Risk of secondary cancers from modern radiotherapy techniques

I en debattartikel framförde nyligen R A Popple att intensitetsmodulerad strålbehandling snart kommer att ersättas av volumetriska rotationstekniker, medan P A Balter argumenterade emot och hävdade att volumetriska tekniker medför olika nackdelar. Eftersom valet av behandlingsmetod kan tänkas ha betydelse för risken att inducera ny cancer i vävnader utanför den tumör som behandlas har vi bett våra kolleger Iuliana Toma-Dasu och Irena Gudowska (båda vid Medicinsk strålningsfysik, Sth univ, samt associerade till Onkologi/patologi, Karolinska Inst) om synpunkter och de har bidragit med följande gästinlägg:

Progress in cancer detection and treatment has led to an improvement in the life expectancy for many cancer patients and has consequently raised the interest in the long-term effects of radiation therapy and especially the risk for secondary cancers. Results have become available from large cohorts of patients treated with conventional radiotherapy and have shown that radiation therapy is associated with a small increase in the risk for secondary cancers in the long-term survivors. This small risk is usually regarded as the inevitable evil associated with radiation therapy, since secondary cancers appear only if radiation therapy has been successful in removing the primary tumour for which often no alternative treatments exists or if they do they have other associated risks.

The introduction of modern treatment techniques like intensity-modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT) or tomotherapy that employ modulation of the beam fluence in order to improve the dose uniformity in the target and the sparing of the surrounding normal tissues has raised further concerns with respect to their impact on the risk for cancer induction. These irradiation techniques have in common that they spread the entrance dose over a larger volume of normal tissues to conform the volume of highly irradiated tissue to the intended target. They also employ longer irradiation times, which could increase the exposure of tissues to leakage and scattered radiation. In addition, the modern irradiation techniques, employing higher precision of dose delivery, are often going hand in hand with image-guided radiation therapy (IGRT) making use of repeated imaging sessions in order to ensure the accurate positioning of the patient, thus increasing the radiation burden.

However, due to the long latency of carcinogenesis, epidemiological studies investigating results from patients treated with these techniques are not yet available as the results would require several decades to mature. Nevertheless, the impact of these techniques has been explored in theoretical studies which have shown that although dependent on the theoretical risk model used, the redistribution of the dose in normal tissues associated with the risk for secondary cancer, leads to differences in the total risk which are very small in comparison to the low levels from conventional radiotherapy.

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