Impact of jaw position on sparing organs at risk in three-1 [601236]

Impact of jaw position on sparing organs at risk in three-1
dimensional conformal radiation therapy of pancreatic 2
cancer 3
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Abstract 6
Introduction- The objective of this work is to investigate the impact of collimation jaw 7
positioning on dose to organs at risk (OARs) during a 3-dimensional conformal radiotherapy 8
(3DCRT) of pancreatic cancer and postulate a method to minimize OAR dose by proper 9
positioning of the jaws. 10
Methods – Clinically delivered 3DCRT treatment plans of 10 patients optimized with multiple 11
static beams using multileaf collimator (MLC) leaves conformed to a block margin around 12
target, and collimation jaws aligned with outer extent of the block margin were selected. 13
Subsequent plans were generated by displacing the collimation jaws outward in lateral, superior-14
inferior or both directions by 1 and 2 cm without altering the MLCs. Computed doses with 15
unaltered dose normalization were compared against the corresponding doses to OARs and target 16
obtained from the original plans. 17
Results – Outward displacement of the collimation jaws by 1 cm in lateral or/and superior-18
inferior resulted in a significant increase in mean doses to OARs. The increases were found to be 19
proportional to the outward displacement of jaws. Increase in maximum dose to spinal cord was 20
significant in few patients while it was insignificant for all other OARs. 21
Conclusion – Collimation jaws aligned with outer extent of block margin minimize dose to 22
OARs. Any gap between the block margin and collimation jaws can lead to inadvertent delivery 23
of higher dose to OARs. Hence, optimal jaw positioning during radiotherapy planning becomes 24
important to all patient plans. 25
Keywords: 3DCRT, pancreatic cancer, collimation jaws, OARs 26
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Introduction 34
Pancreatic cancer has been projected to become second leading cause of cancer related death in 35
the United States by 2030 1. Surgery alone is not an obvious option for pancreatic cancer 36
treatment because of its aggressive biology, late diagnosis, encasement of large blood vessels 37
and the presence of metastasis 2,3. Despite the high chances of distant metastases of pancreatic 38
cancer, radiotherapy may provide a survival advantage 4. Current radiation prescription dose (~ 39
54 Gy) is not adequate for the tumor control 5,6. One approach for better tumor control is to 40
increase the prescription dose but that option comes at the cost of higher toxicity to OARs 7. 41
Hence reducing dose to OARs becomes extremely important. Intensity modulated radiation 42
therapy (IMRT) is a technique that can spare critical structures well. But some studies suggest 43
that IMRT did not present a significant advantage over three dimensional conformal radiation 44
therapy (3DCRT) in terms of OARs sparing 8. There are also potential problems of low dose 45
spread over larger volume and higher leakage dose due to the usage of higher monitor units 46
(MUs) in IMRT treatment. Nonetheless, selection of radiotherapy treatment modality depends on 47
the staging of tumor and confidence level of a physician among other factors; and 3DCRT 48
remains a widely used modality for treating pancreatic cancer. 49
Usually, tumors have a complex shape and collimation jaws of a linear accelerator (LINAC) 50
alone cannot conform to the targets. Multileaf collimators (MLCs) incorporated in modern 51
LINACs offer a good conformity with the target but leaf transmission leads to higher radiation 52
doses as compared to the collimation jaws. Rounded end MLCs suffer from bigger transmission 53
penumbra and higher leaf-end radiation transmission 9. For the current standard prescribed dose 54
to the target, 3DCRT plans usually meet the clinical standards. However, optimal jaw position 55
can help reduce dose to OARs surrounding the target 10. Dose reduction to OARs can help 56
escalate the prescription dose, or at least reduce the chances of complication to the OARs. 57
Minimum Y-jaw separation required for enhanced dynamic wedge (EDW) in some LINACs may 58
not allow the use of optimal jaw position in some of the 3DCRT plans. This factor as well as a 59
gap introduced between jaws and block margin during planning can increase dose to surrounding 60
critical structures. Here we perform a quantitative assessment of the effect of jaw position on 61
dose to OARs and present a method to minimize dose to OARs during a 3DCRT treatment of 62
pancreatic cancer. 63
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Materials and Methods 65
I. Patient Population 66
Ten pancreatic cancer patients treated with 3DCRT were selected in random for the study. These 67
patients were imaged with a computed tomography (CT) simulator (Phillips Brilliance Big Bore, 68
Amsterdam, Netherlands) for treatment planning. Treatment plans were generated with Eclipse 69
treatment planning system (Version 11.0.47, Varian Medical Systems, Palo Alto, CA) using the 70
analytical anisotropic algorithm (AAA). The patients were treated to 45-50.4 Gy (1.8 71
Gy/fraction) using a 18 MV photon beam in a Varian TrueBeam STx LINAC with rounded leaf 72
end 120 high-definition MLCs. OARs (left and right kidneys, stomach, liver, cord and bowel) 73
and target (planning target volume- PTV) were contoured. PTV volume among the patients 74
ranged from 56 to 896 cc. While uniform 7 mm margin around PTV was enough for intended 75

target coverage in 8 patients, 8 mm was needed in 2 cases. Four to seven static beams (with 76
collimator angles of 00 or 900) were used to generate each treatment plan. The beam angles and 77
EDW (a virtual wedge created by computer controlled Y-jaw movement and dose rate change in 78
Varian LINACs) were used as needed in order to get good target coverage while minimizing 79
dose to OARs. Treatment plan was normalized to cover 95% of PTV volume by 100% of the 80
prescribed dose with the collimation jaws aligned with the outer extent of the block outline as 81
shown in Figure 1(a). Dose to PTV and OARs were evaluated with isodose distributions and 82
dose volume histograms (DVHs). 83
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Figure 1- Collimation jaws (a) aligned with the outer extent of block margin around PTV in a 85
clinical plan (b) displaced by 1 cm in superior inferior direction. 86
II. Research plans 87
Starting from the clinical treatment plan, subsequent research plans were developed by 88
translating two opposite jaws away from the treatment field. The jaws were moved in superior-89
inferior (refer Figure 1(b)), lateral or both directions by 1 cm and 2 cm. Note that the MLC 90
positions were not altered. Dose to the target and OARs was calculated for every research plan 91
keeping the same PTV dose normalization. Preserved dose normalization did not alter MUs used 92
in any of the treatment plans. 93
Mean and maximum doses to the OARs for various jaw positions were obtained from the 94
treatment planning system and compared against the corresponding doses obtained from the 95
delivered clinical plans. As there were differences between the mean and maximum doses to 96
OARs in the research plans in comparison with the clinical plans, the significance of the 97
differences was evaluated using statistical analysis. 98
III. Statistical Analysis 99
Test for normal distribution was performed using Shapiro-Wilk test in the R statistical package11. 100
Statistical significance was tested for normally distributed data using a paired Student’s T-test. 101
For a distribution showing larger deviation from a normal distribution, a Wilcoxon signed-rank 102
test was instead used. Statistical significance was compared against a threshold p-value of 0.05. 103

Results 104
Range of the change in mean dose to target and various OARs due to lateral or superior-inferior 105
displacement of jaws by 1 cm are tabulated in Table 1. Changes in mean doses to OARs were 106
larger than the changes in maximum dose. 107
Table 1: Range of percentage increase in mean dose (% ∆D mean) to PTV and OARs due to 1 cm 108
jaw displacement along the lateral or superior-inferior direction for 10 patients 109
Structure % ∆D mean (Lateral) % ∆D mean (Sup-inf)
Lt. kidney 0.4–2.0 -0.2–0.8
Rt. kidney 0.4–2.9 -0.1–2.0
Bilat. kidney 0.3 –2.1 -0.1–0.9
Cord 0.0–2.6 0.2–1.1
Stomach 0.2 –1.3 0.5 –5.3
Bowel 0.1–1.1 0.2–1.5
Liver 0.3–1.2 -0.0–3.2
PTV 0.0–0.1 -0.1
110
As evident from Table 1, mean dose to most of the OARs increased approximately by 1% due to 111
1 cm outer displacement of jaws. This increase was found up to 5% in some cases. Student t-test 112
and Wilcoxon signed rank test showed that mean dose differences are significant (p < 0.02). 113
Kidneys, stomach and cord doses were most affected by an increased jaw margin. These effects 114
were smaller for larger structures such as liver and bowels, and other structures that are beyond 2 115
cm from the target. There was insignificant change in mean dose to PTV, as expected from the 116
preservation of plan normalization. 117
Increase in mean dose for 1 cm and 2 cm lateral and superior-inferior displacement of jaws 118
averaged over 10 patients are shown in Figure 2 and Figure 3 respectively. The percentage 119
increase in mean dose to the OARs increased linearly with the increase in outward lateral 120
displacement of the jaws. 121

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Figure 2- Effect of outward (later al) displacement of jaws on mean dose to OARs and target 123
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Figure 3- Effect of outward (superior-inferior) displacement of jaws on mean dose to OARs and 126
target 127 0.00.40.81.21.62.02.42.83.23.6
1cm
2cm
Average increase in mean dose (%)
Lt kid Bilat kid Rt kid Cord Stomach Liver Bowel PTV
-20246810121416
Average increase in mean dose (%)
Stomach Liver Bowels Rt kid Cord Bilat kid Lt kid PTV 1cm
2cm

Results from Figure 2 and 3 show that structures lying laterally outwards from the target 128
(kidneys, and liver) were less affected by the superior-inferior displacement of the jaws. 129
Likewise, liver and bowel were less affected by lateral jaw displacement. Even though changes 130
in maximum dose to most of the OARs were small, meaningful changes were observed for cord 131
in few patients. Maximum dose, a limiting parameter for cord, increased up to 3 % (~ 100 cGy) 132
for 1 cm superior-inferior displacement of jaws and increased slightly with the increase in jaw 133
displacement. Outward displacement of all four jaws resulted in a much higher increase in mean 134
dose to the OARs than only with one jaw pair. Increase in mean dose to various structures 135
averaged over 10 patients is presented in Figure 4. 136
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Figure 4- Effect of outward displacement of all four jaws by 1 and 2 cm on mean dose to OARs 138
As evident from Figure 4, increase in mean dose to OARs averaged over 10 patients ranged from 139
1.5 to 4.3 % for 1 cm outward displacement of all 4 jaws and the values almost doubled for 2 cm 140
displacement. But the increase was 0.0% for PTV in both cases. Among the patients studied, the 141
increase in mean dose to stomach for 1 cm jaw displacement was as high as 144 cGy. The 142
increase in mean dose to other structures in some patients ranged as high as 50-100 cGy. Such 143
increases are statistically significant and can have clinical significance in some structures. 144
Increases in mean dose to some of the OARs in few patients were much larger than the averaged 145
values displayed in the figures. As such a large increase may occur in any patient, a careful 146
consideration should be given to minimize it. 147
148 -202468101214161820
1cm
2cm
Average increase in mean dose (%)
Stomach Cord Liver Rt kid Bowel Lt kid Bilat kid PTV

Discussion 149
Our study did not show any obvious correlation between the increase in mean dose to the OARs 150
and any other clinical parameters such as PTV size. It is obvious that PTV shape and location 151
differ with patients. Hence portion of the critical structures lying underneath the gap between the 152
inner extent of MLCs and jaws varied with complexity of the PTV shape. This variation in gap 153
resulted in a different amount of increase in mean dose to the OARs. 154
Generally, MLCs conform to the outer extent of the block margin and a few millimeter gap 155
between inner extent of MLCs and collimation jaws is typical in a 3DCRT planning. However, 156
there can be a different scenario in some cases. As an example, if a planner decides to reduce 157
margin around the PTV, MLCs move to conform to the new block margin leaving the jaws in 158
their original position. Some treatment planning systems such as Eclipse do not allow a merger 159
of fields with different jaw defined field sizes. Hence the usage of subfields in a field does not 160
reduce gap between block margin and jaws and leakage dose around the treatment target does 161
not decrease. In addition, the use of an EDW in Varian machines requires a minimum of 4 cm 162
Y-jaw separation. Hence an EDW might not permit the optimal position of jaws for small 163
targets. There effects should be carefully considered during treatment planning. Without a 164
careful consideration, OARs underneath the MLCs can receive an unnecessarily higher dose. 165
The role of optimal jaw positioning in reducing dose to normal structure in 3DCRT based 166
intracranial stereotactic radiosurgery (SRS) planning has been investigated and our results are in 167
line with the results from Han et al. 12 168
169
Conclusions 170
Collimation jaws aligned with the outer extent of the block margin offer the optimal position in 171
minimizing dose to OARs in the 3DCRT. Any outward displacement of jaws from the optimal 172
position can increase dose to OARs surrounding the treatment target. This effect is patient 173
specific and depends on the gap between the inner extents of MLCs and the jaws and shape of 174
the target relative to the OARs. Even a small outward displacement of jaw may lead to a 175
substantial increase in dose to critical structures that may be of clinical significance. 176
Based on the study, we highly recommend that the collimation jaws be pulled all the way in to 177
match with the block outline around target in all external beam plans including 3DCRT plans. 178
Reduction in dose to normal structures not only lowers the chances of normal tissue complication 179
including the risk of secondary cancer, it also helps keep the door open for prescription radiation 180
dose escalation or a boost treatment for a better tumor control . 181
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