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Introduction

Review of treatment plans is a key step in the radiotherapy process that determines the characteristics of the plan selected for treatment and consequently how patients are treated [1-3,4]. However, the review is not easy since there are many aspects to be considered [5-7]. Not only it should include reviewing dose distribution and dose-volume histograms (DVHs) of target volumes and organs at risks (OARs), but also should reviewing plan parameters, such as, gantry angle, optimization conditions, and control points [8-10].

Currently there are two types of plan review, physician review and physics review [3,11-14]. Physician review is performed by the physician(s) in charge of the patient. This review is to see whether the treatment plan meets clinical requirements [15]. Physics review is performed by a physicist after the treatment plan been transferred to the treatment management system. This review is to make sure there are no errors in the treatment plan and during plan transfer, i.e., to check the safety of the treatment plan [16-19]. However, neither review involves the quality control of plan design [20]. After a treatment plan completed, all parameters affecting plan quality, especially beam arrangement and optimization conditions, should be checked [21-25] as early in the workflow as possible and not rely solely on physics review at the end of treatment planning. In addition, if a problem was found after the plan been transferred to the treatment management system, there would require a lot of rework. For these reasons, a review step by a well-experienced medical physicist right after the plan is designed should be established. It can be called independent physics pre-review, which is different from physician review and physics review afterwards. This study was to investigate how independent physics pre-review affects plan quality and planning time.

Materials and Methods

In this study four senior medical physicists participated in the independent physics pre-review. The requirements for senior medical physicists were good at external treatment planning, at least 15 years clinical working experience and have senior professional title. Once enrolled on the study, all participants would receive the same pre-review items, which can be seen in appendix part.

Independent physics pre-review takes place after a treatment plan is newly designed by a qualified dosimetrist or physicists, which were showed in Fig. 1. If the reviewing physicist considers the plan quality is satisfactory and is hard to improve further, physicist will approve the plan, and the plan will enter next step, physician review. Otherwise, reviewing physicist will disapprove the plan, and give advice to the dosimetrist/physicist on how to improve the plan. The latter will modify the plan per advice. After revision, the plan will be reviewed by the same reviewing physicist again. This loop will continue till the plan is approved.

Items of independent physics pre-review

The items of independent physics pre-review were determined through discussion among physicists in our department. The items for IMRT, VMAT, TOMO, CA, CRT and electron techniques are listed in the appendix of Table A1-6. The clinical techniques for different sites are listed in the appendix of Table A7.

The clinical site specific technique would have had more impact in improving the plan quality, a list of recommended clinical treatment techniques for different clinical sites can be found as following:

Data collection

For this study, plans by independent physics pre-review from February 2021 to June 2021 were collected. For plans disapproval, their corresponding disease types, techniques used, problems and causes for disapproval and planning time were counted. The homogeneity [24] and conformity [26] of target volumes and dose to OARs between disapproved and approved plans were statistically analyzed.

Data analysis

To determine whether there is a significant difference of dosimetric parameters between disapproved and approved plans, a Chi square test was performed. The threshold for statistical significance was set at p < 0.05 (two-tailed). All statistical analyses were performed using SPSS Version 19.0 (SPSS Inc., Chicago, IL).

Results

Statistics of independent physics pre-review plans

From February 2021 to June 2021, for three months, there were 1447 plans involved into pre-reviewed, including 138 IMRT, 1233 VMAT, 53 TOMO, 3 CA, 10 CRT and 10 Electron plans, which were showed in Fig. 1. The ratio of the number of IMRT, VMAT, TOMO, CA, CRT and Electron plans to total pre-reviewed plans is 9.54%, 85.21%, 3.67%, 0.21%, 0.69% and 0.69%, respectively. VMAT is the most used technique. Therefore, the independent physics pre-review results related to the VMAT techniques will be mainly presented and discussed in the paper.

Statistics of problem plans found by independent physics pre-review

Table 1 shows the statistics of problem plans found by independent physics pre-review. From the table, it can be seen that the ratio between the number of problem plans and total pre-reviewed plans is 4.56%. For TOMO, VMAT and IMRT technique, this rate is 9.43%, 4.79%, and 1.45%, respectively.

Figure 2 shows the percentage of problem plans classified by diseases. The top three diseases with highest percentages of problem plan were lung cancer, breast cancer and liver cancer, of which problem plans account for 16.67%, 13.64% and 10.61%, respectively, of total problem plans found by independent physics pre-review.

Statistics of problems found by independent physics pre-review

Figure 3 displays the statistics of problems for VMAT plans found through independent physics pre-review. The top problem found by independent physics pre-review was dose distribution improper. Example disapproval and approved dose distributions by independent physics pre-review for plans of sarcoma tumor were showed in Fig. 4(a) and 4(b), respectively. After revision, the dose of normal tissue in Fig. 4(b) was significantly less than that of Fig. 4(a). The secondary problem was the improper setting of gantry angle. Example setting of disapproval and approved gantry angles by independent physics pre-review for plans of lung cancer were showed in Fig. 5(a) and 5(b), respectively. After adjustment, the setting of gantry angle in Fig. 5(b) was more reasonable than that of Figu 5(a), because modified gantry angle was more limited to longest side of target volumes, and less normal tissue was penetrated.

The dose distribution problems include unmet target prescription dose, dose to OARs too high, improper conformity of target volumes, improper homogeneity of target volumes, improper maximum dose, and slow dose fall-off outside target volumes. Detail problem percentages of unreasonable dose distribution are displayed in Fig. 6. The top three problems were dose to OARs too high, improper homogeneity of target volumes, and unmet target prescription dose, of which number of plans account for 67.57%, 18.92%, and 8.11%, respectively, of all plans with dose distribution problems.

Cause analysis of problem plans

The percentage of causes for VMAT plans are displayed in Fig. 7. The top three causes were improper setting of optimization conditions, pseudo organs drawn wrongly or unreasonable, and unreasonable arc range. These three causes resulted in 67.71%, 13.56% and 13.56% problem plans, respectively.

Dosimetric difference between disapproval and approved plans

The treatment plans of lung cancer, with highest percentage of problem, as an example, the dosimetric difference between disapproval and approved plan by independent physics pre-review were shown in Table 2. The forth column in the table lists the p-values. A p-value of < 0.05 is considered clinically significant. As shown in Table 2, compared to disapproval plans, approved plans statistically provide significant decrease in lung all V₅, V₂₀ and D_mean, and no significant reduction in Cord D_max and Cord PRV D_max. These comparisons show that independent physics pre-review can effectively ensure the quality of treatment plan.

Planning time of modifying the disapproval plans

The statistics of planning time of modifying disapproval plans were showed in Fig. 8. From disapprovement to approvement, 50% plans can be revised and passed physics review within 5.57 working hours, 95% plans can be finished within 11.83 working hours, and the longest time for passing independent physics pre-review was 26.6 working hours.

Discussion

The results of Fig. 1 and Table 1 display the statistics of pre-reviewed plans and rate of problem plan found by independent physics pre-review. IMRT, VMAT and TOMO plans account for 98% of total reviewed plans. Approximately, 4.79% of VMAT plans were disapproved by independent physics pre-review and this value was in line with disapproval rate of all plans. 9.43% of TOMO plans were disapproved in independent physics pre-review, and this value was higher than the results of other techniques. This may be because TOMO technique was often used to design more complex treatment plans. The complexity increases the difficulty of the plan design, which requires more experienced, knowledge and skilled ability of dosimetrist/physicist.

Figure 2 show that plan of lung cancer has the highest percentages of problem plan. The main problem was the low dose of lung not strictly restricted caused by the improper setting of optimization condition or by pseudo organs drawn unreasonably. This indicates that for low dose of lung, it needs more attention by dosimetrist/physicist. For plans of breast cancer, the secondary percentages of problem plan, its main problem was no strictly protections of OARs, including the high dose to thyroid or humeral head, too much low dose to the lung, heart or contra-lateral breast.

The proper setting of optimization conditions was very important in plan design, which was displayed in Fig. 7. In addition to the reasonable arrangement of the field, setting of optimization condition largely determined the quality of the plan. If settings of optimization condition were no proper, they would most likely cause the problem of dose to OARs too high, followed by problem of dose homogeneity, which were displayed in Fig. 6. Independent physics pre-review found that even if some OARs meet the clinical requirements, their dose can be further reduced by reasonable setting of optimization conditions.

Independent physics pre-review is a critical step in ensuring the plan quality. Whether the setting of field/arc reasonable and whether settings of optimization conditions reasonable, and whether control point reasonable were all involved in independent physics pre-review. Independent physics pre-review could eliminate suboptimal treatment plans. It can review whether threatment plan has optimization space, that is, whether the dose of OARs is as low as possible on the premise of ensuring the prescription dose of target volumes.

Face-to-face communication between reviewing physicist and planning dosimetrist/physicist helps the dosimetrist/physicist really understand the causes leading to the problems and how to make timely and effectively modification to the plan. The statistics of planning time for modifying disapproval plans was displayed in Fig. 8. The planning time only include working time, excluding the weekends and holidays. It can be counted that the average time of physics review from disapprovement to approvement was 5.51 working hours. 50% plans can be revised and passed physics review within 5.57 working hours. 95% plans need more than 11.83 working hours to finish. The longest time for passing independent physics pre-review was 26.6 working hours, and the corresponding plan was a design of protecting hippocampus plan in whole-brain irradiation with simultaneous integrated boost to metastatic tumors. The complexities of the plan lead to a long optimization time, which increased the time for the plan from disapprovement to approvement. Such time cost is considered to be acceptable in our department.

Despite the benefits described above, the independent physics pre-review also has limitation, and its evaluation efficiency is not as high as the automatic evaluation software. This may increase the waiting time from designing plan to treatment for a patient. Therefore, further work to develop intelligent physical pre-review software is necessary.

Conclusions

This study shows that independent physics pre-review can improve plan quality, and most independent physics pre-review can be finished within acceptable time.

Conflict of Interest Statement

The authors have no relevant conflicts of interest to disclose.