Liver tumors are one of the leading causes of cancer-related mortality worldwide (1). Surgical resection with or without interventional therapy, target therapy, or systematic chemotherapy is the primary treatment option for hepatocellular carcinoma (HCC) or oligometastasis, but over 80% of patients present with unresectable tumors (2, 3). For those who are not operable, radiation therapy is another choice. However, radiation-induced liver disease (RILD) limits the use of radiotherapy in the long term.

Recently, the rapid development of radiotherapy techniques, such as intensity-modulated radiation therapy (IMRT) or volumetric-modulated arc therapy (VMAT), has expanded the indications of radiotherapy for liver tumors. Stereotactic body radiotherapy (SBRT) is an alternative option for early-stage HCC patients or oligometastases who are not eligible for surgery or interventional therapy, and it can also be used for locally advanced HCC (4, 5).

SBRT delivers high doses of radiation in a few fractions with better accuracy to the tumor target. Previous studies have shown that the success of SBRT is due to a higher biological effective dose (BED) and sparing of normal tissue (5). Current clinical investigations have unveiled SBRT’s effectiveness, manifesting in promising local control rates (1-year LC 56%–100%) and overall survival rates (1-year OS 32%–94%) (6–8), Additionally, SBRT has exhibited potential in slowing the progression of disease from an oligometastatic state to a polymetastatic one (9).

Different methods can be used for performing SBRT, including 3D-RT, IMRT, DCAT, VMAT, and Cyberknife. Among all these methods of delivery, VMAT is a type of rotational radiotherapy. VMAT is superior to other modalities due to its better dose distribution and treatment efficiency (10–15).

However, high-dose delivery to the target tissue can cause late effects, which can significantly damage nearby normal tissues (16–18). Therefore, it is crucial to reduce the dosage of the organs at risk (OARs) in SBRT treatment plans. It has been shown that the LC rate depends on the size and number of lesions, while the OS rate is strongly associated with liver function before treatment (19).

The quality of radiotherapy planning largely depends on the experience and skills of the medical physicists (20, 21). Manually setting and adjusting parameters is the mainstream in radiotherapy planning and optimization. It has been shown that optimization strategies greatly affect dose outcomes in liver SBRT plans (10). Therefore, we developed an automatic stereotactic body radiation therapy planning (ASP) program to improve the overall plan quality and consistency, prevent bias caused by different physicists while reducing the reliance on personal experience or skills, and accelerate the entire process. The ASP program has been previously evaluated in both the lung and liver with better reproducibility and repeatability (22–24). In this study, we aimed to investigate the efficacy of our ASP program in optimizing the liver SBRT plans by increasing the number of total half arcs in VMAT plans.

To our knowledge, this is the first study to use the self-coding automatic software program to generate the liver SBRT plan and compare the efficacy by adding the number of half arcs in the VMAT plans. This system was previously validated in lung tumors (22, 24), and this ASP for each plan can be similarly optimized. Thus, we can minimize the impact of the experience or clinical preference of different physicists and reduce the deviations caused by manual adjustments.

In the past few years, IMRT has been the most commonly used method to generate SBRT plans (27). Recently, VMAT became popular due to its more flexible gantry rotation, variable dose rate, and dynamic multileaf collimator movement (28). Thus, VMAT can improve the dose distribution and shorten the treatment duration in both conventional and SBRT radiotherapy plans (29). Recent studies showed that a partial-arc SBRT plan can better protect normal tissues without sacrificing the target dose in lung SBRT (30). Considering that the anatomical location of the liver tumor is mostly unilateral, we set up the half-arc rotating to cover the whole liver instead of using a full arc. Then, we used D_2cm , R_50% , CI, and HI index to analyze the dosimetric characteristics. D_2cm and R_50% were normally used to evaluate the low-dose spillage in the SBRT plan, according to RTOG0813 (31). In the standard criteria, the maximum dose of D_2cm should be limited to 50%–77% of the prescription dose, and the R_50% value should be stipulated to <2.9–5.9. In our plans, the D_0.03cc of D_2cm was approximately 53%–54% due to the relatively larger size of liver tumors, which was acceptable. Seven of the thirty-one patients did not meet the R_50% criteria in HA1. Using multiple arcs helped four of the seven the suggested goal in HA2 to HA4. On the other hand, as the number of arcs increased (from one to three arcs), the CI of PTV improved, indicating that increasing arcs enhanced conformity. However, the four-arc plan did not improve the CI. Thus, more flexible dose delivery in the VMAT plan can limit dose spillage without compromising conformity.

Multiple concentric rings outside the PTV were used for dose fall-off limitations, which have been studied among stereotactic radiosurgery plans (32, 33). In this study, we conducted 10 concentric rings evaluating dose fall-off and measured the dosimetric parameters for each ring. Analyzing the D_0.03cc of rings indicated that using two half-arcs enhanced normal tissue safety. However, no further progress was observed in the HA3 or HA4 optimization plans. Moreover, the dose fall-off curve based on the D_mean of rings was steep from ring 1 to 4 and then became mild from ring 4 to 10. Previous studies have demonstrated the advantages of the SBRT plan with heterogeneous plan optimization (24). Thus, we loosened the limitations of HI when establishing the optimization characteristics, providing more conformal radiation plans. In previous studies, multiple arcs increased beam-on-time (34, 35). In this study, the longer beam-on time was observed after using more arcs.

The most dangerous toxicity in liver SBRT is RILD. As a classic parallel organ, the risk of RILD depends on the dose of irradiation and the volume of the irradiated organ (18). Several studies have established different models of dose limitation to minimize the risk of RILD. A group initially used the dose constraint of at least 700 cc of normal liver <15 Gy in three fractions, and no patient experienced grade 3 liver or intestinal dysfunction (36). This clinical trial included patients with liver metastases; however, patients with primary HCC are more susceptible to RILD due to hepatic dysfunction or cirrhosis. Some studies suggested that for patients in the Child-Pugh B stage, the dose to one-third of the uninvolved liver was restricted to ≳18 Gy, and the dose to ≧500 cc of the uninvolved liver was restricted to <12 Gy in five fractions (37, 38).

Meanwhile, the mean dose of a normal liver is also required, according to the QUANTEC report in 2010. The mean dose should be <15 Gy for liver metastases in three fractions and <20 Gy for liver metastases in six fractions (18). In our study, the mean dose of 700 cc of the normal liver and the mean dose of the normal liver were below the constraints. D_500cc ranged from 6.71 Gy to 6.29 Gy which met the limitations mentioned above. D_mean ranged from 9.27 Gy to 9.33 Gy. Increasing the number of arcs (one to four arcs) decreased D_500cc , D_700cc , and D_mean , resulting in a lower probability of RILD.

The right side of the kidney is anatomically adjacent to the liver. Previous studies showed that, as a mixed serial and parallel organ, the kidney obeys a volume–dose-response relationship. Cassady found that a total dose was associated with a 5% and 50% risk of injury after 5 years of 18–23 Gy and 28 Gy radiotherapy, respectively (39). Nevertheless, Cheng et al. showed that a dose of 9.8 Gy was associated with a 5% risk of kidney toxicity (40). In 2010, the AAPM published a recommendation for kidney dose limitation. Based on the threshold dose, 200 cc of the kidney volume should be exposed to less than 16 Gy in three fractions or 17.5 Gy in five fractions (41). Based on RTOG1112, the mean dose of bilateral kidneys should be less than 10 Gy. In this study, the mean dose of the right kidney ranged from 0.83 Gy to 1.15 Gy in HA1 to HA4. Among the 31 patients, only one patient had a D_mean of 4.9 Gy. From HA1 to HA4, D_0.03cc ranged from 13.27 Gy to 14.23 Gy, and D_2cc ranged from 5.98 Gy to 7.80 Gy. D_mean reduced by 4.46% and 0.43% from HA1 to HA2 and from HA2 to HA3, respectively. Using a multiple-arc plan may help reduce the dose to the kidney (HA1 to HA4) when D_0.03cc is significantly decreased.

Among the serial organs, the spinal cord, duodenum, colon, and stomach were the most critical organs. Kopek et al. found that the mean maximum dose of 1 cc of duodenum (D_1cc ) was significantly higher for patients with grade ≥2 ulceration or stenosis (37.4 Gy vs. 25.3 Gy). However, patients receiving a dosage lower than 25.3 Gy only experienced grade 0 or 1 duodenal toxicity (42). Bae et al. also indicated that D_0.03cc can be a valuable predictor of gastroduodenal toxicity, as the D_0.03cc of 35 Gy and 38 Gy were respectively associated with a 5% and 10% probability of developing severe gastroduodenal toxicity (43). Thus, we used D_0.03cc and D_5cc to evaluate the safety of the duodenum, colon, stomach, and small bowel according to RTOG1112 and TG101 recommendations. The D_0.03cc or D_5cc of each organ met the criteria. Compared with HA1, the D_0.03cc and D_5cc of the gastrointestinal organs decreased in HA2. Meanwhile, only the D_0.03cc and D_5cc of the stomach continued to reduce in HA3. These results indicate that gastrointestinal OARs can be reduced by increasing the number of partial arcs, and the effect decreases by using three arcs.

Adding several arcs improved and helped spare the OARs. The stomach, kidneys, and bowels were the most significant ones. Normal liver was also protected by decreasing maximum dose in D_700cc and D_500cc , respectively.

However, it is important to acknowledge certain limitations in our study. Firstly, this was a retrospective analysis, potentially introducing selection bias. Secondly, our investigation was confined to a single-center setting, relying on a limited patient pool. The relatively large tumor sizes within our cohort may restrict the generalizability of our findings to specific patient groups. Lastly, the optimized plans have yet to be implemented for patient treatment. Assessing the clinical outcomes of patients treated with these optimized plans is essential for verifying their organ-sparing benefits. Moreover, comparing our proposed approach with alternative methods may hold promise for future research endeavors.

In the present study, we used an ASP to generate SBRT plans for oligo liver tumors. The process was more objective and had less dependence on physicians’ skills or preferences, which can help rule out manual bias. We comprehensively compared multiple-arc plans by using ASP. Using more arcs improved conformity but sacrificed planning homogeneity. In addition, increasing half-arcs improved dose distribution and dose fall-off setting. A sharper dose of fall-off planning showed prominent benefits in protecting OARs. However, the advantages were mostly found in two or three half-arc plans in the present study. Only the liver, kidney, and bowel were protected in the four-arc plan. All in all, using ASP may improve the consistency of the liver SBRT plan, and using three to four half arcs may improve plan conformity with better protection of surrounding OARs. On the other hand, the beam-on time was prolonged. Considering both treatment quality and efficiency, a three-arc plan is suitable for clinical application.

The raw data supporting the conclusions of this article will be made available by the authors, without undue reservation.

The studies involving humans were approved by the Clinical Research Committee of the study institute. The studies were conducted in accordance with local legislation and institutional requirements. The ethics committee/institutional review board waived the requirement of written informed consent for participation from the participants or the participants’ legal guardians/next of kin because this is a retrospective study without any intervention during patients’ previous treatment.

SZ: Conceptualization, Data curation, Formal Analysis, Investigation, Methodology, Validation, Writing – original draft. WZ: Data curation, Formal Analysis, Writing – original draft. NZ: Investigation, Writing – original draft, Data curation, Methodology. JY: Data curation, Formal Analysis, Writing – original draft. MX: Investigation, Writing – review and editing. WL: Funding acquisition, Validation, Writing – review and editing. NC: Supervision, Validation, Writing – review and editing. JX: Conceptualization, Funding acquisition, Supervision, Validation, Writing – review and editing.