<p><strong>Purpose: </strong>The purpose of this technical study was to evaluate how the effect of changing beam energies for one to multiple fractions of a patient’s plan affected the overall dose delivered to the planning target volume (PTV) and surrounding organs at risk (OAR’s). <strong></strong></p><p><strong>Method:</strong> In this study, twenty-eight patient plans from treatment sites including the oesophagus, prostate, lung, spine, rectum, bladder, chest, scapula, and breast were evaluated in the Philips Pinnacle treatment planning system (TPS), of these 14 were originally planned with 15MV and 14 with 10MV. Each of these plans were substituted with a single to multiple fractions with 10MV and 15MV respectively while keeping the original monitor units the same.</p><p><strong>Results:</strong> It was determined that when the number of fractions of the substituted beam energy remained at one fifth or less of the overall fractions a change of dose of less than 2% to the PTV could be maintained. The OAR’s dose, when the plan had 20% of its fractions substituted with a different energy, were found to change by on average up to 3.5% and 2.3% for original plan energies of 15MV and 10MV respectively. The dose change calculated in the TPS was then verified using ion chamber measurements for bladder and oesophagus treatment plans. <strong></strong></p><p><strong>Conclusion:</strong> Results appear to indicate that the site of treatment was not an important factor when changing energy but the overall number of fractions versus the number of fractions substituted with an alternative energy was fundamental. These results may be clinically useful when a radiotherapy department have machines with different photon energies. In the event of a break down, when a patient needs to be urgently treated, it may be possible to treat them on another machine with a different energy, without an immediate recalculation in the TPS. This decision would depend upon the percentage of fractions of their overall treatment needing to be treated before the machine was repaired</p><p>-------------------------------</p><p><strong>Cite this article as:</strong><br />Hawke S, Torrance A, Tremethick L. Evaluation of planned dosimetry when beam energies are substituted for a fraction of the treatment course. <em>Int J Cancer Ther Oncol</em> 2013; <strong>1</strong>(2):01014.</p><p><strong>DOI</strong>: <a href="http://dx.doi.org/10.14319/ijcto.0102.4" target="_self">http://dx.doi.org/10.14319/ijcto.0102.4</a><br /><br /></p
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