Analysis of dose distribution reproducibility based on a fluence map of in vivo transit dose using an electronic portal imaging device

In external radiotherapy, cervical and breast cancers are normally irradiated with 25 fractions, and nasopharyngeal cases with 33 fractions. Treatment planning is performed only before irradiation and is used until the end of the treatment period. Patient-specific quality assurance (PSQA) to confirm the treatment planning system (TPS) calculation results [1, 2]. In addition, patient dose verification can be completed by in vivo dosimetry [3].

PSQA and in vivo dosimetry evaluation on Halcyon 2.0 can be performed with an electronic portal imaging device (EPID) [4–6]. The advantages of the EPID aS1200 are that it has high sensitivity and resolution, as well as better response linearity than the previous version [7]. The gamma index (GI) can be used for the quantitative evaluation of PSQA and in vivo dosimetry using the EPID [1, 8]. Recently, EPID has been integrated with treatment planning system (TPS), which has made PSQA on Halcyon 2.0 more effortless and efficient to execute [9]. According to Laugeman, Heermann [5], the Halcyon 2.0 with 6 MV Flattening Filter Free (FFF) photons had an output difference of about 1% when PSQA was performed again after one month.

The EPID in Halcyon 2.0 continuously records the transit dose from the patient, which is then formed into a two-dimensional (2D) fluence map. The fluence map is related to the inter-treatment consistency [4, 10, 11] and dose distribution in patients [12, 13]. Dosimetry of in vivo treatment delivery errors can be calculated based on the fluence map [5, 7, 14]. In addition, a fluence map analysis of the transmission of each fraction can detect changes in the dose distribution in patients [3, 10–12]. The EPID in Halcyon 2.0 has been used in several studies to determine the in vivo dosimetry (IVD) and assess the effectiveness of daily treatment based on the transit dose fluence map from day to day [5, 15]. According to Kim, Huq [4], IVD using EPID has good consistency and can potentially be used to monitor dose distribution discrepancy during treatment.

IVD research conducted by Matsushita, Nakamura [16] found a decrease in the gamma passing rate (GPR) value in nasopharyngeal cases. This decrease indicates a change in the dose distribution in patients when compared to the first fraction [17]. Morphological changes impact a patient's dose distribution, allowing for an increase in the dose for the organ at risk (OAR) [11, 13, 18–20]. Increases and decreases in body weight are a form of morphological change [18, 19]. Weight loss is a common factor that can occur during radiotherapy [11].

Changes in the position of internal organs and the body's contours can be influenced by body weight, thus impacting the attenuation of fluence in the body [19]. The research of Choi, Jo [21] showed that the average D_95% at the target was 98% due to an increase of 2 cm or more in body contours. In addition, D_max reached 107% due to shrinkage of 1.5 cm or more in the body contour. Therefore, the use of in vivo dosimeters with the EPID can be considered in the replanning process [13].

In vivo dosimetry evaluation using the EPID was carried out by Matsushita, Nakamura [16] and Kang, Li [3]. Matsushita, Nakamura [16] used TrueBeam STx and PerFRACTION software for the evaluation of the nasopharynx, and then a weight correlation test was carried out. Kang, Li [3] used the EPID on Halcyon 2.0 for pelvic cases. Both studies were conducted using the composite calculation method. In our research, a field-by-field evaluation will also be carried out.

1.1. Aim

2.1. Subject characteristics

2.2. Patient-specific quality assurance measurements

2.3. Tolerance limit (TL) and action limit (AL)

TG-218 recommends that the minimum values for the action and tolerance limits be 90% and 95%, respectively. The calculation of the action limit is based on the value of $\unicode{x02206}{\rm{A}},$ which is the difference between the upper and lower action limits [4, 21]. The calculated action limit was determined with the following equation (TG-218):

$$\begin{eqnarray*}&&\unicode{x02206}{\rm{A}}={\beta }\sqrt{{{\sigma }}^{2}+{(\mathop{{\rm{x}}}\limits^{{\unicode{x00305}}}-{\rm{T}})}^{2}}\end{eqnarray*}$$

therefore

$$\begin{eqnarray*}&&{\rm{AL}}\left( \% \right)=100-\unicode{x02206}{\rm{A}}\end{eqnarray*}$$

where the $\beta$ value based on the AAPM TG-218 is 6. $\mathop{x}\limits^{{\unicode{x00305}}}$ is the mean value, $T$ is the target value (for GPR is 100), and ${{\sigma }}^{2}$ is the variance [4, 21]. In addition, the tolerance limit value was obtained from the lower control limit value. Therefore, the tolerance limits were determined according to the following equation (TG-218) [21]:

$$\begin{eqnarray*}&&{\rm{TL}}\left( \% \right)={\rm{Centre\; line}}-2.660{\rm{\cdot }}\mathop{{\rm{mR}}}\limits^{{\unicode{x00305}}}\end{eqnarray*}$$

where

$$\begin{eqnarray*}&&{\rm{Centre\; line}}=\frac{1}{{\rm{n}}}\displaystyle \sum _{1}^{{\rm{n}}}x\end{eqnarray*}$$

and

$$\begin{eqnarray*}&&{\rm{Moving\; Range}}(\mathop{{\rm{mR}}}\limits^{{\unicode{x00305}}})=\frac{1}{{\rm{n}}-1}\displaystyle \sum _{{\rm{i}}=2}^{{\rm{n}}}\left|{x}_{{\rm{i}}}{-x}_{1-1}\right|\end{eqnarray*}$$

where $x$ is the value of GPR, and ${\rm{n}}$ is the number of subjects.

2.4.  In vivo dosimetry

2.5. Body weight

2.6. Statistical test

3.1. Patient-specific quality assurance (PSQA)

In general, the tabel 1 explain the mean GPR value using EPID is greater than 95% for all GI criteria. Additionaly, GI with criteria of 3%/3 mm and 3%/2 mm tend greater than 99.80%. Table 2 shows the lowest TL and AL values for the EPID for all cases were 99.78% and 97.55%, respectively. But by anatomical site it is 99.72% and 96.65% for nasopharynx. For Delta4, the lowest TL and AL values were 98.59% and 94.79%, respectively. But by anatomical site it is 97.46% and 92.05% for breast. For the EPID the lowest values of the tolerance and action limit were in line with the 3%/2 mm criteria and the gantry rotation condition. However, for 3%/2 mm the value of the tolerance and action limit measurements for the EPID and Delta4 met the requirements of AAPM TG-218.

Table 1. Calculation of mean and standard deviation from PSQA using EPID and Delta4.

	3%/3 mm			3%/2 mm			2%/2 mm			2%/1 mm
Case	EPID (R)	EPID ($0{\rm{^\circ }}$)	Delta4	EPID (R)	EPID ($0{\rm{^\circ }}$)	Delta4	EPID (R)	EPID ($0{\rm{^\circ }})$	Delta4	EPID (R)	EPID ($0{\rm{^\circ }})$	Delta4
Cervix	99.97 ± 0.13	100.00 ± 0.00	99.79 ± 0.31	99.95 ± 0.16	99.99 ± 0.01	99.65 ± 0.45	99.82 ± 0.40	99.93 ± 0.19	99.46 ± 0.56	96.83 ± 2.48	99.10 ± 0.93	99.46 ± 0.56
Breast	99.98 ± 0.07	99.99 ± 0.02	99.48 ± 0.67	99.94 ± 0.30	99.99 ± 0.04	99.04 ± 0.91	99.90 ± 0.17	99.96 ± 0.09	98.99 ± 1.00	95.96 ± 3.31	97.82 ± 1.51	98.99 ± 1.00
Nasopharynx	99.95 ± 0.39	99.99 ± 0.08	99.93 ± 0.16	99.93 ± 0.55	99.98 ± 0.05	99.77 ± 0.37	99.86 ± 0.60	99.96 ± 0.09	99.77 ± 0.36	96.47 ± 2.84	98.76 ± 0.87	99.77 ± 0.36
All Cases	99.96 ± 0.26	99.99 ± 0.05	99.74 ± 0.47	99.94 ± 0.40	99.99 ± 0.04	99.49 ± 0.70	99.86 ± 0.45	99.95 ± 0.13	99.41 ± 0.76	96.41 ± 2.92	98.56 ± 1.23	99.41 ± 0.76

EPID (R) is a measurement performed on a rotating gantry following treatment conditions. EPID ($0{\rm{^\circ }}$) refers to measurement in static gantry conditions at an angle of $0{\rm{^\circ }}.$

Table 2. Calculation of tolerance limit and action limit from PSQA using EPID and Delta4.

Parameters	Case	3%/3 mm			3%/2 mm			2%/2 mm			2%/1 mm
		EPID (R)	EPID ($0{\rm{^\circ }}$)	Delta4	EPID (R)	EPID ($0{\rm{^\circ }}$)	Delta4	EPID (R)	EPID ($0{\rm{^\circ }})$	Delta4	EPID (R)	EPID ($0{\rm{^\circ }})$	Delta4
Tolerance Limit (TL)	Cervix	99.92	100.00	99.41	99.89	100.00	99.04	99.51	99.76	98.80	92.69	97.90	98.80
	Breast	99.93	99.99	98.27	99.77	99.97	97.46	99.72	99.85	97.26	91.20	94.85	97.26
	Nasopharynx	99.82	99.96	99.80	99.72	99.93	99.27	99.52	99.86	99.28	92.52	97.05	99.28
	All Cases	99.88	99.98	99.16	99.78	99.96	98.59	99.58	99.83	98.45	92.16	96.60	98.45
Action Limit (AL)	Cervix	99.20	100.00	97.77	99.01	99.94	96.59	97.37	98.80	95.35	75.84	92.24	95.35
	Breast	99.57	99.91	94.92	98.16	99.73	92.05	98.83	99.40	91.50	68.65	84.10	91.50
	Nasopharynx	97.64	99.51	98.98	96.65	99.68	97.41	96.33	99.41	97.41	72.85	90.90	97.41
	All Cases	98.41	99.68	96.74	97.55	99.74	94.79	97.16	99.19	94.21	72.24	88.64	94.21

EPID (R) is a measurement performed on a rotating gantry following treatment conditions. EPID ($0{\rm{^\circ }}$) refers to measurement in static gantry conditions at an angle of $0{\rm{^\circ }}.$

In the EPID measurements, the rotational gantry had a lower value than the static gantry. Measurements in EPID (R) and EPID $\left(0{\rm{^\circ }}\right)$ conditions can be seen when the calculation of gamma index with criteria of 2%/2 mm and 2%/1 mm. The statistical analysis of the PSQA results is shown in table 3 for 3%/3 mm and 3%/2 mm. Measurements with the EPID and Delta4 were significantly different (p < 0.05), except for few subjects. For nasopharynx cases the action limit using Delta4 is lower than others. In addition, the measurements with EPID (R) had significantly lower values than those of EPID $\left(0{\rm{^\circ }}\right).$

Table 3. Statistical test of the difference in the results of the calculation of the gamma index from PSQA (p-value).

Case	3%/3 mm			3%/2 mm
	EPID ($0{\rm{^\circ }}$) versus Delta4	EPID (R) versus Delta4	EPID (R) versus EPID ($0{\rm{^\circ }}$)	EPID ($0{\rm{^\circ }}$) versus Delta4	EPID (R) versus Delta4	EPID (R) versus EPID ($0{\rm{^\circ }}$)
Cervix	0.000	0.000	0.012	0.000	0.000	0.005
Breast	0.000	0.000	0.000	0.000	0.000	0.000
Nasopharynx	0.139	0.001	0.011	0.000	0.000	0.005

EPID (R) is a measurement carried out on a rotating gantry following treatment conditions. EPID ($0{\rm{^\circ }}$) refers to a measurement in static gantry conditions at an angle of $0{\rm{^\circ }}.$

3.2.  In vivo dosimetry results

Figures 1 and 2 show the average GPR value of each fraction, which represents the results of the calculations with composite and field-by-field calculations. The figure 1 explains that the use of criteria 3%/3 mm and 3%/2 mm achieves the same pattern of GPR. However, the use of 3%/2 mm criteria produces a GPR lower than 3%/3 mm criteria. Based on figure 2, cases of nasopharynx have the highest correlation for each subject which has decreased GPR during irradiation treatment, which is characterized by a negative correlation value. Breast is the case with the lowest correlation value for each subject.

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There are no criteria for in vivo transit dose, therefore our reference use a tolerance limit of 95%, following the limit for PSQA recommended by APPM TG-218. The use of PSQA tolerance is used as a guide in determining the timing of replanning with a new CT scan, thereby reducing the possibility of changing the dose to the patient. 95% is the limit for a patient's condition that can still be treated but still needs to paid attention. Figure 1 shows that using field-by-field and composite has a difference in fractional time when the average GPR value is smaller than 95%. In the case of the cervix, the mean value of GPR was below 95% after fraction 17 was achieved with the composite and field-by-field methods and GI criteria of 3%/2 mm and 3%/3 mm, respectively. In the case of the nasopharynx, the GPR value decreased faster than 95% after fraction 11 for both composite and field-by-field methods, with GI criteria of 3%/3 mm and 3%/2 mm, respectively. In the breast cases, the value of GPR was below 95% after the second fraction, with parameters of 3%/3 mm. In contrast, the GPR value was consistently below 95% when using 3%/2 mm.

The correlation tests for the mean GPR value and the fraction of radiotherapy for each case are shown in table 4. Based on the results, the correlations between the GPR and various fractions were found to be significant. However, the use of the field-by-field approach exhibited a more significant correlation compared to the composite method for the cervix and breast. In contrast, in the case of the nasopharynx, the correlation between the composite and field-by-field methods was such that they had the same values. In addition field-by-field analysis allows there to be no influence from other fields that can superimpose the fluence map [27].

Table 4. Mean correlation of the average GPR value with the increase in fractions.

Case	Composite		Field-by-Field
	3%/3 mm	3%/2 mm	3%/3 mm	3%/2 mm
Cervix	−0.757*	−0.747*	−0.861*	−0.83.6*
Breast	−0.630*	−0.624*	−0.799*	−0.766*
Nasopharynx	−0.975*	−0.976*	−0.976*	−0.977*

(*) has a significant correlation values (p < 0.05).

3.3. Body weight

In the cervical and nasopharyngeal cases, each subject's body weight during the irradiation period tended to decrease, while in the breast cases, it tended to increase. Figure 3 shows the correlation value of weight to each fraction. A positive correlation value indicates that there was an increase in body weight during the irradiation period. A negative correlation indicates the occurrence of weight loss.

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Subjects 3 and 10 of the cervical cases did not have weight data because of the patients' conditions. This was also the case for the subject of 16 nasopharyngeal cases. In addition, subjects 4, 11, 14, and 16 of the breast cases had a correlation value of 0 because there was no change in body weight. The most significant correlation (p < 0.05) was in the nasopharynx, followed by the cervix, and then the breast. Table 5 shows the correlations value betweent body weight and fraction.

Table 5. List of weight and fraction correlation value.

No subject	Nasopharynx	Cervix	Breast
1	−0.889*	−0.907*	0.436
2	−0.359	−0.756	−0.775
3	−0.995*		−0.488
4	−0.939*	−0.872
5	−0.731	−0.918*	−0.775
6	−0.531	−0.266	0.964*
7	−0.935*	0.879*	0.378
8	−0.911*	−0.866	0.775
9	−0.926*	−0.859	0.707
10	−0.967*		−0.707
11	0.408	−0.836
12	−0.957*	0.898*	0.956*
13	−0.986*	−0.866	−0.139
14	−0.655	−0.525
15	−0.948*	−0.898*	0.866
16		−0.775
17	−0.991*	−0.525	0.219
18	−0.974*	−0.632	0.233

(*) has a significant correlation values (p < 0.05).

3.4. Correlation of body weight with in vivo dosimetry

Figure 4 shows the correlation between body weight and GPR for each subject. In the case of the cervix and nasopharynx, the correlation value between GPR and body weight is dominant positively. In contrast, the dominant breast had a negative correlation. Based on the image visualization, some subjects had different correlation values between the 3%/3 mm and 3%/2 mm criteria. In addition, the correlation results for the field-by-field method for nasopharyngeal and cervical cases were better than for composite cases.

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Correlation value between body weight and GPR is listed in tables 6 and 7 for composite and field-by-field, respectively. Based on the number of subjects, the nasopharyngeal case had the most significant correlation. There was no significant correlation between GPR and body weight in the breast cases. However, for the nasopharyngeal and cervical cases, the field-by-field subjects had a more significant correlation than the composite subjects.

Table 6. Correlation value between GPR and body weight, calculation GPR using composite.

No subject	Nasopharnyx		Cervix		Breast
	3%/3 mm	3%/2 mm	3%/3 mm	3%/2 mm	3%/3 mm	3%/2 mm
1	0.750	0.768*	0.470	0.463	0.149	0.066
2	0.452	0.359	0.533	0.560	−0.775	−0.775
3	0.358	0.580	—	—	0.293	0.293
4	0.907*	0.893*	0.803	0.803	—	—
5	0.151	0.241	0.661	0.708	−0.775	−0.775
6	−0.248	−0.310	−0.266	−0.393	−0.210	−0.374
7	0.536	0.542	−0.900*	−0.910*	−0.771	−0.815
8	0.960*	0.969*	−0.667	0.000	−0.775	−0.775
9	0.778	0.804	0.968	0.968	−0.707	−0.707
10	0.168	0.887*	—	—	−0.707	−0.707
11	−0.408	−0.309	0.978*	0.969*	—	—
12	0.447	0.577	−0.296	−0.296	−0.712	−0.702
13	0.832*	0.915*	0.258	0.258	0.100	0.100
14	−0.655	−0.655	−0.515	−0.498	—	—
15	0.837*	0.837*	−0.206	−0.210	0.000	0.000
16	—	—	−0.500	−0.500	—	—
17	0.889*	0.888*	0.629	0.650	−0.155	−0.120
18	0.875	0.849	0.527	0.158	−0.299	−0.408

(*) has a significant correlation values (p < 0.05).

Table 7. Correlation value between GPR and body weight, calculation GPR using field-by-field.

No subject	Nasopharnyx		Cervix		Breast
	3%/3 mm	3%/2 mm	3%/3 mm	3%/2 mm	3%/3 mm	3%/2 mm
1	0.807*	0.802*	0.764	0.617	0.406	0.389
2	0.359	0.359	0.705	0.703	−0.775	−0.775
3	0.649	0.648	—	—	0.098	0.098
4	0.918*	0.912*	0.894*	0.894*	—	—
5	0.172	0.285	0.675	0.758	0.775	0.258
6	−0.306	−0.285	−0.131	−0.131	−0.454	−0.569
7	0.720*	0.732*	−0.866*	−0.845*	−0.929	−0.943
8	0.964*	0.982*	0.866	0.866	−0.775	−0.775
9	0.835*	0.853*	0.866	0.866	−0.707	−0.707
10	0.972*	0.972*	—	—	−0.354	−0.354
11	0.000	−0.408	0.953*	0.942*	—	—
12	0.539	0.620	0.289	0.289	−0.366	−0.609
13	0.859*	0.874*	−0.775	−0.775	0.000	0.000
14	−0.655	−0.655	−0.478	−0.505	—	—
15	0.837*	0.837*	−0.426	−0.506	−0.289	−0.289
16	—	—	0.866	0.866	—	—
17	0.845*	0.815*	0.525	0.525	0.015	−0.008
18	0.852	0.853	0.738	0.949*	−0.180	−0.233

(*) has a significant correlation values (p < 0.05).

The results of the PSQA measurement using the EPID have been validated by the measurement results from Delta4. Both detectors have tolerance and action limits that are much more refined than those recommended by the AAPM TG-218. In the results of Delta4, the values of the AL and TL were lower than the EPID measurement in both the rotational and static gantry. However, there was a significant difference in the results (p < 0.05) between the EPID and Delta4, except for the nasopharynx. This difference is in line with the research conducted by Laugeman, Heermann [5], who reported that this difference is caused by the influence of the EPID resolution, which is better than that of Delta4 [1].

In contrast, PSQA measurement using the EPID is more effective with a rotational gantry because there is a significant difference (p < 0.05) between static and rotational gantry

measurements, especially using the 3%/2 mm criterion, which leads to a p-value < 0.01. When measured with a rotating gantry, the movement of the gantry and collimator can be affected by the force of gravity. Therefore, measurements with gantry rotation should include correction of gantry and collimator movement, which is expected to be closer to the irradiated state of the patient [15, 21].

An IVD study of each fraction was also conducted using the EPID. The GPR values in the nasopharyngeal, cervical, and breast cases decreased with increasing fractions. Changes in GPR can be caused by morphological disparities, such as differences in body weight, body contour, volume of OAR, and tumour [6, 27, 28]. These morphological changes resulted in inaccurate dose distribution on the target and potentially increased doses of OAR, which is in line with the findings reported by Bak, Skrobala [29] and Bhandari, Patel [30]. Dose distribution changes due to patients' morphological changes can be observed based on the GPR values. Choi, Jo [21] found that the GPR value decreases when there is a change in body contour. Furthermore, there was a change in the magnitude of Dmax and D95% in instances of PTV. Therefore, the value of GPR and patient classification to identify morphological changes can be an additional consideration for the decision of replanning [11].

Based on figure 2, each subject has a variation of correlation value between GPR and irradiation fraction. This is in line with Inui's study of prostate cases, with subjects without changes in GPR and subjects with major changes [31]. In addition, Purwati, Suhaimi [32] research by in cases of the nasopharynx showed a dominant decrease in GPR during treatment.

The nasopharyngeal case exhibited a decrease in the GPR value faster than the cervical and breast cases as explained in figure 1. This is in line with research conducted by Matsushita, Nakamura [16], who showed that the GPR value of the nasopharynx has a steeper curve than that of the cervix. In the case of breasts, almost all fractions had a GPR value of < 95%. Therefore, the tolerance of breasts post mastectomy is sufficient for the use of a GPR value of 90%.

This study only examined the effect of body weight on the GPR value and ignored other factors. Based on the assessment of body weight for each subject, it was found that body weight correlated with GPR. This is consistent with a study conducted by Matsushita, Nakamura [16], in which, for the nasopharynx, the correlation values between GPR and changes in body weight or neck volume were −0.77 and −0.74, respectively.

For cases of the nasopharynx, GPR value decreased for all subjects. Furthermore, twelve subjects have significantly changed of body weight (p < 0.05) as described in table 5. Based on the correlation between body weight and GPR, nine subjects had a significant correlation (p < 0.05). Therefore, body weight correlates with GPR value as explained in tables 6 and 7. According to Ghosh, Gupta [25], during irradiation of HNC patients, there was a significant decrease in primary and parotid gland volumes (p < 0.01 and p < 0.05, respectively). These decreases in volume both resulted in a significant decrease in the neck circumference size (p < 0.01), which is positively correlated with body weight [18]. Therefore, changes in body weight in the nasopharynx have a strong correlation with body separation or body contour [29]. A study by Burela, Soni [33] also found a significant reduction (p < 0.05) in PTV and parotid structures when repeated CT was performed mid-treatment. In a study by Bak, Skrobala [29], there was a decrease in the right parotid gland by 20.9% and in the left by 36.6%. Additionally, Brown, Owen [34] found a decrease in parotid gland volume ipsilaterally and contralaterally by 21.7% and 20.3%, respectively, and Ho, Marchant [35] showed a 29.7% and 28.4% reduction in ipsilateral and contralateral volumes, respectively.

In addition, figure 2 and tables 6 and 7 described that the GPR correlation to the irradiation fraction of each subject for breast case was lower than the cervix and nasopharynx. Statistically, no breast cases correlated significantly (p < 0.05) between body weight and GPR. This supported Kang's study which described that the GPR value can be influenced by the respiratory cycle and the position of the setup [3]. Based on figure 4, the subject of the dominant breast case had a negative correlation value, which contrasted with the cases of the nasopharynx and cervix, both of which had positive correlations. The negative correlation in breast cases can be caused by the dominant breast subject gaining weight, as shown in figure 3(b) and table 5. In breast cases, in general, patients gain weight, and chemotherapy is also associated with weight gain in breast patients [33, 36].

GPR changes in cervical cases were lower than in nasopharyngeal cases, and five subjects had significant changes in body weight (p < 0.05). However, only four subjects had a significant correlation (p < 0.05) between body weight and GPR. This situation in line with Inui study which stated that the changes in rectal gas volume have contributed to the GPR value [31]. The correlation between body weight and GPR values in cervical cases found here is similar to the result of Kim's study on the relationship between GPR and separation [4]. Specifically, Kim found that the decrease in GPR values in gynaecological cases occurred due to changes in SSD or the separation of 1.5–2.0 cm at the isocentre, which was measured based on lateral images from kilovoltage Cone-Beam Computed Tomography (kV-CBCT).

This study also found that the correlation between GPR and body weight, which was determined using a field-by-field evaluation method, was stronger than that of a composite evaluation. This is because, in the composite evaluation, fluence from other fields could affect the GPR value [21]. Therefore, IVD verification using the EPID is more accurate when performed with the field-by-field evaluation approach.

This study used a 95% threshold to consider the time required for replanning. For cervical and nasopharyngeal cases, it is possible to consider replanning in fractions 17 and 11, respectively. However, the average GPR value for each fraction in cervical cases is ≥ 90%, making it possible not to replan. Replanning is done to optimise the target dose and OAR [13]. According to Figen, Colpan Oksuz [37] study showed that the average HNC case was replanned in fraction 15, which provided information on tumor shrinkage (35.5%) and weight loss (35.5%) based on the data obtained. In addition, Bak, Skrobala [29] showed that 84.1% of the main reason for replanning is shrinkage of the body contour.

Therefore, it should be noted that changes in body weight are one of the causes of changes in the dose distribution in patients 18, 19]. However, the drawback of this study is that it cannot provide recommendations when replanning is carried out based on information from body weight. This is because complete weight data are needed. There is no information on body weight data in the first fraction or during the CT simulator that can be used as an initial reference. Therefore, it does not calculate the difference in weight loss or gain of the patient. In addition, it is necessary to validate the relationship between GPR and dose changes.

Although the nasopharynx, cervix, and breast have unequal reproducibility, they have similarities in how the GPR value decreases as the fraction increases. The subject's body weight was associated with a decrease in the GPR value, with the nasopharyngeal case having the highest correlation and the breast case the lowest. In addition, breast cases had the lowest correlation with body weight. The correlation between body weight and GPR is stronger when field-by-field evaluation is used rather than composite evaluation. Furthermore, information on GPR values can be used as a consideration for replanning related to changes in dose distribution. The results of the PSQA met the tolerance and action limit recommended by the AAPM TG-218, and the PSQA measurement with the EPID was found to be more accurate with gantry rotation conditions.

This study was supported by PUTI Research Grant Universitas Indonesia with contract number NKB-1139/UN2.RST/HKP.05.00/2022, and for provision of facilities by the Radiation Oncology Integrated Service Installation (IPTOR) of Dr Cipto Mangunkusumo hospital.

All data that support the findings of this study are included within the article (and any supplementary files).

This study was approved by ethical committee Faculty of Medicine Universitas Indonesia with number Ket-221/UN2.F1/ETIK/PPM.00.02/2022.