Direct simulation of fields to mesentary and other abdominal lymphoma manifestations should only by done for palliative treatment with a relatively short goal for symptoms relief. Much movement oflesions in the mesentary must be expected as well as a low accuracy by X-ray on the simulator.
For direct simulation, the field borders are placed with adequate margins to the tumor to include microscopic growth in the tumor border zone (1 cm), penumbra and set-up variation (minimum 1–1.2 cm) and assumed internal movement. In addition, there is uncertainty because during palpation or X-ray, tumor borders can almost never be defined with certainty. One must depend on information from radiological diagnostics which is transferred to the simulator image (for example the relation of the tumor to bone structures).
The fields are often large.
Treatment should be done based on CT-guided dose planning.
The actual or orginal tumor volume (for curative treatment of localized indolent lymphomas or after limited chemotherapy for early stages of HL and aggressive NHL) defines GTV.
- CTV is generated with a 1 cm margin in all directions, but is limited to the contact surfaces where infiltration is not expected such as bone structures.
- A 2-3 cm margin is made for internal movement for ITV. ITV should, again, be limited. For example, if the movement in toward the retroperitoneal structures such as the kidney and spine is significantly less that in the peritoneal cavity.
- Standard field set-up is with anteroposterior beams, but in many cases, the extent of the tumor in the kidney region, side differences in kidney function etc. necessitate other field set-up with better shielding of healthy renal tissue, for example a diagonal field or side field.
CT dose plan, mesenterial lymph nodes
CT dose plan, paraaortal region
Gonads present in the primary field, but not within the target volume, must be shielded by blocks in the filter holder or by using a multileaf collimator. At Oslo University Hospital HF, lead blocks have traditionally been used rendering 10 half value layers. The standard blocks used previously for the scrotum (in men) and bladder bladder (both women and men) are no longer used. The leaves from the multileaf collimator can be enhanced by using and extra lead layer to give the same effect.
For girls and women of fertile age, shielding of the ovaries and/or ovariopexy should be considered. Ovariopexy is the surgical relocation of the ovaries out of the small pelvis to the midline behind the uterus. Surgical clips should indicate where the ovaries are located. Only then is it possible to exclude the ovaries from the target volume.
In addition to shielding, it is important to consider use of close shielding against diffuse radiation, which mainly occurs in the filter holder and multileaf collimator. This applies to both the gonads that lie in the primary field but are shielded with blocks or multileaf collimator and for gonads that lie outside but near the primary field. At Oslo University Hospital today, a lead belt is used to pull the scrotum away from the field for unilateral irradiation in the pelvic region, as well as a gonadal shield attached to the treatment table. For symmetric irradiation in the pelvic region, a scrotum cup is used (5 mm of lead under and on the side of the scrotum) with a 3 cm lead block on top. The ovaries are shielded from diffuse spreading if they are in the primary field or near it with a gonadal shield attached to the treatment table.
Standard fractionation and total dose for curative treatment is given below. These are also guidelines for palliative treatment, but must be modified individually.
For Hodgkin's lymphoma stage I-IIA without risk factors: 2 Gy x 10
Otherwise for Hodgkin's lymphoma: 1.75 Gy x 17
For curative treatment of indolent non-Hodgkin lymphoma: 2 Gy x 15
For aggressive NHL: 2 Gy x 20.
- For large abdominal fields where doses to the kidneys are limited, fractionation into 1.2 Gy x 15-16 or 1.5 Gy x 12-13 may be considered.