There are many similarities between how 3D Pelvis and Abdomen cases are planned and so these two tutorials have been combined. This tutorial will be written primarily for the Pelvis, and information specific to an Abdomen case will be italicized and in brackets.
Pre-simulation
Pelvis cases are commonly seen with a prescription of 45 Gy in 25 fx. Depending on the site, the physician may contour volumes, just set apertures, or do a combination of both.
You may need to import ancillary images for a 3D pelvis plan. The physician may request for a PET, MRI, or a previous CT scan.
Simulation
Patients are simulated in either the supine or the prone position. If simulated in the supine position, the patient will lie on the table to which an inflated vac lock bag is attached. There are pegs at the head of the table for the patient to grab onto. Once the patient is appropriately positioned, the therapists will remove air from the vac lock bag, and doing so will create a mold/outline of the patient’s body contour for treatment reproducibility.
The physician may request to simulate the patient in the prone position while lying on a belly board to reduce bowel toxicity. The belly board that is designed with an opening for the belly of the patient to fall into and will displace the bowel superiorly and anteriorly. Thus, less bowel will be present near the treatment area.
[Patients are generally simulated in the supine position. The patient will lie on the table to which an inflated vac lock bag is attached. Once the patient is appropriately positioned, the therapists will remove air from the vac lock bag, and doing so will create a mold/outline of the patient’s body contour for treatment reproducibility. The physician may occasionally opt for an abdominal compression by which a pressure plate dwells over and gently presses on the abdomen to reduce organ motion from breathing.]
In either position, the physician may also request that the patient be simulated with a comfortably full bladder as a full bladder will push the bowel superiorly out of the treatment area and also decrease dose to the bladder. This setup technique is commonly seen in prostate and rectal cancer patients. The therapists will proceed to align the patient and use the lasers of the CT machine to place radiopaque BBs on the laterals and AP (supine) or PA (prone) of the patient on the same Z plane. The CT scan will then take place using a slice thickness of 2.5 mm, and the images will then be pushed to the treatment planning system (TPS).
Contouring
Create a structure set that includes the following structures:
- GTV – Gross Tumor Volume
- CTV – Clinical Target Volume
- PTV – Planning Target Volume
- Body
- Bladder
- Two unique structures: “LT Femoral Head” and “RT Femoral Head”
- Two unique structures: “LT Kidney” and “RT Kidney”
- Small Bowel
- Large Bowel
- Bowel
- Rectum
- Potentially the spinal cord, liver, penile bulb, and/or stomach if they are close to the target volume
- Pacemakers are not normally a concern with pelvic treatments but it is good practice to check if the patient has one. Pacemakers can receive no more than 2 Gy max point dose.
[Create a structure set that includes the following structures:
- GTV – Gross Tumor Volume
- CTV – Clinical Target Volume
- PTV – Planning Target Volume
- Body
- Esophagus
- Heart
- Liver
- Two unique structures: “Lt Kidney” and “Rt Kidney”
- Stomach
- Three unique structures: “Lung LT”, “Lung RT”, and “Whole Lung – GTV”
- Spinal Cord
- Pacemakers are not normally a concern with abdomen treatments but it is good practice to check if the patient has one. Pacemakers can receive no more than 2 Gy max point dose.]
The physician will contour the “GTV”, “CTV” and “PTV”. The physician may re-name these and/or contour additional structures. If the CTV is contoured, but the PTV is not, ask the physician for instructions to create the PTV. For example, the physician may request for an outer 0.5 CM margin around the CTV. On the other hand, the physician may not contour, but decide to only set apertures. Do NOT modify physician drawn contours.
Planning Setup
Refer to the link below for a broad setup overview:
3D pelvis cases are generally treated using three or four fields of the same isocenter. A 4-field technique may be preferred if the PTV is more centrally located, whereas a 3-field technique may be preferred if the PTV is located predominately anteriorly or posteriorly. Oblique fields, though not as common, may also be utilized for a pelvis depending on the location of the PTV.
- [The dosimetry on a low lying esophageal tumor may benefit from using LPO (~115°) and RPO (~225°) wedged fields as the the depth of normal tissue for which the beam traverses to achieve adequate coverage will be lesser for oblique fields compared to lateral fields. Thus, these oblique fields will have a lesser entrance dose, and also reduce the amount of lung dose compared to a 4 field AP/PA and LT/RT LAT opposed pairs.]
Here are combinations of field orientations you may select from:
- AP and PA fields.
- AP, LT LAT, and RT LAT fields (90° apart)*.
- AP, PA, LT LAT, and RT LAT fields (90° apart), also referred to as a “four field box”. If a four field box is used for a prostate case, it will typically be for palliation*.
- AP, LAO, LPO, PA, RPO, and RAO (60° apart), or a field orientation of seven fields (4 anterior & 3 posterior). Used for 3D prostate cases when the insurance fails to approve the IMRT technique.
Field orientations marked with an asterisk* are most commonly used.
If the physician is expected to draw apertures, and will be utilizing an opposing field orientation (e.g. AP/PA and/or LT LAT/RT LAT), it is fair to prepare only one half of the opposing field pair. For example, for an AP/PA pair, remove the PA field because chances are likely that the physician will request of you to “oppose” his AP field aperture and then proceeding to make subtle adjustments, as opposed to re-drawing the aperture for the PA beam.
- Properly name the fields based on the beam number, orientation, and gantry angle (e.g. 15 LT LAT G90, 01 AP G0). Remember to check for previous treatments in the process of naming beams.
- Align the fields to the PTV and round the coordinates to the nearest decimal place (e.g. +2.4 CM shift, not +2.48 CM). If you presume the shifts to yield minor or negligible advantages, leave the fields at a shift of 0 CM.
- Rotate the collimator to either 0° or 90°. At 0°, the MLC leaves will fit to the PTV moving from left to right/right to left. At 90°, the MLC leaves will fit to the PTV moving from superior to inferior/inferior to superior. If the physician is expected to draw the apertures, ask about the collimator rotation BEFORE he/she starts working.
- Consider a 6X vs 15X comparison based on the size of the patient and the location of the PTV. Mix and match 6X fields and 15X fields if doing so is dosimetrically justifiable.
3-Field Planning
If the physician drew the apertures for this plan, skip the subsequent portion on MLC adjustment. Remember that you will NOT edit a physician’s work without his/her approval. The following section undertakes the assumption that the dosimetrist will create the MLC pattern for the PTV.
- Select the target structure to be the PTV.
- Select circular margin, collimator coordinate system, leaf edge-contour meet point = outside, closed leaf meeting position = center, and only check “optimize collimator jaws”.
- Give a 0.7 CM margin with this tool for penumbra to ensure full dose to the PTV. Using zero margin or a larger margin may be used depending on the dosimetry on a case-by-case basis. Remember to evaluate the dose to the PTV and OARs when determining the margin.
After using this tool, you should see that the field size, on the axial view and the BEV, have been adjusted to include the entirety of the PTV. In the BEV, adjust the field size so that the edges align with the MLC leaves so that leaves are not partially exposed which may require you to open the field size by millimeters. The difference between the field edge and the MLC leaves should only be off by only a few millimeters.
After you or the physician are finished setting apertures, place your reference point in the center of the PTV and calculate. Manually move the reference point to a location which provides adequate coverage while meeting the dose constraints of the OARs.
A different method of achieving adequate coverage without continually moving the reference point is by normalizing to a target volume. As the reference point will not be the prescription point unlike the previous method, it may be placed anywhere within the PTV for now. In the dose prescription tab, normalize the prescription to 95/95 or 100/95 with the “PTV” as the target volume, and the TPS will achieve the set normalization. Then, by moving the reference to the 100% isodose line, and changing the normalization back to “100% at reference point”, you can find the exact location to achieve the coverage you were looking for. For many simple 3D plans, it is unconventional to normalize the prescription to a target volume, even though it is very commonly seen in VMAT planning. If the hotspot is too high, you can:
- Adjust the weighting of the fields*.
- Use wedges*. Refer to the link below for more information on wedges.
- Use the field-in-field technique. Refer to the link below for more information on field-in-field.
- Settle for a lesser PTV coverage and consult with the physician to explain your justification.
Techniques marked with an asterisk* are most commonly used.
The following below will discuss the planning techniques as if the beam orientation consisted of: AP, LT LAT, and RT LAT. Thus, if you are utilizing a PA beam because your PTV is located more posteriorly, read the explanation as if AP is PA.
Adjusting the weighting of the fields and using wedges are the preferred planning techniques for this beam orientation. The weighting of the beams will generally favor the AP beam, and the weighting between the lateral beams will be made so that the dose distribution is as close to symmetrical on the axial view along the central axis of the AP beam.
Wedges are most commonly equipped onto the lateral fields with the thick end pointing in the AP direction. The wedge is orientated in this position to tilt the dose distribution away from where the beam divergence overlaps. In doing so, what would normally be regions of high dose at the overlap will be reduced, and the posterior aspect of the PTV will also receive greater dose.
The more anterior or posterior the PTV is located is usually indicative of greater weighting onto the AP beam, and also indicative of using a thicker wedge on the lateral beams.
Dose constraints will be listed at the bottom of this page.
4 Field Planning
If the physician drew the apertures for this plan, skip the subsequent portion on MLC adjustment. Remember that you will NOT edit a physician’s work without his/her approval. The following section undertakes the assumption that the dosimetrist will create the MLC pattern for the PTV.
- Select the target structure to be the PTV.
- Select circular margin, collimator coordinate system, leaf edge-contour meet point = outside, closed leaf meeting position = center, and only check “optimize collimator jaws”.
- Give a 0.7 CM margin with this tool for penumbra to ensure full dose to the PTV. Using zero margin or a larger margin may be used depending on the dosimetry on a case-by-case basis. Remember to evaluate the dose to the PTV and OARs when determining the margin.
After using this tool, you should see that the field size, on the axial view and the BEV, have been adjusted to include the entirety of the PTV. In the BEV, adjust the field size so that the edges align with the MLC leaves so that leaves are not partially exposed which may require you to open the field size by millimeters. The difference between the field edge and the MLC leaves should only be off by only a few millimeters.
After you or the physician are finished setting apertures, place your reference point in the center of the PTV and calculate. Manually move the reference point to a location which provides adequate coverage while meeting the dose constraints of the OARs.
A different method of achieving adequate coverage without continually moving the reference point is by normalizing to a target volume. As the reference point will not be the prescription point unlike the previous method, it may be placed anywhere within the PTV for now. In the dose prescription tab, normalize the prescription to 95/95 or 100/95 with the “PTV” as the target volume, and the TPS will achieve the set normalization. Then, by moving the reference to the 100% isodose line, and changing the normalization back to “100% at reference point”, you can find the exact location to achieve the coverage you were looking for. For many simple 3D plans, it is unconventional to normalize the prescription to a target volume, even though it is very commonly seen in VMAT planning. If the hotspot is too high, you can:
- Adjust the weighting of the fields*.
- Use wedges*.
- Use the field-in-field technique.
- Settle for a lesser PTV coverage and consult with the physician to explain your justification.
Techniques marked with an asterisk* are most commonly used.
Adjusting the weighting of the fields is the preferred planning technique for this field orientation. The ideal dose distribution will be a uniform distribution such that the isodose lines are near symmetrical along the central axi of the fields. You do not want hotspots veering off to one quadrant or predominantly on one side of the PTV. The field weighting will typically favor the AP/PA as the depth to the PTV is normally lesser in the AP/PA compared to from LT/RT LAT.
[The following will discuss the planning techniques as if the beam orientation consisted of: AP, PA LPO, and RPO. This setup may be seen for a low lying esophageal cancer. Thus, if you are opting for a LAO and LPO because your PTV is located more anteriorly, read the explanation while substituting LPO and RPO for LAO and LPO respectively.
Adjusting the weighting of the fields and using wedges are the preferred planning technique for this beam orientation. The weighting between the oblique fields will be made so that the dose distribution is as close to symmetrical on the axial view along the central axis of the AP/PA beam. Overall, beam weighting for a 4-field with obliques will require some trial and error to achieve the ideal dose distribution before using wedges.
Wedges are most commonly equipped onto the oblique fields with the thick end pointing in the direction of the PA. The wedge is orientated in this position to tilt the dose distribution away from the overlap of the PA, LPO, and RPO beams. In doing so, what would normally be regions of high dose at the convergence will be reduced, and the anterior aspect of the PTV will also receive greater coverage.]
In general, a 3D pelvic plan must meet the following constraints:
- At least 95% of the prescription dose covers at least 95% of the PTV.
- At least 99% of the GTV should receive at least 100% of the prescription dose.
- The maximum hot spot or “3D Dose Max” cannot exceed 110% prescription dose.
- The rectum has four constraints:
- The volume of the rectum that receives 60 Gy should be below 50%.
- The volume of the rectum that receives 65 Gy should be below 35%.
- The volume of the rectum that receives 70 Gy should be below 10%.
- The volume of the rectum that receives 70 Gy should be below 10cc.
- The bladder has four constraints:
- The volume of the rectum that receives 65 Gy should be below 50%.
- The volume of the rectum that receives 70 Gy should be below 35%.
- The volume of the rectum that receives 75 Gy should be below 25%.
- The volume of the rectum that receives 80 Gy should be below 15%.
- The small bowel has four constraints:
- The volume of the small bowel that receives 35 Gy should be below 180 cc.
- The volume of the small bowel that receives 40 Gy should be below 100 cc.
- The volume of the small bowel that receives 45 Gy should be below 65 cc, no more than 150 cc.
- The max point dose of the small bowel should be no greater than 50 Gy (0.035 cc point dose).
- The volume of each femoral head that receives 50 Gy cannot exceed 10%.
- The mean dose to the penile bulb cannot exceed 52.5 Gy.
- The spinal cord should not exceed a max point dose of 45 Gy.
- The kidneys have six constraints:
- The volume of each kidney that receives 15 Gy cannot exceed 55%.
- The volume of each kidney that receives 20 Gy cannot exceed 32%.
- The volume of each kidney that receives 23 Gy cannot exceed 30%.
- The volume of each kidney that receives 28 Gy cannot exceed 20%.
- The mean dose to each kidney should not exceed 18 Gy.
- If the mean dose to one kidney exceeds 18 Gy, the other kidney will have a new constraint: the volume of this kidney that receives 6 Gy cannot exceed 30%.
- The liver has two constraints:
- The volume of the liver that receives 28 Gy cannot exceed 100%.
- The mean dose to the liver should not exceed 38 Gy.
[In general, a 3D abdomen plan must meet the following constraints:
- At least 95% of the prescription dose covers at least 95% of the PTV.
- At least 99% of the GTV should receive at least 100% of the prescription dose.
- The maximum hot spot or “3D Dose Max” cannot exceed 110% prescription dose.
- The spinal cord should not exceed a max point dose of 45 Gy
- The whole lung – GTV has four constraints:
- The volume of the whole lung – GTV that receives 20 Gy cannot exceed 30-35%.
- The volume of the whole lung – GTV that receives 10 Gy cannot exceed 45%.
- The volume of the whole lung – GTV that receives 5 Gy cannot exceed 65%.
- The mean dose to the whole lung – GTV cannot exceed 20-23 Gy.
- The heart has three constraints:
- The volume of the heart that receives 40 Gy cannot exceed 100%.
- The volume of the heart that receives 45 Gy cannot exceed 67%.
- The volume of the heart that receives 60 Gy cannot exceed 33%.
- The esophagus has seven constraints:
- The volume of the esophagus that receives 35 Gy cannot exceed 50%.
- The volume of the esophagus that receives 50 Gy cannot exceed 40%.
- The volume of the esophagus that receives 55 Gy cannot exceed 67%.
- The volume of the esophagus that receives 65 Gy cannot exceed 33%.
- The volume of the esophagus that receives 70 Gy cannot exceed 20%.
- The max point dose to the esophagus should not exceed 70 Gy.
- The mean dose to the esophagus cannot exceed 34 Gy.
- The kidneys have six constraints:
- The volume of each kidney that receives 15 Gy cannot exceed 55%.
- The volume of each kidney that receives 20 Gy cannot exceed 32%.
- The volume of each kidney that receives 23 Gy cannot exceed 30%.
- The volume of each kidney that receives 28 Gy cannot exceed 20%.
- The mean dose to each kidney should not exceed 18 Gy
- If the mean dose to one kidney exceeds 18 Gy, the other kidney will have a new constraint: the volume of this kidney that receives 6 Gy cannot exceed 30%.
- The liver has two constraints:
- The volume of the liver that receives 28 Gy cannot exceed 100%.
- The mean dose to the liver should not exceed 38 Gy.]
