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E14. Optimization of thoracic irradiation: An overview
El4. Optimization of thoracic irradiation: An overview Roger W: Bvhardt Medical College of Wisconsin, Milwaukee, WI, USA Local control rates after radiation therapy for non-small cell lung cancer (NSCLCI h a ~ languished in the range of 2qO.. even w h e n combined with chemotherapy Recent clinical data emerging from dose escalation trials using 3D conformal and stereotactlc radiotherapy techniques suggest that total dose of radiation up to 5qO.. higher than given m the past are needed for 8qO.. or better local control, especially for tumors larger than 3 cm Up to the present time. limitations of diagnostic imaging svstems and radiation therapy treatment dehverv systems, as well as poor understanding of the radiation blolog~ of N S C L C has hampered further improvement in local/regional tumor control The effectiveness of radiation therapy promises to undergo SlgnlfiC,:U'lt improvement over the next decade through better delineation of the tumor target volume w i t h the use of M R I and function imaging (PET) combined with application of radiation using improved dehverv svstems such as 3D conformal radiation therapy 13D CRT). mtensltv modulated radiation therapy (IMRT) and CT-gulded a d a p t l ~ tomotherapy Newer understanding of the radloblolog2v of N S C L C w i l l also permit the design of fractlonatlon schemes such as hypofractlonatlon that will increase the biologic effectiveness of radiation therapy while preserving normal tissue tolerance Further. improvements m patient immobilization, target positioning and target motion control will permit more accurate target acquisition More uniform application of radiation beam lnhomogenelty correction and selection of beam energy( s ) will further optimize treatment A number of clinical trials of 3D CRT h a ~ now reported that total doses as high as l q.~Gv can be safely given and all suggest improved tumor control Cause specific survival decreases with the size of the gross tumor volume (GTV) Dose response c u r ~ s generated from the clinical data suggest that doses biologically equivalent to 84 5 Gv at 2 q~Gv/fractlon are needed for a 5qO.. 3-vear tumor control probablhty ITCP) a n d t o lq.~ Gv are needed for an 8qO.. TCP Hypofractlonatlon bears promises as a means to Increase the biologic effectiveness of treatment bv reducing the impact of tumor repopulatlon for the prolonged treatment times that would be required to get to 8q~ to l q.~Gv at 2 q~Gv per fraction Hypofractlonatlon increases the daily fraction size and avoids the 1 6% per dav loss of local control for treatment courses that go bevond 5 - 6 weeks For each w e e k shortened down to 5 weeks TCP can theoretically be increased bv 11% Some vahdatlon
of [his theoretical advantage has been shown from the experience w i t h small numbers of large fractions used for stereotactlc irradiation of small primary tumors (..Tcm) in medically inoperable patients U s i n g advanced multifield arrangements, sophisticated immobilization and gating systems, local control rates from 67 to lq.O., at 1-3 vears have been reported with acceptable tolerance for regimens that range from 45 Cr¢ m 3 fractions to 6q~Cr¢ m 1 q~ fractions Improved target volume definition with CTfI~IRI and functional imaging has permitted better avoidance of geographic misses of ttmaor targets, treatment of the '*biologically relevant" target, and reduction of radiation dose exposure to critical normal tissue N e w radiation treatment planning svstems ITPSI. requiring significant computing power, are able to h a ~ diagnostic imaging data from CT. M R I and PET **fused" into the TPS so that the data thev provide corresponds to the patient treatment position This capability, as well as growing use of corrections for increased electron transmission through low density lung. better understanding of dose perturbations at the lung/tumor interface, and adjustments for beam energ2v permit further optimization of dose d e l l ~ r v Standardization of tumor target designations and the language describing them. to account for microscopic tumor extension, internal target and organ motion, patient set-up error and beam geometry w i l l assure greater uniformity m reporting treatment information Dose volume histograms can be calculated using the newer TPS's and graphically display coverage of both target and avoidance structures Finally. choice of the appropriate target volume for treatment has an important impact on outcome I f one attempts to treat both involved sites and potentially involved sites that are not documented by imaging or surgical staging. target volumes become impossibly large, especially i f dose escalation is to be attempted Clinical data now suggests that elective irradiation of uninvolved sites is not necessary since failure in these sites is less than 3-4°.. ff left untreated This is an important observation, since even using 3D and INIRT techniques, one could not fashion a viable treatment plan for dose escalation, because the volume to be treated would defeat any potential advantage provided bv the techniques IMRT is just at its lnfancv m being applied to N S L C L Some reports have appeared suggesting a theoretical advantage m terms of higher doses being dehverable to the tumor with lower doses to lung and esophagus compared to 3D plans for
523 the same patient Howe~r, there is not vet enough clmlcal data to validate that this translates to improved outcomes More still needs to be done in terms of finding a practical means for motion control, especially employmg functional lmagmg data that is suitable for all levels of radiation therapy practice The use of PET isotopes other than FDG, mav also h a ~ application in terms of identifying areas of tumor hypoxla and in estimating proliferation rate As new centers usmg proton beams begin to open around the country there
will be the opportunity to evaluate whether the "Bragg peak" of proton beams can provide more precise and normal tissue sparing treatment In summary, just consldermg the new developments m radiation therapy technolog.~, lmagmg and treatment plannmg systems, the future looks ripe for significant optimization of NSCLC treatment Combined with advances in svstemlc treatment, the next decade could see greater progress in defeating NSCLC than that seen on the previous 3q~ vears
of [his theoretical advantage has been shown from the experience w i t h small numbers of large fractions used for stereotactlc irradiation of small primary tumors (..Tcm) in medically inoperable patients U s i n g advanced multifield arrangements, sophisticated immobilization and gating systems, local control rates from 67 to lq.O., at 1-3 vears have been reported with acceptable tolerance for regimens that range from 45 Cr¢ m 3 fractions to 6q~Cr¢ m 1 q~ fractions Improved target volume definition with CTfI~IRI and functional imaging has permitted better avoidance of geographic misses of ttmaor targets, treatment of the '*biologically relevant" target, and reduction of radiation dose exposure to critical normal tissue N e w radiation treatment planning svstems ITPSI. requiring significant computing power, are able to h a ~ diagnostic imaging data from CT. M R I and PET **fused" into the TPS so that the data thev provide corresponds to the patient treatment position This capability, as well as growing use of corrections for increased electron transmission through low density lung. better understanding of dose perturbations at the lung/tumor interface, and adjustments for beam energ2v permit further optimization of dose d e l l ~ r v Standardization of tumor target designations and the language describing them. to account for microscopic tumor extension, internal target and organ motion, patient set-up error and beam geometry w i l l assure greater uniformity m reporting treatment information Dose volume histograms can be calculated using the newer TPS's and graphically display coverage of both target and avoidance structures Finally. choice of the appropriate target volume for treatment has an important impact on outcome I f one attempts to treat both involved sites and potentially involved sites that are not documented by imaging or surgical staging. target volumes become impossibly large, especially i f dose escalation is to be attempted Clinical data now suggests that elective irradiation of uninvolved sites is not necessary since failure in these sites is less than 3-4°.. ff left untreated This is an important observation, since even using 3D and INIRT techniques, one could not fashion a viable treatment plan for dose escalation, because the volume to be treated would defeat any potential advantage provided bv the techniques IMRT is just at its lnfancv m being applied to N S L C L Some reports have appeared suggesting a theoretical advantage m terms of higher doses being dehverable to the tumor with lower doses to lung and esophagus compared to 3D plans for
523 the same patient Howe~r, there is not vet enough clmlcal data to validate that this translates to improved outcomes More still needs to be done in terms of finding a practical means for motion control, especially employmg functional lmagmg data that is suitable for all levels of radiation therapy practice The use of PET isotopes other than FDG, mav also h a ~ application in terms of identifying areas of tumor hypoxla and in estimating proliferation rate As new centers usmg proton beams begin to open around the country there
will be the opportunity to evaluate whether the "Bragg peak" of proton beams can provide more precise and normal tissue sparing treatment In summary, just consldermg the new developments m radiation therapy technolog.~, lmagmg and treatment plannmg systems, the future looks ripe for significant optimization of NSCLC treatment Combined with advances in svstemlc treatment, the next decade could see greater progress in defeating NSCLC than that seen on the previous 3q~ vears