D epartment of radiation Oncology，University of Maryland School of Medicine USA

The war againstcancer is a war not yet won. Over the years we have made significant progress in cancertreatments. However,viewing from the end point-the survival rate,the results remain verydisappointing. Of the all patients diagnosed with cancer in the world,approximately 45%will eventually die as a result of recurrence and/or metastasis. Radiation therapy is oneof the major modalities for cancer treatment,it had contributed significantly in savingthe lives of patients inflicted with cancer and improving their quality of life.However,it also has its share in the disappointiong statistics with all other modalities.Over the last 5 decades,radiation therapy has gone through many advances both in equipmentand in treatment methods. The impact,however,falls short as compared with theexpectations. It is possible that the efficacy of radiation therapy has reached itslimits. It is also possible that what we have done in the past has some major shortcomingsin its methodology,which may require revolutionary improvement. Conformal radiationtherapy as a new treatment technique provides us some hope in answering these keyquestions.
Clinical studiesindicate that local-regional recurrences,as the first event of relapes,account for 83% ofthe treatment failures in prostate[Fuks,et al]and 70% of treatment failures in head and neck regions[Leibel,et al].This evidence suggests that,in many cases,distantmetastasis may be spawned from local recurrence on the primary tumor.Consequently,improving local tumor control will enhance loog-term survival and reduced thecost of health care[Suitand Westgate]. Tothis goal,3 D conformal radiation therapy has been developed to maximally irradiate thetumor while sparing surrounding structures. Higher doses than that conventionallyadministered to the patient can be safely delivered to local regional sites.
The goal ofconformal radiation therapy is to conform the high dose volume to the clinical targetvolume while at the same time sparing he surrounding normal structures. Although the goalis not new to radiation therapy,it is only recently that the goal can be realized. Viewingthe conventional treatment methods being practiced in most radiation therapy treatmentfacilities over the world,it is surprising that how little we have accomplished inreaching this goal. In many sites,the treatment techniques have not changed for decades.
There are manyfactors that contributed to the seemingly sudden interests in conformal therapy. Therealization of the need for dose escalation to enhance local control provided us theclinical bases for developing conformal techniques. The introduction of digitallycontrolled linear accelerators,especially the introduction of multileaf collimator. madedynamic delivery of radiation reattempt possible.The advances in computer technology andgraphical capabilities made 3 D treatment planning,3 D dose calculation and treatment planoptimization feasible. Advances in diagnostic imaging allowed us to delineate treatmenttarget more precisely. Knowledge gained in biological modeling allowed us to evaluatetreatment plans with more clinical relevance. Armed with these knowledge and tools,we cannow look more critically on our own practice and make fundamental changes when necessary.
It is importantto note that conformal therapy,as a term is not very well defined.Viewing from the way ofpractice,there are two types.(1)Using 3 D volumetric information from CT/MRI images todefine the target and to determine treatment field arrangements. Uniform beam intensitiesor wedged beam intensities are used for the beams;(2)Using computer optimization based onthe 3 D volumetric information to determine not only beam arrangements but also the beamintensities of each beam. None uniform beam intensities are delivered to the target fromeach individual beam. The two methods represent dramatic differences in both the operationand the outcome. The former is a natural extension from conventional practice. I do notexpect may differences in terms of the treatment outcome as compared with the traditionalmethods. The latter,also termed Intensity Modulated Radiation Therapy (IMRT),presents arevolutionary change from the conventional practice. With intensity modulation,the abilityto shape the high dose volume is greatly enhanced. Many leaders in the field believe thatIMRT is the future for radiation therapy.
Whether or notconformal therapy,especially IMRT,is going to make a difference on the end point of cancertreatment is a hypothesis remain to be proven. To date,most of the development has been incomputer optimization of treatment plans and predictions of what dosimetric and biologicaloutcome might result from these plans. The results,while very appealing,may also beunattainable.Inadequate attention has been devoted to whether the optimized treatmentplans can be accurately executed. Our ability in delivering the optimized treatment plansis still very limited. Due to the fact that the main goal in conformal radiation therapyis to improve the physical characteristics of the treatment,these questions and drawbacksprovide significant challenge and opportunities for radiation oncologists and medicalphysicists.

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(收稿日期 1998-06-20) (Cedric Yu)