การเปรียบเทียบเชิงรังสีคณิตระหว่างแผนรังสีรักษาเทคนิครังสีปรับความเข้มเชิงปริมาตรแบบเต็มพื้นที่และครึ่งพื้นที่ลำรังสีในผู้ป่วยมะเร็งปากมดลูก

Abstract

Introduction: Cervical cancer ranks as the second most prevalent cancer among Thai women, with an age-standardized incidence rate (ASR) of 14.4 per 100,000 women. Due to the high five-year overall survival (OS) rate for both early-stage and locally advanced-stage cervical cancer patients, the radiation therapy technique has continuously developed to increase the radiation dose to the tumor while decreasing the dose to Organ at Risk (OARs). Volumetric Modulated Arc Therapy (VMAT) is a novel treatment technique that provides equivalent target coverage compared to conventional IMRT while significantly reducing dose to OARs including bowel, rectum, bladder, and femoral head. Furthermore, VMAT enhances conformity and homogeneity indices to the Planning Target Volume (PTV), along with reducing average monitor units and shortening delivery time. However, a general drawback of VMAT is the concern regarding low-dose exposure to normal tissues. Due to high overall survival rate of cervical cancer patients, the reduction of radiation-induced toxicity for many OARs in cervical cancer radiotherapy is a significant concern. Therefore, this study aims to develop a treatment planning technique to improve plan quality in VMAT for cervical cancer. Methods and Materials: Previously treated twenty-eight cervical cancer patients were retrospectively randomly selected and divided into two groups: 1. Adjuvant radiotherapy group: 14 patients, and 2. Definitive radiotherapy group: 14 patients. For each patient, three different planning techniques were used: 1. Auto field volumetric modulated arc therapy (AF-VMAT), 2. Modified fixed field volumetric modulated arc therapy (MF-VMAT): 2.1 Full field with 35-degree collimator rotation (FFcol35), and 2.2 Half field with 35-degree collimator rotation (HFcol35). Dose constraints for the target were evaluated based on the guidelines provided by the International Commission on Radiation Units and Measurements (ICRU 83). For all treatment plans, the PTV parameters including V107%, V100%, V95%, Conformity Index (CI), and Homogeneity Index (HI) were maintained as statistically insignificant and met the criteria outlined in ICRU83. In this study, we compared all planning techniques in three parts: (1) Organ at Risk (OARs) and whole-body dose: with the dose constraints and evaluated based on the guidelines provided by the Radiation Therapy Oncology Group (RTOG 1203 and RTOG 0418), including bowel bag V40Gy, rectum V40Gy, bladder V45Gy, femoral head V30Gy, and whole-body V20Gy, (2) Treatment plans efficiency: number of monitor units (MUs), number of control points, beam on time, maximum leaf travel, average maximum leaf travel, and maximum leaf travel per gantry rotation, (3) Treatment plans accuracy: patient-specific quality assurance (PSQA) was performed by the new Model 1220 ArcCHECK (Sun Nuclear Corporation, Melbourne, Florida, USA) and SNC patient software version 8.5.1.9 to evaluate the delivery accuracy of the plans. All plans were delivered using the Elekta Synergy® linear accelerator on the same day. The gamma passing rates (GPR) with a 10% dose threshold were determined for both absolute dose (AD) and relative dose (RD) using gamma criteria of 3%/3mm and 3%/2mm, respectively. Results: (1) OARs and whole-body dose: bowel bag (p-value = 0.001, 0.000), rectum (p-value = 0.002, 0.003), left femoral head (p-value = 0.001, 0.004) and whole-body (p-value = 0.000, 0.000) received a statistically significant dose reduction when using the HFcol35 plan compared to AF-VMAT and FFcol35, respectively. (2) Treatment plan efficiency: HFcol35 exhibited a statistically significant increase in both number of Monitor Unit (MUs) and control points (p-values = 0.000), while beam-on time, maximum leaf travel, average maximum leaf travel, and maximum leaf travel per gantry rotation were statistically significant decreased (p-values = 0.000) compared to both AF-VMAT and FFcol35. (3) Treatment plan accuracy: the average gamma passing rate was higher in the HFcol35 plan for both absolute dose (AD) (p-value = 0.001, 0.004) and relative dose (RD) (p-value = 0.000, 0.000) for 3%/3mm and 3%/2mm gamma criteria compared to AF-VMAT plan, respectively. Conclusion: The HFcol35 is an effective planning technique in VMAT for cervical cancer patients. It significantly reduces OARs doses and decreases the spread of low doses to normal tissues compared to AF-VMAT and FFcol35. Half-beam designs in HFcol35 are essential for generating the double concave dose distribution, which reduces the maximum MLCs travel and relative to MLCs speed, resulting in an increase in plan delivery accuracy. This is demonstrated by a significant increase in the average gamma passing rate for both 3%/3mm and 3%/2mm gamma criteria compared to AF-VMAT. Although HFcol35 has higher MUs and control points, it shows a significant decrease in beam-on time compared to AF-VMAT and FFcol35. Therefore, HFcol35 offers advantages in dosimetric comparison, plan quality, plan efficiency, and plan delivery accuracy for cervical cancer VMAT treatment planning.