Contribution of CT to Quantitative Radiation Therapy Planning

Transcription

1 123 Satish C. Prasad1 Miljenko V. Pilepich Carlos A. Perez Received January ; accepted after revision August 1 1, 198. All authors: Division of Radiation Oncology, Mallinckrodt Institute of Radiology, Washington University School of Medicine, 1 S. Kingshighway Blvd., St. Lous, MO Address reprint request to S. C. Prasad. AJR 136: , January 1981 o361-8o3x/81/1361-o123 $. American Roentgen Ray Society Contribution of CT to Quantitative Radiation Therapy Planning The contribution of computed tomography (CT) in radiotherapeutic treatment of lung cancer was evaluated. Radiation therapy ports prepared for patients on the basis of routine diagnostic radiographs and without CT scan Information were reviewed after CT scanning. Of patients, 1 3 (26%) required alteration of treatment ports on the basis of the additional CT scan information. In 1 1 patients (22%) the changes resulted in an increase in field size to cover the tumor adequately, and in two patients (4%-) the field size was reduced to spare normal tissue. A three-phase study was completed to evaluate the impact of CT on quantitative parameters of two-dimensional treatment plans on 2 of the patients. Treatment plans in the absence of CT scan and without lung transmission correction were compared with treatment plans where CT InformatIon was used and lung transmission corrections were performed. Numerical results for local efficiency and nonuniformity factor were compared for conventional, CT unoptlmized, and CT optimized plans. Of the 2 patients, 1 4 (6%) had poorer local efficiency and 16 (64%) had nonuniformity factor exceeding % when treatment was planned without CT information. CT-optimized plans improved local efficiency in I 6 (64%) of 2 patients and reduced nonuniformity to within % in 21 (84%). It is suggested that in the treatment planning of patients with lung cancer, CT scan information Is essential for accurate determination of dose distribution and optimization of therapy. The efficacy of computed tomography (CT) in radiotherapy treatment planning has been investigated by several authors and this subject is still evolving [1.7]. CT scans provide patient outline, tumor and internal structures configuration, and location information which is useful in treatment planning. Moreover, CT numbers, which are related to attenuation coefficients, can be used for dose computations which take into acount tissue heterogeneity [8]. These developments have given new impetus to development of treatment planning methods to optimize dose distribution. At present, there is no universally accepted method of describing a radiation therapy plan in quantitative terms. Although a radiotherapist is guided by his clinical judgment and has access to computer-assisted two-dimensional displays of isodose curves, he lacks numerical specification of relevant parameters that may guide him in selecting one treatment plan from another. A quantitative twodimensional treatment plan was first suggested by Ellis and Oliver [9] and subsequently implemented by computer by Jones and Washington and others [1-12]. The necessary parameters suggested by these authors for dose optimization are maximum tumor dose, minimum tumor dose, average tumor dose, local efficiency, and a nonuniformity factor. The local efficiency is the ratio of integral dose over the (tumor) to that oven the patient s contour. The nonuniformity factor is a measure of the variation in dose distribution within the. In radiotherapy the aim is to give adequate dose uniformly over the volume and minimize the dose to normal and adjacent critical structures. Thus higher local efficiency and smaller nonuniformity imply a better treatment

2 124 PRASAD ET AL. AJR:136, January 1981 TABLE 1 : Tumor Histology and Staging. Histology Squamous cell carcinoma Adenocarcinoma Oat cell carcinoma Undifferentiated carcinoma Total tage II Stage Ill Stage iv 6 (4) 3 (3) 2 Recurrent Total ii (6) Note-Numbers in parentheses denote patients with T4 or N3 lesions as defined by the RTOG moditication ot the AJC staging system plan. Despite the rather appealing nature of these parameters, little attention has been paid to them by radiotherapists. Most commercially available computers do not provide estimates of local efficiency and the nonuniformity factor. We present the results of a study of the efficacy of CT scanning in radiotherapy planning in patients with lung cancer. Subjects and Methods The series consists of consecutive unselected patients with bronchogenic carcinoma who had at the time of initiation or radiotherapy undergone CT scanning of the chest. These patients were seen at the Mallinckrodt Institute of Radiology during and Histologic diagnosis and staging are listed in table 1. The staging system is a modification of the American Joint Committee Staging System used by the Radiation Therapy Oncology Group. First, routine diagnostic information on patients was used to select radiotherapy beam portals for their treatment and no CT scan information was used. We then assessed the impact of CT in terms of its ability to identify the tumor location and size and the comespending changes in treatment portals for the patients. CT scans on patients were performed on EMI CT- and CT- whole body scanners with flat couch inserts and no bolus material to simulate therapy condition. The scans were transferred to a floppy disc by a disc transfer module interfaced directly with the whole body scanner. Enlarged hand copies of the scans in gray scale were obtained by an EMI RAD-8 treatment planning system [8], which is 4 interfaced with a diagnostic display console. The hard copies of CT scans provided accurate information about the size and location of tumor in transverse cuts. Treatment portals based on routine diagnostic information on all patients were reviewed and CT scans on these patients were used to determine if changes were required in the treatment portals. The second part of our study consisted of assessing the impact of CT on quantitative aspects of two-dimensional treatment plans on 2 patients. These 2 patients were the consecutive first-half of the patients. This part of our study was divided into three phases. The initial phase, called conventional treatment planning, consisted of treatment planning without the aid of CT information. Routine diagnostic information and conventional method of lead-wire contouring were used to obtain two-dimensional treatment plans through the central plane of tumor. Isodose distributions were computed using either Artronix PC-i 2 or RAD-8 treatment planning computers without any correction for lung transmission (fig. 1 A). The tumor outline was drawn based on diagnostic posteroantenor and lateral radiographs (fig. 2). In the second phase, the CT scan through the center of the tumor was used for computation of isodose distribution with a lung transmission correction. These computations were done for the same group of 2 patients studied in the first phase. Enlarged hard copies of the scans in a gray scale were obtained by the EMI RAD-8 treatment planning system. This system uses CT scan data to perform pixel-by-pixel tissue inhomogeneity corrections [8]. The hard copies of CT scans provided accurate information about patient contour, lung outline, spinal cord position, and location of tumor in the transverse cuts. In a few large patients, the lateral body contour was not seen on the CT scan due to a limitation in the size of the CT image reconstruction ring to 4 cm. On patients with lateral cut off, the contours were completed using actual patient measurements. In the second phase, the isodose distributions were computed using a beam configuration identical to that used in the conventional treatment plan phase. The CT scan was used to delineate tumor and lung outline. An example of this plan for the patient previously illustrated is shown in figure 1 B. Thus the CT-aided treatment plan (fig. 1 B) reflects changes in dose distribution due to lung transmission corrections and CT information regarding location and configuration of tumor and sensitive structures (fig. 3). The third and final phase of our study consisted of optimizing the treatment plans on the same group of 2 patients with CT information to achieve the most desirable dose distribution which was clinically practical. An example of optimized plan for the same patient is shown in figure 1 C where wedged beams were used. The local efficiency and the nonuniformity factor were calculated for optimized plans to estimate the improvement compared to conventional treatment plans. The local efficiency and the nonuniformity, expressed in pencentage, are defined as: nonuniformity where local efficiency = factor patient : dose, x,, : dose, x,, : average dose - dose, x,, E dose, x,, average dose = : dose, x,, x 1 x 1 Dose, is the dose over a small element of,,, for a given treatment plan. These parameters were computed for conventional (phase one), CT unoptimized (phase two), and CT optimized (phase three) treatment plans on 2 patients. The at various isodose intervals were measured with a planimeten and the quantity (dose, x,) calculated. The summation of these quantities over and patient s cross-sectional together with the relations defined above yielded the local efficiency and the nonuniformity factor.

3 AJR:136, January 1981 CT IN RADIATION THERAPY PLANNING 12 Results This study showed that CT scans clearly identified the location and size of tumor in 37 of patients. Of patients who had their therapy portals planned without CT information, 1 1 patients (22%) needed an increase in treatment field size to cover the tumor adequately. On the other hand, field size was reduced for two patients and 37 patients (74%) did not require any change in treatment field size. These numbers are in reasonable agreement with the results of Emami et al. [2] who reported that CT scans lead to an increase in treatment field size in 1 of 32 lung patients and no change in treatment field size in 2 of 32 patients. The local efficiency and the nonuniformity factor calculated from treatment plans developed in each phase of our study of 2 patients are summarized in table 2. With external beam therapy, on the average, the local efficiency is of the order of 3%-3%. Comparison of conventional treatment plans with CT unoptimized plans revealed that local effi- B Fig. 1 -A, Conventional treatment plan with no lung transmission correction. Target shown as hatched s in center of contour was drawn on basis of conventional chest radiographs. Hatched outside contour surface represents compensating filter and spinal cord block. Local efficiency and nonuniformity factor are 27.7 and 8.7, respectively. B. CT unoptimized treatment plan with lung transmission correction. Beam configuration and weights were identical to conventional plan. Hatched within patients contour represents as defined by CT scan. Accurate lung transmission correction could be made because lung outline and density could be obtained directly from CT scans. Hatched outside patient contour represents compensating filter and spinal cord block. Local efficiency and nonuniformity factor are 2. and 6., respectively. c, CT optimized plan with lung transmission correction. Wedged beams resulted in marked improvement in uniformity of dose and reduced dose to normal structures. Local efficiency and nonuniformity factor are 38.9 and 3.2, respectively. TABLE 2: Local Efficiency and Nonuniformity Factor for Three- Phase Treatment Plans Type ot Treatment Plan Locai Efficiency (average ± one SD) Nonuniformity Factor (average ± one SD) Conventional 29.9 ± ± 4.9 CT unoptimized 26. ± ± 1 2. CT optimized 33.2 ± ± 1.4 Note-There were 2 tr eatment pians in each group ciency was decreased in 1 4 (6%) of 2 plans and increased in 1 plans (4%) when CT information was used. Comparison of the local efficiency of conventional plans with CT optimized plans indicated that 1 6 (64%) of 2 plans had higher local efficiency while nine (36%) of 2 plans had lower local efficiency when optimized CT plans were used. The local efficiency of several optimized plans increased by more than % compared to conventional plans; however,

4 126 PRASAD ET AL. AJR:136, January 1981 A Fig. 2.-Posteroantenior (A) and lateral (B) radiographs used to outline for conventional treatment plan (fig. 1 A). Tumor (arrows): histology was large cell undifferentiated carcinoma of lung. Tumor not identified on lateral radiograph. and 1C). Tumor (arrow). on the average this improvement was not statistically significant. This is reflected in table 2 in the average values of local efficiency for conventional and optimized plans which are not statistically different. As an example, the local efficiency of treatment plans in figures 1 A, 1 B, and 1 C are 27.7%, 2%, and 38.9%, respectively. The decrease in local efficiency for unoptimized CT plan is partly due to lung transmission corrections which resulted in higher dose to normal tissue. We also investigated the correlation of change in local efficiency with change in for individual plans. The results of such a comparison for conventional vs. optimized plans are shown in table 3. There is a strong direct correlation between change in and change in local efficiency. Thus, local efficiency influenced by changes in and by optimization of plan. B NON-UNIFORMITY EJ FACTOR Conventional CT plan Fig. 4.-Percentage of treatment plans with nonuniformity in range of to % for conventional, CT unoptimized, and CT optimized plans. plan (unoptimized) CT plan (optimized) Significantly higher local efficiency can be achieved only by minimizing normal tissue dose. For centrally located tumors this is not possible with conventional photon irradiation. However, MV photon beams will give better distribution than 4-6 MV photons. Charged particle beams, such as negative pi-meson, which deliver a low dose to first

5 AJR:136, January 1981 CT IN RADIATION THERAPY PLANNING 127 TABLE 3: Correlation of Change in Target Area with Change in Local Efficiency for Conventional vs. Optimized Plans Treatment Plan with Increase in Decrease in. Unchanged Total Number of Treatment Plans Change in Lo cal Efficiency Increase Decrease 2 16(64) 9(36) Note-A change of more than 2% in between the conventional and optimized plans was used to categorize treatment plans. Good correlation was found between an increase in local efficiency with an increase in and a decrease in local efficiency with a decrease in. Numbers in parentheses are percentages. few centimeters of tissue and a high dose in a localized Bragg-peak region would improve local efficiency significantly higher than photon beams. The nonuniformity factor describes the variation in dose distribution within the. Table 2 indicates that, on the average, optimized plans had the least nonuniformity and the unoptirnized CT plans had the greatest nonuniformity factor. CT scans give better localization and an accurate lung outline as well as density of lung tissue than conventional nadiographs. This information, when used to generate unoptirnized plans with a lung transmission correction, resulted in significantly increased nonuniformity factors as compared to those calculated from conventional plans where no lung correction was used. These increases were particularly significant in cases where the tumor size and location determined by CT scan differed markedly from that of the conventional radiographic method. A nonuniformity factor of less than % was considered acceptable for purposes of this study. The percentage of treatment plans with nonuniformity factors in the range of to % are shown in figure 4 for the three types of treatment plans. The larger nonuniformity factors of unoptimized CT plans is reflected in figure 4 where 36% of unoptirnized plans had nonuniformity values in the range of to %, compared to 68% for conventional plans. Optimization of the CT plans reduced the nonuniformity in dose distribution and 84% of plans had nonuniformity factor below % (fig. 4). Comparisons of changes in with the changes in nonuniformity showed that an increase in increased nonuniformity in unoptimized CT plans. This was expected because of inadequate volume coverage by treatment fields in several unoptimized plans and also because of lung transmission correction in unoptimized plans. In the optimized plans an increase in had little influence on the nonuniformity factor. Discussion On the basis of the comparison of results for conventional, CT unoptimized, and CT optimized two-dimensional treatment plans we concluded that significant improvement in treatment planning of lung carcinoma can be achieved by using CT information. Better localization of tumor and lung 1 3 outline together with lung transmission correction will give the radiotherapist an improved and more accurate descniption of dose distribution within the and the patient s normal structures. In conventional planning, where diagnostic films are used and no CT scan information is used, and lung transmission correction cannot be easily calculated. It has been shown that in treatment plans based on poor localization of lung outline, such as from an anatomical atlas or conventional tomograms, the lung transmission correction may result in significantly large errors [ ]. Furthermore, the often described focusing transit dosimeter for lung correction [1 ] has various limitations and is not very convenient for routine use. In view of these limitations, in routine therapy planning, most radiotherapists ignore dose perturbations due to increased lung transmission and prescribe dose based on conventional treatment plans. Our results, however, show that in the treatment of lung patients conventional treatment plans are in reality more likely to have nonuniform dose distribution with lower local efficiency. Thus conventional lung plans give a radiotherapist a misrepresentation of actual dose distribution within patients and he is likely to change his treatment strategy in the presence of more accurate information. In our opinion, patients with bronchogenic carcinoma should have treatment plan based on CT scan information. The optimization of dose distribution in radiotherapy planning was proposed long before the advent CT [9]. Unfortunately, until recently, delineation of tumor and adjacent structures within a patient has been a weak link in treatment planning and has impeded progress toward dose optimization. CT scanning has given a new level of precision in radiotherapy planning by providing accurate anatomic information. This has renewed interest in optimization procedunes that deliver adequate radiation dose to the tumor volume while minimizing dose to normal structures [16]. Bjarngand [1 7] has suggested that optimization can play an important role in decision-making process in radiation thenapy planning. Ragan and Perez [3] have shown that good correlation exists between quantitative parameters of an optimized plan, such as local efficiency and nonuniformity factor, and a radiotherapist s judgernent in selection of a radiotherapy plan. Thus optimization of a therapy plan based on CT scan and its execution in daily treatment is, in our opinion, an essential step toward better tumor control in radiotherapy management. Although the results for local efficiency and nonuniformity described here are based on two-dimensional treatment plans, the method can be extended to three-dimensional plans where multiple CT slices with isodose distributions in these planes are used. REFERENCES 1. 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