selected carefully, based on tumour site, histology and the applied LQ model. To account for uncertainties in LQ parameter estimates, exploring a range of values is recommended. Electronic supplementary material The online version of this article (10.1186/s13014-018-1040-z) contains supplementary material, which is available to authorized users.
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