The war against cancer is a war not yet won. Over the years we have made significant progress in cancer treatments. However,viewing from the end point-the survival rate,the results remain very disappointing. 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 one of the major modalities for cancer treatment,it had contributed significantly in saving the 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 equipment and in treatment methods. The impact,however,falls short as compared with the expectations. It is possible that the efficacy of radiation therapy has reached its limits. It is also possible that what we have done in the past has some major shortcomings in its methodology,which may require revolutionary improvement. Conformal radiation therapy as a new treatment technique provides us some hope in answering these key questions.

Clinical studies indicate that local-regional recurrences,as the first event of relapes,account for 83% of the 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,distant metastasis may be spawned from local recurrence on the primary tumor. Consequently,improving local tumor control will enhance loog-term survival and reduced the cost of health care［Suit and Westgate］. To this goal,3 D conformal radiation therapy has been developed to maximally irradiate the tumor while sparing surrounding structures. Higher doses than that conventionally administered to the patient can be safely delivered to local regional sites.

The goal of conformal radiation therapy is to conform the high dose volume to the clinical target volume while at the same time sparing he surrounding normal structures. Although the goal is not new to radiation therapy,it is only recently that the goal can be realized. Viewing the conventional treatment methods being practiced in most radiation therapy treatment facilities over the world,it is surprising that how little we have accomplished in reaching this goal. In many sites,the treatment techniques have not changed for decades.

There are many factors that contributed to the seemingly sudden interests in conformal therapy. The realization of the need for dose escalation to enhance local control provided us the clinical bases for developing conformal techniques. The introduction of digitally controlled linear accelerators,especially the introduction of multileaf collimator. made dynamic delivery of radiation reattempt possible.The advances in computer technology and graphical capabilities made 3 D treatment planning,3 D dose calculation and treatment plan optimization feasible. Advances in diagnostic imaging allowed us to delineate treatment target more precisely. Knowledge gained in biological modeling allowed us to evaluate treatment plans with more clinical relevance. Armed with these knowledge and tools,we can now look more critically on our own practice and make fundamental changes when necessary.

It is important to note that conformal therapy,as a term is not very well defined.Viewing from the way of practice,there are two types.(1)Using 3 D volumetric information from CT/MRI images to define the target and to determine treatment field arrangements. Uniform beam intensities or wedged beam intensities are used for the beams;(2)Using computer optimization based on the 3 D volumetric information to determine not only beam arrangements but also the beam intensities of each beam. None uniform beam intensities are delivered to the target from each individual beam. The two methods represent dramatic differences in both the operation and the outcome. The former is a natural extension from conventional practice. I do not expect may differences in terms of the treatment outcome as compared with the traditional methods. The latter,also termed Intensity Modulated Radiation Therapy (IMRT),presents a revolutionary change from the conventional practice. With intensity modulation,the ability to shape the high dose volume is greatly enhanced. Many leaders in the field believe that IMRT is the future for radiation therapy.

Whether or not conformal therapy,especially IMRT,is going to make a difference on the end point of cancer treatment is a hypothesis remain to be proven. To date,most of the development has been in computer optimization of treatment plans and predictions of what dosimetric and biological outcome might result from these plans. The results,while very appealing,may also be unattainable.Inadequate attention has been devoted to whether the optimized treatment plans can be accurately executed. Our ability in delivering the optimized treatment plans is still very limited. Due to the fact that the main goal in conformal radiation therapy is to improve the physical characteristics of the treatment,these questions and drawbacks provide significant challenge and opportunities for radiation oncologists and medical physicists.

方晋平校对

(收稿日期1998-06-20)